¶ Chapter 2 — DENTAL MATERIALS
Reference Edition
This chapter is part of the Air Force Dental Laboratory Manual (2005) – Digitally Restored Edition.
This edition preserves the original publication while correcting OCR errors, restoring formatting, reconstructing damaged tables where necessary, and improving digital readability.
The technical content has not been rewritten, modernized, expanded, or altered.
It is provided as a professional reference. Modern instructional material is published separately throughout DentalTechnology.org.
¶ Section 2A — Overview
¶ 2.1. Introduction
This chapter describes the composition, properties, and use of materials in a dental laboratory. Information on the use of the materials is also discussed in those chapters dealing with specific laboratory procedures.
¶ 2.2. Knowledge of Dental Materials
2.2.1. Almost all dental materials are obtained from a commercial manufacturer. Each manufacturer furnishes recommendations for handling and storage of its product so the desired results are consistently obtained. The dental laboratory technician must know which material is needed to do a good job, the way it is handled, how it reacts, and how it is stored to maintain its physical properties.
2.2.2. Knowledge of materials is not only necessary to routinely perform laboratory tasks successfully, but to evaluate a failure so it won't be repeated. A failure means wasted laboratory time, additional clinic time, and physical discomfort for the patient.
¶ Section 2B — Gypsum Materials
¶ 2.3. Introduction
Gypsum is the common name for calcium sulfate dihydrate.
2.3.1. Gypsum products are more frequently used in laboratory procedures than any other group of compounds. Controlled variations in the manufacture of gypsum products yield a group of dental materials that include plaster, dental stone , die stone, casting investment, and soldering investment.
2.3.2. Each substance is a carefully formulated powder that has the particular combination of physical properties to do a specific job. When the prepared powder is mixed with the proper amount of water, the blend initially forms a fluid paste that gradually hardens into a solid. In the fluid paste state, the mixture can be poured into molds or otherwise shaped. As gypsum sets, dense masses of crystals form and heat is liberated. This liberation of heat, called an exothermic reaction, happens while all gypsum products are setting.
¶ 2.4. Physical Properties
(NOTE: See Table 2.1 for an analysis of the physical properties of gypsum materials.)
2.4.1. Crushing Strength. Crushing strength or compressive strength is the measure of the greatest amount of compressive force that can be applied to a substance without causing it to fracture. The strength of a gypsum product increases rapidly as it hardens. Because the relative amount of water left in the set material has a distinct effect on strength, the following kinds of gypsum product strengths (wet and dry) are recognized:
2.4.1.1. Wet Strength. This is the strength of the material with excess water still present in the set up mass.
2.4.1.2. Dry Strength. This is the strength of a dried gypsum specimen. Twenty-four hours after setting, the compressive strength of a gypsum specimen left to dry will double.
2.4.2. Setting Time. The setting time is the time required for the material to set or harden. It is divided into the following stages:
2.4.2.1. Initial Set. The time starts when the powder is mixed with water and ends when the material becomes solid enough to remove from the tray and trim without distortion.
2.4.2.2. Final Set. This is the time required for full crystallization to occur. All exothermic heat dissipates and the mass reaches about half its potential crushing strength.
Table 2.1. Physical Properties of Gypsum Materials.
| ITEM | A | B | C | D | E | F |
|---|---|---|---|---|---|---|
| Material | Setting Time | Heat Resistance | Technique | |||
| Normal Setting Expansion | Hygroscopic Expansion | Thermal Expansion | ||||
| 1 | Plaster Initial: | 7 - 13 minutes Final: 45 minutes | NA | As low as possible | NA NA | |
| 2 | Stone (Hydrocal) | Initial: 8 - 15 minutes Final: 45 minutes | ||||
| 3 | Die Stone | Initial: 15 minutes Final: 25 - 30 minutes | ||||
| 4 | Soldering Investments | Initial: 8 - 12 minutes Final: 18 - 22 minutes | Matched to the melting temperature of the solder | Matched | to the expansion of the metals being soldered | |
| 5 | Gold- Casting Investments | Initial: about 12 minutes Final: 35 - 45 minutes | Matched to the burnout and casting temperature of the metal being cast | Thermal Expansion Technique: Semihygroscopic and thermal expansion must compensate for gold shrinkage (about 1.4 percent) | ||
| 6 | Hygroscopic Expansion Technique: Hygroscopic expansion, pattern wax expansion, and thermal expansion must total about 1.4 percent. | |||||
| 7 | Chrome- Nickel System Investment | Initial: 8 - 12 minutes Final: about 20 minutes | Special, gypsum-bound investment for the Ticonium® system | Combined semihygroscopic and thermal expansion must compensate for shrinkage of chrome-nickel (about 1.7 percent). | ||
2.4.3. Setting Expansion. A gypsum product enlarges in volume as it sets. This enlargement is called setting expansion and usually amounts to a fraction of 1 percent. A gypsum material sets up in air or in contact with water. The setting expansion varies, depending on the conditions the material is exposed to (Table 2.1).
2.4.3.1. Normal Setting Expansion. A gypsum product expands predictably when it is allowed to solidify unconfined in a normal room temperature environment. A setting expansion that takes place under these conditions is called normal setting expansion.
2.4.3.2. Hygroscopic Setting Expansion. Hygroscopic setting expansion occurs when a gypsum material is allowed to solidify under water. A hygroscopic expansion can be expected to more than double a normal setting expansion. In some dental procedures, a gypsum product solidifies in limited contact with water. For example, an investment is sometimes made to set against a wet ring liner. This expansion is greater than the normal setting expansion, but it is not as great as a hygroscopic expansion. A setting expansion that occurs as a result of limited contact with water is called semihygroscopic expansion.
2.4.3.3. Thermal Expansion. This kind of expansion occurs as the result of a gypsum product being heated. The amount of thermal expansion is proportional to the temperature.
¶ 2.5. Effect of Selected Variables on Crushing Strength
The strength of set gypsum products can be directly affected by several variables under the control of the technician:
2.5.1. Water-Powder Ratio. The crushing strength lowers as more water is used in the mix. Gypsum products are porous, and the greater amount of water increases porosity because there will be fewer crystals formed per unit of volume of the material.
2.5.2. Mechanical Mixing. Longer and more rapid mixing, up to a maximum of 1 minute, results in greater strength. However, overmixing breaks down the forming crystals and reduces the crushing strength of the end product.
2.5.3. Chemical Modifiers. In general, chemical modifiers reduce crushing strength. However, borax can act to increase the surface hardness of the material.
¶ 2.6. Effect of Selected Variables on Setting Time
The setting time of a gypsum product can be affected directly by certain variables the dental technician can control. These variables must be applied with extreme care. In gaining a more desirable setting time, other physical properties, such as strength, may be adversely affected as follows:
2.6.1. Water-Powder Ratio. A longer setting time is required when more water is used in the mix. Conversely, the setting time is reduced when less water is used in the mix.
2.6.2. Water Temperature. As the temperature of the water used in the mix is raised from 32 to 85 °F, the setting time is shortened. When the water is between 85 and 120 °F, the setting time is lengthened. If boiling water is used and the mixture is maintained at about 212 °F, the material will not set at all.
2.6.3. Mixing. The setting time is shortened as the mixture is stirred (spatulated) either for a longer time or at a faster rate.
2.6.4. Accelerators and Retarders.
2.6.4.1. An accelerator is a substance that, when added to a gypsum product, decreases the setting time. Conversely, a retarder increases the setting time. The manufacturer uses these substances to standardize the setting behavior of a product. At times, accelerators or retarders may be used to alter the usual setting behavior of a product.
2.6.4.2. Potassium sulfate and common table salt are accelerators; vinegar, potassium citrate, and borax are retarders.
2.6.4.3. Unfortunately, accelerators and retarders also change properties other than setting time, and they tend to reduce both setting expansion and crushing strength. For this reason, chemical accelerators or retarders should never be used with casting or soldering investments because a predictable setting expansion is important in these materials. Manipulating the water temperature, mixing time, and mixing rate are safer ways of controlling setting time than using chemicals.
2.6.4.4. There are a few laboratory procedures where using a specific accelerator is acceptable. One outstanding example is when slurry water is used to accelerate plaster or dental stone mixes in cast mounting procedures. Slurry water is a concentrated suspension of gypsum particles in water made by catching the runoff from a cast trim ming machine. The suspended gypsum particles are allowed to settle, and about two-thirds of the water is siphoned away. The object is to develop a more highly concentrated suspension when the sedimentary calcium sulfate dihydrate particles are reagitated. Each of these calcium sulfate dihydrate particles acts as a center of crystalline formation.
2.6.4.5. Depending on the concentration of the suspension, you can expect much shorter setting times when you use slurry water than when you use plain water.
¶ 2.7. Effect of Water-Powder Ratio and Mixing Time on Setting Expansion
The manufacturer strictly controls the setting expansion of a gypsum product by using a carefully measured amount of chemical modifiers. The manufacturer recommends standard proportioning and mixing procedures that make physical properties, including setting expansion, predictable. In the case of investments, setting expansion is such a sensitive factor that deviating from the manufacturer's directions is a questionable practice. Always be aware that a number of gypsum's properties are interdependent. For example, steps taken to change setting time can also alter setting expansion. If there is good reason to change a gypsum material's normal setting expansion, follow these guidelines:
2.7.1. Thick mixes (less water) tend to result in increased setting expansion and vice versa.
2.7.2. Long mixing times tend to increase setting expansion and vice versa.
¶ 2.8. Modeling Plaster
2.8.1. Manufacturing Process.
2.8.1.1. Gypsum is converted into model plaster by grinding it into small particles and then heating it slowly in open vats to drive off the water of hydration. Under a microscope, the plaster is seen to be made up of rough irregular crystals. Each crystal contains a definite proportion of water. This is called water of crystallization or water of hydration. The amount of water eliminated by heating has a bearing on the behavior of the plaster when it is again mixed with water in the laboratory.
2.8.1.2. A special process is used to ensure plaster made for dental use has suitable working properties. These properties must always be uniform throughout a batch of material and from one batch to another.
2.8.1.3. One of the most important requirements of plaster is that it must set or harden within definite time limits. The amount of setting expansion must also be from 0.2 to 0.3 percent. A setting expansion of 0.3 percent is the maximum amount allowed by the American National Standards Institute (ANSI) of the ADA's Specification Number 25 for model plaster.
2.8.2. Model Plaster's Uses. Model plaster has many uses in the laboratory. It is used for constructing a matrix, flasking a denture, attaching casts to an articulator, and as an ingredient in some investments. The initial setting time for most dental plasters is from 4 to 12 minutes. The final setting time is approximately 20 to 45 minutes.
¶ 2.9. Impression Plaster
This is a plaster that has been specially compounded for making impressions of the mouth, as follows:
2.9.1. Impression plaster must behave differently than model plaster. It must be able to set much faster to reduce the time it is held in the patient's mouth. Because a plaster impression cannot spring around an undercut as it is withdrawn from the mouth, it must be broken into pieces and reassembled outside the mouth. For this reason, it must be weak and brittle.
2.9.2. Impression plasters are rarely used in dentistry today due to the availability of hydrocolloids and elastomers. Impression plaster must have a very low setting expansion of 0.13 percent because an impression that changes size significantly is inaccurate. Various accelerators and retarders are added to control the setting time of plaster, and coloring agents are often added to distinguish one gypsum product from another.
2.9.3. Today, impression plaster is mainly used to obtain bite registrations for dentures or orienting a fixed partial denture in the mouth for a solder index.
¶ 2.10. Dental Stone
2.10.1. Dental stone is medium strength plaster that is stronger and more resistant to abrasion. It is used primarily for casts (such as diagnostic casts), opposing arch casts, and complete and partial denture working casts.
2.10.2. Dental stone is made by autoclaving the gypsum under pressure and then grinding it into a hemihydrate powder. The particles are more prismatic and regular in shape. For this reason, dental stone requires less water in mixing and sets more slowly. When set, it is harder, much more dense, and has a higher crushing strength than model or impression plaster. The average setting expansion is approximately 0.12 percent.
2.10.3. The manufacturer colors dental stone to make it easy to distinguish from plaster. The initial setting time of a typical dental stone product is from 8 to 15 minutes. The final set takes approximately 45 minutes.
¶ 2.11. Die Stone (Improved Stones)
2.11.1. Improved stones are specially processed forms of gypsum products used to make crown, onlay, and inlay dies. They are harder, more dense than dental stone, and have a 0.08 to 0.18 percent setting expansion. They are also colored to distinguish them from plaster.
2.11.2. Because the amount of setting expansion is critical, it is important to use the water-to-powder ratio the manufacturer recommends. These high strength plasters are made by first boiling the gypsum in a 30-percent calcium chloride solution before autoclaving and then grinding the stone into very fine particles. Some manufacturers use a 1 percent solution of sodium succinate, or they add resin particles to increase the hardness of the stone.
¶ 2.12. Investment Materials
Investments are products used to form molds for molten metal and to relate pieces of metal to one another prior to soldering. Investments are composed of a refractory (heat-resistant) substance, like cristobalite or quartz, and a binder. Common binders are gypsum, phosphate, and silicate compounds. As a result, investments are often described as gypsum, phosphate, or silicate bound.
2.12.1. Investments with a high cristobalite content expand more than those with a high percentage of quartz. Depending on what metal is to be used, some casting investments need significant expansion to compensate for metal shrinkage, and their refractory component needs to contain a higher amount of cristobalite. When low expansion is required (such as for soldering investments), the refractory component will be high in quartz.
2.12.2. Investments are supposed to withstand heat without decomposing. Depending on the binder, they become more or less able to resist heat-induced breakdown.
2.12.3. Overheated, gypsum-bound investments liberate sulfur dioxide which makes the casting brittle. To minimize sulfur dioxide liberation, gypsum-bonded investment molds are recommended to burn out below 1300 °F. Also, molten metals thrown (cast) into those molds should have casting temperatures below 1950 °F.
2.12.4. The company that produces Ticonium chrome alloy makes a special gypsum-bound investment that withstands a 1350 °F burnout temperature and a casting temperature of 2600 °F. Barring this kind of exception, phosphate and silicate bound investments have excellent high heat resistance and are commonly used when casting or soldering temperatures exceed 1950 °F. Some more recent investments can be used as “all-purpose” investments. They have a high silicate bound makeup and use burnout temperatures of 1500 to 1600 °F.
¶ 2.13. Inlay Investment
2.13.1. Inlay investments are usually gypsum bound. Inlay investments are commonly used for investing many different kinds of fixed restorations cast in conventional golds.
2.13.2. When molten gold alloy is cast into a mold, it cools and solidifies. As it cools, it shrinks. The amount of shrinkage is approximately percent. If nothing is done to compensate for this shrinkage, the casting will be too small. The mold space must be enlarged so the molten metal is cast into a space that is 1.4 percent oversize. As the molten metal solidifies and shrinks, the casting attains the correct size.
2.13.3. Techniques have been devised to use setting and thermal expansion characteristics of investments to compensate for cast metal shrinkage. In one technique, high heat (1290 °F) is used to produce the majority of the required expansion. In another technique, the hygroscopic expansion of the investment is responsible for most of the compensation.
2.13.4. Inlay investments tend to fall into two broad categories depending on how they are used--high heat technique investments (above 1300 °F) and low heat technique investments (1300 °F or less). One type of low heat technique is used with a high water content called a hygroscopic technique. This technique creates additional expansion at a lower temperature burnout.
¶ 2.14. Soldering Investment
2.14.1. A soldering investment is similar in composition to a casting investment with a quartz refractory. An investment with a quartz refractory expands less than one having cristobalite as the heat resistant component.
2.14.2. Minimal normal setting expansion is a desirable soldering investment characteristic. A soldering investment does not expand nearly enough to compensate for the shrinkage of molten gold and should not be used for casting purposes. Like casting investments, soldering investments are made with gypsum or high heat binders. The heat resistance of the binder is matched to the anticipated soldering temperature. As a rule of thumb, a soldering procedure that takes place above 1950 oF requires an investment with a high heat binder.
¶ 2.15. Investments for Chrome Alloys
2.15.1. High-Heat, Chrome-Alloy Investment. A high-heat, chrome-alloy investment is made to withstand a much higher heat than the 1300 °F normally used in eliminating wax for casting gold. Such an investment consists of a quartz powder mixed with an ethyl silicate liquid and is used with the high melting range of chrome alloys (2700 to 2800 °F).
2.15.2. Low-Heat, Chrome-Alloy Investment.
2.15.2.1. A low-heat, chrome-alloy investment is gypsum bound and has a silica refractory component. It is similar to the investment used for casting gold. A low-heat, chrome-alloy investment is used as part of the system for producing Ticonium chrome alloy castings. Ticonium metal is used throughout the Air Force Dental Service for RPD frameworks.
2.15.2.2. The burnout temperature of ticonium investment molds is 1350 oF, and the casting temperature of ticonium metal is 2500 to 2600 oF.
2.15.2.3. There is a sulfur dioxide liberation problem associated with gypsum bound investments at high burnout or casting temperatures. One way to combat this problem is to increase the percentage of refractory material relative to the gypsum binder in an investment formula. Ticonium metal shrinks 1.7 percent as it solidifies. The investment and burnout techniques are balanced to furnish that amount of expansion in the mold.
¶ 2.16. Investments for Ceramic Gold Alloys
2.16.1. Gypsum-bonded investments are not adequate for casting ceramic golds. The expansion is not high enough, and the gypsum decomposes under the high temperatures. Instead, investments containing magnesium oxide and soluble phosphate should be used.
2.16.2. The dissolved phosphate reacts with magnesium oxide to form a matrix of magnesium phosphate which binds silica particles together much the same as gypsum binds low heat investments. Phosphate-bound investments are coarse in particle size, heat resistant, strong, and sometimes difficult to remove from castings. The investment is sluggish and sets rather rapidly with a working time of 3 to 4 minutes. All-purpose investments have a smaller particle size; therefore, a smoother casting can be made.
¶ 2.17. Rules for Handling Gypsum Materials
2.17.1. Use Clean Equipment. Always use a clean mixing bowl and spatula. Hardened particles left in the bowl from a previous mix alter the setting time and weaken the material. As little as 0.1 percent of the hardened particles in a mix of casting investment reduces the setting time and alters the thermal or hygroscopic expansion. The best time to clean a bowl and spatula is while the plaster is still soft and easy to remove.
2.17.2. Tumble the Contents. Tumbling helps ensure an even distribution of the investment constituents.
2.17.3. Add the Powder to the Water. The powder is always added to the water; the water is never added to the powder. Place the required amount of water into the bowl and then sift the powder into the water until the powder forms an island. The powder gradually absorbs the water; consequently, the mixture is free of lumps and air. Because tap water contains contaminants, use only distilled water.
2.17.4. Measure the Water and Weigh the Powder. To ensure the properties of any gypsum product are maintained, an accurate water-to-powder ratio must be obtained. Weigh the powder and measure the volume of water before mixing the gypsum material.
2.17.5. Mix Well. Ensure all powder is spatulated into the water. As mixing proceeds, the water and powder form a mixture of creamy consistency. (To avoid excessive incorporation of air into the mix, do not whip the mix.)
2.17.6. Vacuum-Mix the Materials. Phosphate-bound investments release ammonia gas when mixing. Vacuum-mixing removes gas and air from the mix. Avoid gas entrapment by holding the mix under vacuum for 30 seconds. (Gas entrapment in the mold results in nodules on the casting.)
2.17.7. Never Add to a Mix. Adding to a mix interferes with the setting mechanism and results in a weak and distorted product. It is better to begin a new mix.
2.17.8. Use Good Equipment. A scarred or cracked plaster bowl allows minute particles of material to lodge in the cracks. These particles could contaminate and spoil the mix.
2.17.9. Do Not Contaminate the Material. Never allow water or other contaminants to fall into a bin containing gypsum material. One drop of water can adversely affect the entire batch.
2.17.10. Know the Material. An aged investment can ruin a piece of work. Be aware that investments have batch numbers and expiration dates stamped on them. Contact the manufacturer if any problems are suspected with your investments. Another good practice is to keep investments rotated, with the oldest packs being used first.
¶ 2.18. Storing Gypsum Materials
2.18.1. Improper Storage.
2.18.1.1. When gypsum material is exposed to air, it absorbs water. The water may alter its working qualities and make it unfit for use. When plaster or stone is exposed to air for a short period of time, it sets faster than usual. If it is exposed for a longer period, it may set very slowly and be weak when it's set.
2.18.1.2. A prolonged period of storage in an unsealed container may alter the physical properties of casting investments, greatly changing the setting time, setting expansion, and reducing the crushing strength.
2.18.1.3. The setting time of casting and soldering investments is listed on the container along with the physical properties expected when the recommended powder to water ratios are used. This data is based on fresh material as it leaves the factory. It does not apply to aged batches of material that have been improperly stored.
2.18.1.4. If an investment takes an unusually long time to reach an initial set (more than 20 minutes), the entire batch must be discarded. A prolonged setting time is a warning that some or all of the desirable physical properties may have been lost or so altered as to render the investment unfit for use.
2.18.2. Proper Storage.
2.18.2.1. Gypsum material must be properly stored. The storage problem is more acute in a humid climate than in a dry one. All gypsum products must be stored in a sealed container in a dry room.
2.18.2.2. A systematic plan for withdrawing older stock from the supply room should be used. To minimize prolonged periods of storage, large quantities must not be stockpiled due to the danger of deterioration.
2.18.2.3. Some authorities also recommend that still another factor be taken into account when casting investments are stored. The heavier constituents (for example, quartz) settle to the bottom of the container, thereby altering the working properties of the investment. Therefore, investments should be tumbled before use, either mechanically or by hand, to make sure the powder is evenly mixed throughout.
¶ 2.19. Proper Handling of Plaster and Dental Stone Casts
2.19.1. Erosion of Casts.
2.19.1.1. A well-poured cast can be ruined by contact with water because hardened stone is soluble in water in a ratio equal to or less than 1 part stone to 500 parts of water. When a stone cast is immersed in water, an erosion process begins immediately on the surface of the stone. The erosion is noticeable in as short a period as 10 minutes. This can be shown in the laboratory by suspending a stone cast in water so part of the cast is submerged, while part of it remains out of the water. In 10 minutes, the erosion of the submerged part will be evident because of its pitted appearance.
2.19.1.2. The time necessary to produce a noticeable effect depends on the mineral content of the water, temperature of the water, and density of the stone. A poured impression should never be submerged in tap water because of the harmful effect it has on stone.
2.19.2. Saturated Calcium Sulfate Dihydrate Solution (SDS) Preparation.
2.19.2.1. SDS is a clear, true solution of water and a maximum amount of dissolved dihydrate (set) gypsum product. Cast surfaces exposed to SDS do not erode nearly as much as cast surfaces bathed in tap water. If a cast must be soaked for more than 1 or 2 minutes, SDS should be used.
2.19.2.2. SDS is made by immersing fragments of gypsum casts in water for about 5 days. A saturated solution consists of about 0.2 grams of dihydrate in 100 cc of water.
2.19.2.3. If a slurry water suspension is left to settle out for 3 to 4 days, the clear fluid above the sediment is SDS. For use, siphon off the SDS into another container without agitating the sediment layer.
2.19.2.4. SDS can be made from plaster, dental stone, or gypsum bound investment, whichever is best suited for the kind of cast you expect to wet.
2.19.3. Wetting Casts.
2.19.3.1. Occasionally, casts require quick superficial wetting (for example, cleansing cast surfaces)
SDS must be used instead of tap water for this purpose.
2.19.3.2. When a cast is shaped on a cast trimmer, gypsum slurry splashes onto its surface. If this slush layer is allowed to dry, it is hard to remove and cast damage could occur. As the slurry buildup accumulates, rinse the cast in a suitable container of SDS to remove the slurry. The SDS must be changed often or it will also turn into concentrated gypsum slurry.
2.19.3.3. When outright cast soaking must be done in conjunction with a laboratory procedure, the cast must not be completely submerged in SDS. Total immersion slows down the soaking process because air trapped in the cast cannot readily escape. Instead, the fluid level should be maintained below the tissue surface of the cast. A cast can be moistened in this manner in 20 to 30 minutes.
2.19.3.4. The wetting process can be seen gradually working up from the base of the cast to the tips of the teeth, much the same as oil dampens the wick in a lamp. If relief wax has been placed on the cast, there is danger of the escaping air from the cast lifting the wax from the stone. Instead of setting the cast on its base, set it on its end in the SDS.
¶ Section 2C — Dental Waxes
¶ 2.20. General Information
2.20.1. Wax compounds used in dentistry are mixtures of individual waxes of natural or synthetic origin. As with all other dental materials, each component in the mixture is selected to give the specific properties best suited for the procedure being performed. Depending on the purpose the wax serves, modifiers are included to change the melting range, increase or decrease stickiness, or impart a distinguishing color.
2.20.2. Dental waxes are supplied in various shapes, sizes, colors, and compositions. Become familiar with their uses and manipulation and be prepared for variations in the behavior of different waxes supplied by manufacturers.
2.20.3. See Table 2.2 for the types of waxes used in the laboratory.
Table 2.2. Types of Dental Waxes.
| Item | Material | Use | Remarks |
|---|---|---|---|
| 1 | Baseplate Wax | Denture wax-ups, fill the tongue space of a lower impression, other uses miscellaneous. | Supplied in medium or hard types. Most baseplate wax sheets are about 1 mm thick (18 ga). |
| 2 | Inlay Wax | Wax patterns: inlays, onlays, crowns, and pontics; RPD frame wax-ups. | Highest requirements for accuracy of any wax. Supplied in medium and hard types. |
| 3 | Ivory Wax | For waxing acrylic resin jackets and compression-molded acrylic veneers. | Nonpigmented inlay wax. |
| 4 | Wax Forms | RPD patterns: spiral retention posts, sprues, external finish lines, etc. | Same characteristics as the softer inlay waxes. |
| 5 | Sheet-Casting Wax | Relief to create areas under RPD acrylic resin retention grids. | 24 ga = 0.51 mm 26 ga = 0.40 mm 28 ga = 0.32 mm 30 ga = 0.25 mm |
| 6 | Sticky Wax | To hold broken pieces together prior to pouring an indexing cast. | Breaks with a “snap” at room temperature; shows very little flow when cool. |
| 7 | Utility Wax | Beading impressions prior to boxing. | Tacky at room temperature. |
| 8 | Boxing Wax | Damming impressions for controlled pouring of casts. | Supplied in strips 1 1/2 inch wide by 12 inch long. |
| 9 | Blockout Wax | To block out undercuts in RPD fabrication. | Flows easily, sticks to a cast well, cuts cleanly. |
| 10 | Beeswax | To seal a refractory cast. | Use at around 290 °F. |
¶ 2.21. Groups of Waxes
Most dental waxes fall generally into three functional groups; impression, pattern, or processing, as follows:
2.21.1. Impression Waxes. These waxes are used primarily by the dentist at the chair. They have low melting points and flow fairly easily at mouth temperatures. They can be distorted very easily and require extreme care in handling. Examples of impression waxes are corrective wax and jaw movement recording wax.
2.21.2. Pattern Waxes. These waxes are used by the dentist and laboratory technician.
2.21.2.1. They are used to form the molds in which prosthodontic restorations are made. Examples of pattern waxes are inlay wax (paragraph 2.23), baseplate wax (paragraph 2.22), wire wax (paragraph 2.25.1), preformed wax (paragraph 2.25.1), and sheet-casting wax (paragraph 2.25.3). With the notable exception of inlay wax, almost all of the pattern waxes are meant to be used in controlled thicknesses.
2.21.2.2. Gauge (ga) is a measure of thickness. The term is applied to the diameters of metal wires and wax forms having circular and semicircular cross sections (for example, wire wax). Gauge is also used when talking about sheet metal and sheet wax thicknesses.
2.21.2.3. Unfortunately, manufacturers don't always use the same gauge standard. Even if the discussion is limited to wax, the thickness of wax shapes with the same gauge number can vary between two manufacturers. Table 2.3 shows a portion of the Brown and Sharpe Gauge Scale for nonferrous (non-iron containing) sheets and wire. Notice that as gauge numbers get smaller, the thickness increases.
Table 2.3. Brown and Sharpe Gauge Scale.
| Item | Gauge Number | Inches | Millimeters |
|---|---|---|---|
| 1 | 10 | 0.1019 | 2.59 |
| 2 | 12 | 0.0808 | 2.05 |
| 3 | 14 | 0.0641 | 1.63 |
| 4 | 16 | 0.0508 | 1.29 |
| 5 | 18 | 0.0403 | 1.02 |
| 6 | 20 | 0.0320 | 0.81 |
| 7 | 22 | 0.0253 | 0.64 |
| 8 | 24 | 0.0201 | 0.51 |
| 9 | 26 | 0.0159 | 0.40 |
| 10 | 28 | 0.0126 | 0.32 |
| 11 | 30 | 0.0100 | 0.25 |
| 12 | 32 | 0.0080 | 0.20 |
2.21.3. Processing Waxes. These waxes are used primarily for fabricating prosthodontic restorations. Examples are sticky wax (paragraph 2.26), utility wax (paragraph 2.27), boxing wax (paragraph 2.28), blockout wax (paragraph 2.30), and beeswax (paragraph 2.32).
¶ 2.22. Baseplate Wax
2.22.1. Composition. Baseplate wax is composed mainly of beeswax, paraffin, and coloring matter. The ingredients are melted together, cast into blocks, and then rolled into sheets. A typical baseplate wax might contain 50 parts of yellow beeswax, 6 parts of gum mastic, 3 parts of prepared chalk, and 4 parts of vermilion.
2.22.2. Requirements. There are several requirements for a baseplate wax. The wax must be fairly rigid at mouth temperature under biting pressure. It must be capable of holding porcelain or acrylic teeth in position, but must not be brittle. The wax should maintain a uniform consistency throughout a normal range of room temperatures as well as at mouth temperature.
2.22.3. Types. Baseplate wax is supplied in two types, hard and medium. The hard wax is indicated for warmer climates because it resists flow at higher temperatures. At cold temperatures, it might be too brittle and have a tendency to crack. The medium wax is indicated for low temperatures, but might exhibit too much flow in a warmer environment.
2.22.4. Uses. Baseplate wax is used for occlusion rims, as a boxing for matrices, for filling the tongue space of lower impressions, in complete and partial denture waxups, and for many miscellaneous purposes. Most baseplate wax sheets are about 1 millimeter (mm) (18 gauge [ga]) thick.
¶ 2.23. Inlay Wax
2.23.1. General Composition. Inlay wax consists of paraffin (to make up the bulk); gum dammar (to improve the smoothness in molding and to render the wax more resistant to flaking and cracking); and carnuba (to control the softening point and hardness of the wax).
2.23.2. Requirements for Use in Dental Procedures. Inlay wax is one of the most carefully compounded of all the dental waxes. It should have the following qualities: high accuracy in reproducing every detail of a cavity or crown preparation; ease of carving without chipping or flaking; workable in the mouthat body temperature and in the laboratory at room temperature; dimensionally stable when transferred from one temperature environment to another; strong enough in thin areas to withstand the ordinary stresses of investing; and finally, the ability to burn out cleanly from the mold at ordinary burnout temperatures without leaving a solid residue.
2.23.3. Types of Inlay Wax. There are three types of inlay wax—Type A, a hard or low flow wax used in some indirect methods; Type B, for the direct technique of pattern making or intraoral use; and Type C, for the indirect technique or laboratory use.
¶ 2.24. Ivory (or White) Wax
Ivory or white wax is an inlay wax containing no color pigment. It is especially useful for waxing acrylic jacket patterns. It does not leave a colored residue in the plaster mold which might discolor the resin of the jacket crown.
¶ 2.25. Casting Waxes for Partial Dentures
2.25.1. Preformed Wax (Round and Half-Round Cross Section)
Preformed wax is supplied by the manufacturer in a variety of shapes and sizes suitable for use in constructing the wax pattern for a partial denture framework. Some of the round forms (wire wax) can also be used for spruing fixed prosthetic units.
2.25.2. Inlay Wax. When waxing frameworks, inlay wax is primarily used to free flow and carve those parts of the pattern that join preformed components to each other. Inlay wax is also used to sprue patterns.
2.25.3. Sheet-Casting Wax. Sheet-casting wax is very similar to baseplate wax (paragraph 2.22). At room temperature, sheet-casting wax possesses the properties of toughness and pliability and sufficient tackiness to adhere to the cast and stay where it is placed.
2.25.3.1. Gauge. Sheet-casting wax is manufactured in several thicknesses or gauge. The most common sizes are 24, 26, 28, and 30 gauge.
2.25.3.2. Color. Although manufacturers supply the wax sheets in several colors to distinguish waxes of different consistencies and handling characteristics, there is no standardization of colors among manufacturers. For example, one brand of green wax may be entirely different in working properties from the green wax of another manufacturer.
2.25.3.3. Uses. Sheet-casting wax can be used when a definite thickness of wax is needed. Its principal use is with RPD work to provide relief of the residual ridge on the master cast. It is often combined with one thickness of baseplate wax to produce a palate of uniform thickness in a complete denture.
¶ 2.26. Sticky Wax
Sticky wax is composed of beeswax, paraffin, and a considerable amount of natural resin. The resin gives the wax its adhesiveness and hardness. An important property of sticky wax is that it breaks under pressure instead of bending or distorting. This property makes it useful for joining the parts of a broken denture or holding together the structural parts of a wrought wire clasp while it is invested for soldering.
¶ 2.27. Utility Wax
Utility wax is an extremely pliable wax that is marketed in rope form. It is plastic and somewhat tacky at room temperature, which makes it usable without heating. Most importantly, utility wax is used for beading impressions before pouring the cast. It is sometimes used in impression techniques before pouring the cast to build up the impression tray borders.
¶ 2.28. Boxing Wax
Boxing wax is a specially prepared wax, supplied in strips 1 1/2 inches wide by 12 inches long. It is primarily used to box impressions. Most boxing waxes do not require heating; they are pliable enough at room temperature to be formed into desired shapes.
¶ 2.29. Low-Fusing Impression Wax
Low fusing impression wax is specially compounded to flow under controlled pressure in the mouth. It is melted in a water bath and painted on the tissue surface of an individual impression tray as a corrective liner for final complete and RPD impressions. Because the wax is easily distorted, low-fusing wax impressions must be handled with the utmost care. Fingers must never touch the tissue side of the impression, including the periphery. When the impression is rinsed, a gentle stream of room temperature water should be used. A separator is not necessary when the cast is poured.
¶ 2.30. Undercut (Blockout) Wax
Undercut wax has physical properties that allow it to be built up around an abutment tooth and then easily carved with surveying tools. Undercut wax is made by combining beeswax, resin, and kaolin. It is usually supplied in small, wide-mouthed jars. The formula for making this kind of wax is shown in Chapter 8, paragraph 8.42.1.4.1.
¶ 2.31. Disclosing Wax
Disclosing wax has a very low fusing range. It flows readily under pressure and is used to detect points of unequal pressure when seating many kinds of castings. Disclosing wax is melted on the tissue side of a casting and then held in place under pressure. It flows away from the pressure points and discloses them for corrections.
¶ 2.32. Beeswax
Refined beeswax is supplied in cakes or bars. It is used in molten form (280 to 300 °F) as a dip for sealing refractory casts. To prevent cracking, casts must be heated and dehydrated before they are dipped. Subsequent sealing of refractory casts provides a satisfactory surface for attaching wax and plastic patterns, and prevents absorption of moisture when invested for casting RPD frames.
¶ Section 2D — Impression Materials
¶ 2.33. Introduction
2.33.1. A variety of impressions are made in the dental clinic. Each variety requires a material of slightly different properties. In complete denture work, a material is needed that accurately registers all the denture-bearing areas. In partial denture work, there is an additional requirement. The material must be capable of registering both tooth and soft tissue undercuts. In many dental impression procedures, two materials and sometimes even three are used in sequence to take advantage of the most favorable qualities of each.
2.33.2. A useful table of impression materials is shown in Table 2.4.
Table 2.4. Types of Impression Materials.
| Item | Material | Use | Characteristics |
|---|---|---|---|
| 1 | Modeling Plastic | Preliminary complete denture impressions, final impressions, and final impression trays. | Rigid at room temperature, but begins to distort when it gets warm. |
| 2 | Low-Fusing Impression Wax | Specialized impressions. | Very easily distorted by warmth or pressure. Must be handled with care. |
| 3 | Zinc Oxide-Eugenol Paste | Final complete denture impressions and jaw relationship records and for stabilizing baseplates. | Holds dimensional stability well. Rigid. |
| 4 | Rubber Base (Polysulfide) | Final complete denture impressions and impressions for fixed prosthetic units. | Stains clothing badly and indelibly. Extremely accurate and durable. Can be poured twice if necessary. Elastic. |
| 5 | Hydrocolloid, Reversible | RPD and fixed prosthetic impressions. In a tougher form, it is used in the laboratory for cast duplication procedures. | Highly susceptible to drying. Should be poured within 10 minutes after the impression is made. Can be broken down and reused many times. Elastic. |
| 6 | Hydrocolloid, Alginate | Preliminary impressions for diagnostic cast and final RPD impressions. Can be used as a matrix to make temporary fixed prosthetic units. | Highly susceptible to drying. Should be poured within 10 minutes after the impression is made. Cannot be reused like the reversible type. Elastic. |
¶ 2.34. Impression Compound
2.34.1. Impression compound is a material that can be softened by heat into a soft plastic mass and then hardened by cooling it with either a stream of cold water or a blast of air. It is used in the clinic for preliminary impressions, to make custom impression trays, and to modify stock trays. Because it does not accurately spring around an undercut and return to its former shape, impression compound has very limited use in partial denture work.
2.34.2. Although impression compound is a basic material in the clinical phases of prosthetic dentistry, it is not often used in the laboratory. The laboratory technician uses it periodically to attach a cast to its mounting in an articulator.
2.34.3. Impression compound is marketed in several colors and designated by the manufacturer as high, medium, or low fusing. However, there is no uniformity among manufacturers as to exactly what constitutes high or low fusing. One brand of “high-fusing” impression compound may have about the same fusing range as another brand labeled “medium-fusing,” and the two may be the same or different colors. In general, high-fusing impression compounds flow at approximately 135 to 140 °F while low-fusing types may flow at 115 °F. Several of the manufacturers make a “tray” compound which is high fusing (about 140 °F). It is almost always black in color and is the type of impression compound most suitable for a custom impression tray.
2.34.4. Impression compound is one of the few impression materials an amalgam die can be packed against. Any of the gypsum materials can be poured into a compound impression without the need of a separator. The material is supplied in the form of cakes and sticks.
¶ 2.35. Low-Fusing Impression Wax
2.35.1. Low-fusing impression wax is specially formulated to flow under controlled pressure in the mouth. It is melted in a water bath and painted in an individual impression tray as a corrective liner for final impressions. It is also used for reline impressions for complete dentures and RPDs.
2.35.2. Low-fusing wax impressions must be handled with extreme care in the laboratory because the wax is so easily distorted. Fingers must never touch the tissue side of the impression, including the periphery. If an impression is rinsed, it should be done very carefully with room-temperature water. A separator is not necessary when a cast is poured.
¶ 2.36. Impression Paste
2.36.1. Impression paste is usually supplied as two separate components, a base and a hardener. The base and hardener are mixed together in specific proportions to form a paste. Impression paste is rigid when it sets, and it does not spring over undercuts. Its principal ingredients are zinc oxide and eugenol or lauric acid. Impression paste is used primarily as a corrective material inside an individual impression tray.
2.36.2. One use of impression paste is to reline impressions for both complete and RPDs. Occasionally, it is used in immediate denture work as a lining for a sectional impression. It can also be used to provide a lining for a complete denture record base to make it fit the cast and the mouth more accurately. A separator is not required when the cast is poured into an impression made with this material.
¶ 2.37. Elastomeric Impression Materials
These materials are supplied as two-part systems; a base paste and an accelerator paste. Some manufacturers supply the accelerator as a liquid. When the two are mixed in the correct proportions, the resulting mixture polymerizes into a rubbery state. Elastomeric materials are not reversible and can be used only once. They are used primarily for fixed prosthetic units (crowns or onlays), although they can also be used as corrective liners in complete denture impressions and RPD bases. Pastes and accelerators from different brands of elastomeric materials must never be cross-mixed. Materials must be carefully handled because the stains that some of them produce on contact with clothing and towels are impossible to remove. Four different types of elastomeric impression materials are available; polysulfides, silicones, polyvinylsiloxanes, and polyethers, as follows:
2.37.1. Polysulfides. The basic ingredient of a polysulfide impression material is polysulfide rubber with various fillers, pigments, and modifiers. The polysulfides are easily recognized because one of the two pastes is usually dark and the other paste is white in color. This material has a very characteristic odor. The polysulfides are more commonly known as rubber base or polysulfide rubber base impression material. The low viscosity of this material allows accurate registration of the soft and hard tissues. It is most often used in removable prosthodontics.
2.37.2. Silicones (Condensation Reaction Silicones)
2.37.2.1. Silicone impression material consists of silicone and ethyl silicate. This material exhibits significant setting shrinkage and should be used in thin consistent layers.
2.37.2.2. Silicone manufacturers were the first to offer a two-phase impression method. The dentist first makes an impression, either in the mouth or on the diagnostic cast, of the patient's arch, using a stock impression tray and a putty form of the silicone material. The resulting custom-fitted tray is used to carry a wash of the lower viscosity silicone material to the mouth for the final impression.
2.37.2.3. Silicone materials are generally lighter in color and translucent when set, and they have a much more subdued odor than the polysulfides. A disadvantage is that shrinkage occurs if the material is allowed to sit for more than 30 minutes before pouring the impression.
2.37.3. Polyvinylsiloxanes (Addition Reaction Silicones)
Polyvinylsiloxanes are like the conventional silicones in their elastic nature, but they differ in chemical structure and reactions. Because of this, the polymerization shrinkage of polyvinylsiloxanes is well controlled, and a thin uniform thickness of the material is not so significant a requirement for accurate impressions. Polyvinylsiloxanes are also used with a two-stage impression technique. A disadvantage is the rigidity and cost of the material.
2.37.4. Polyethers. The base of this impression material is a polyether compound, and the accelerator is a sulfonic acid. Laboratory studies have shown that polyethers, along with polysiloxanes, are the most accurate of the elastomeric impression materials. However, when set, polyether impression material is very stiff, making it difficult to remove the impression tray from the mouth if large tooth undercuts are present. This is also a problem when trying to separate casts from the impression without breaking off stone teeth.
¶ 2.38. Special Characteristics of Elastomeric Impressions
2.38.1. Polysulfide rubber base and condensation silicone impressions should be poured within 25 minutes of their removal from the mouth. These materials do not have long term dimensional stability. Second pours may be made from these impressions, but the resulting cast does not have the same accuracy as the cast from the first pour. Addition reaction silicone and polyether impressions show great dimensional stability. Pours of casts may be delayed up to 7 days with no significant loss of accuracy. Second pours maintain accuracy of detail and dimension comparable to first pours.
2.38.2. Polyethers are highly hydrophilic (they absorb water) and exposure to liquids must be minimized. They may not be used in die-plating procedures because water sorption leads to unwanted dimensional change. Addition reaction silicones are more difficult to pour or achieve a bubble-free cast due to their hydrophobic nature.
¶ 2.39. Impression Plaster and Soluble Impression Plaster
2.39.1. Impression plaster is a plaster that has been specially compounded by the manufacturer for use in the mouth. It must set rapidly to reduce the time it is held in the mouth. Because plaster will not flex over an undercut as it is withdrawn from the mouth, it must be broken into pieces and reassembled outside the mouth. For this reason, it should be weak and brittle so it will fracture cleanly, and it must have a very low setting expansion to make an accurate impression.
2.39.2. Various accelerators and retarders are added to give the plaster the required properties. In addition, coloring and flavoring agents are often added. Because of the difficulty sometimes experienced in removing a plaster impression from the cast, some impression plasters are made water soluble by adding cornstarch. If a separator is used, the plaster can be dissolved off the cast with hot water. This decreases the possibility of breaking the cast when it is separated from the impression.
¶ 2.40. Hydrocolloids
In partial denture work, an impression material is needed that accurately registers both tooth and soft tissue undercuts. A hydrocolloid material elastically deforms and then returns to its original shape. The undercuts are thus accurately reproduced in the impression. There are two basic types of hydrocolloids, the agar type and the alginate type. They are chemically and physically different and require different handling, but the purposes for which they are used are very similar. They are often referred to as reversible and irreversible, respectively. The agar type can be softened by heat and stiffened by cooling. Because this behavior can be driven either way, it is reversible. The alginate type is a powder that, when mixed with water, hardens by gelling. Because it cannot be softened to be used again, it is irreversible.
2.40.1. Hydrocolloid, Agar Type (Reversible)
There are two different reversible hydrocolloids. One is designed to be used in the mouth for impressions; the other is compounded for duplication use in the laboratory as follows:
2.40.1.1. Impression Type. Impression hydrocolloid is a gelatin-like material that is composed mainly of agar-agar and water. The material is heated in a double boiler or in a special heating syringe to soften it to a thick consistency. It is then tempered, carried to the mouth in a tray, and cooled with 70 °F water to make it set. When it has set, it is removed, and the cast is poured. Impression-type agar can be used for duplicating in the laboratory if laboratory duplicating agar is not available. Its principal use, however, is for making RPD impressions and fixed prosthodontic final impressions. A main disadvantage of this material is that an impression can only be poured one time due to the dimensional change caused by the evaporation of water.
2.40.1.2. Laboratory Type. Laboratory duplicating hydrocolloid is specially manufactured for laboratory use. It is stronger and, therefore, more satisfactory for duplicating than the impression type. It can be used repeatedly if it is properly handled and stored. In order to maintain a precise water balance, heat the material in a stainless steel double boiler. The double boiler has a dome-shaped lid that condenses the water and returns it to the mixture. The water balance of the hydrocolloid is critical and is maintained by the double boiler. Store any unmixed hydrocolloid in a sealed container.
2.40.2. Hydrocolloid, Alginate Type (Irreversible)
2.40.2.1. General Description. The alginate-type hydrocolloid is supplied in the form of a fine powder. The powder is mixed with a prescribed amount of water to form a mixture that, like the agar-type hydrocolloid, is capable of accurately reproducing an undercut of either a tooth or soft tissue. In general, the ingredients used to make an alginate impression material are sodium or potassium alginate, plaster, magnesium oxide, trisodium phosphate, sodium phosphate, and ditomaceousearth as a filler.
2.40.2.2. General Uses. Alginate is used as an impression material for partial dentures. It can be used alone or in conjunction with another material for immediate denture impression and is sometimes used for cast duplication in the laboratory. When used for duplicating, alginate is usually mixed with more than the usual amount of water (2 or 3 times more, depending on how fluid a mix is needed).
2.40.2.3. Handling Requirements.
2.40.2.3.1. Making an accurate cast from a hydrocolloid impression requires following certain rules. The water balance in a gelled hydrocolloid material is critical to its accuracy. When gelled hydrocolloid is exposed to water or air, it changes its dimensions quickly. This is why hydrocolloid impressions must be poured as soon as possible after they are made. (As soon as possible means within 10 minutes after the material is set.)
2.40.2.3.2. Reversible and irreversible hydrocolloids tend to exude a fluid that causes gypsum surfaces to be soft and chalky. Not all gypsum products are affected in the same manner. Some brands of reversible hydrocolloid material require immersing the impression in a 2 percent solution of potassium sulfate before the cast is poured. This procedure is called fixing. It improves the surface qualities of the cast. Of course, manufacturer's directions must be followed.
¶ Section 2E — Denture Base Materials
¶ 2.41. Introduction
2.41.1. A great variety of materials have been used over the years to make denture bases. Today, a plastic material is by far the most universally used. The chemical name is methyl methacrylate; the common name is acrylic resin.
2.41.2. Since it was first introduced in 1937, a considerable amount of refinement and improvement has been made in acrylic resin and in the methods of handling and processing it.
2.41.3. Manufacturers supply acrylic resin as both a powder (polymer) and a liquid (monomer) or in the form of a premixed gel. The powder and liquid form is the one most commonly used. When the material is supplied in this form, the technician adds a measured amount of powder to a specific volume of liquid to form a dough. The dough is then packed into the denture mold, and heat is used to cure (harden) the denture. Dry heat can be used, but curing in hot water is the most commonly used method. Known as polymerization, the cure or hardening of the acrylic resin in the mold takes place by a chemical reaction between the powder and the liquid.
2.41.4. The types of denture base materials are shown in Table 2.5.
Table 2.5. Types of Denture Base Materials.
| Item | Material | Use | Comment |
|---|---|---|---|
| 1 | Heat-Cured Denture Resins (powder + liquid) | Complete and RPD bases. | Improper packing and/or processing result in contamination, breakage, porosity, etc. Heat is required for polymerization. |
| 2 | Heat-Cured Denture Resins (gel) | Complete and RPD bases. | Basic composition is the same as the powder and liquid varieties. Must be refrigerated to inhibit polymerization. |
| 3 | Autopolymerizing Acrylic Resins | Repairs, relines, impression trays, and baseplates. | Heat is not required to induce polymerization. Can be applied by the “sprinkle” method or used in dough form. |
| 4 | Resins for Tinting | Tinting standard pink denture base material. | Pigments may be mixed with polymer powder or packed separately. |
| 5 | Tooth-Colored Resins | Custom denture teeth, temporary fixed prostheses repairs, etc. | Fine polymer powders that come in a variety of natural tooth shades. Available in self-curing or heat-curing forms. |
| 6 | Soft-Lining Resins | Tissue-conditioning denture liners. | Polymerize to a semisoft state. |
| 7 | Vinyl Resins and Polystyrenes | Complete and RPD bases. | Special processing equipment is necessary. |
¶ 2.42. Heat-Cured (Heat-Activated) Denture Resins
Heat-cured denture resins are processed in the dental laboratory, using heat and pressure to obtain a product that meets the requirements of the particular appliance being constructed. Heat-cured denture resins are bought in packages containing powder (polymer) and liquid (monomer). The monomer has an inhibitor to prevent polymerization until activated by heat. Monomer is highly flammable. It is a skin and eye irritant and is known to cause allergic reactions.
¶ 2.43. Acrylic Resin Gel
This is a premixed form of acrylic resin. The manufacturer mixes the powder and liquid at the factory and adds a substance (inhibitor) that prevents polymerization until the resin is placed in the mold and heated. The gel is more homogeneous because it is machine mixed in large quantities. It is somewhat handier and quicker to use, but its shelf life is limited so it must be refrigerated when stored.
¶ 2.44. Autopolymerizing (Chemically Activated) Resins
2.44.1. Composition. Another member of the acrylic resin group used in the dental laboratory is the autocuring or self-curing resin. The basic composition of these autopolymerizing resins is the same as that of heat-cured denture acrylic. The difference is that, instead of using heat to bring about polymerization, a chemical agent (activator) is added to the liquid so the dough polymerizes in about 10 to 20 minutes at room temperature.
2.44.2. Uses. The self-curing acrylic resins are used for most denture repairs. In repair procedures, these resins have a decided advantage over heat-cured resins because the denture does not have to be subjected to a high-curing temperature which often causes the denture base to warp. These resins are also used for impression trays, record bases, and denture base construction. The self-curing resins can be mixed and used as a dough, or they can be applied by the sprinkle method. The liquid provided with the self-curing resins should always be well shaken before it is used because the activators are lighter than the liquid and tend to rise to the top of the bottle.
¶ 2.45. Acrylic Resins Used for Tinting
Methyl-methacrylate resins are used to modify the color of basic pink, denture base plastics. The pigments are used to more closely simulate the colors of natural gum tissue in the finished denture base. Some are applied to the desired areas of the denture mold before the resin material is packed; some are shaped into preformed patterns that are placed in the mold prior to packing.
¶ 2.46. Hard-Lining Resin
This is a slightly different type of acrylic resin which is marketed and intended for one step (clinical) denture relines. The material has been compounded specifically for clinical purposes. It does not lend itself well to laboratory use. It can be used for impression trays if a more suitable material is not available in the laboratory.
¶ 2.47. Tooth-Colored Acrylic Resin
Tooth-colored methyl-methacrylate resins are very similar to denture resin except for the color and finer particle size. Tooth-colored pigments are added to the polymer to simulate natural tooth shades. Tooth-colored acrylic resins are available in heat-curing and autopolymerizing forms. They can be used to make denture teeth, veneer crowns, and temporary restorations. These resins are extensively used to perform repairs.
¶ 2.48. Soft-Lining Resins
Some clinical conditions require a soft, cushion-like liner in the tissue side of denture bases. These soft materials are usually known as resilient liners. It is important for these lining materials to bond well to the denture base, resist tearing, and retain their cushion effect. There are three basic kinds of lining resins; velum acrylic, silicone, and ethyl methacrylate.
2.48.1. Velum Acrylic Resin. Velum resin is a form of acrylic resin that never polymerizes rigidly in the manner of ordinary resin. It is supplied as a powder and liquid. The liquid is much more viscous than regular acrylic liquid. It contains a retarder which prevents the resin from hardening. The principal use of velum resin is in cleft palate prostheses. It is occasionally used for denture reliner when a soft material is needed. Unfortunately, the ingredient that keeps the resin soft is eventually lost. Loss of this ingredient causes the liner to gradually harden. Once it hardens, it must be replaced.
2.48.2. Silicone Resin Liners. These soft liner materials, composed primarily of silicone gum and a liquid or paste hardener, are available as heat-cured or autopolymerizing types. These silicone liners are the most truly elastic of the soft-lining materials. However, these liners have a major disadvantage. They have poor abrasion resistance and are difficult to trim properly. Also, like velum resins, silicone liners do not remain soft indefinitely, although they harden more slowly than other lining materials.
2.48.3. Ethyl Methacrylate (Sof Pac®, Dura Soft®)
Ethyl methacrylate is a two-component, heat-processed resilient material designed for use in the construction of long-term denture relines and maxillofacial prostheses. Ethyl methacrylate bonds to methyl methacrylate, increasing its durability in the mouth. Ethyl methacrylate is processed using standard techniques. The liner can either be used against fresh acrylic in the initial processing of a new denture, or it can be cured against an existing denture base. Ethyl methacrylate offers easy polishing and finishing. It can easily be trimmed with a carbide bur or arbor band. It is then polished to a luster using a cloth wheel, pumice, and polishing compound.
¶ 2.49. Tissue Conditioners
Until the patient's oral tissues return to a healthy state, the dentist uses tissue conditioners as a temporary soft liner for dentures that require relining. Tissue conditioners must be changed at 3 to 4 day intervals. Some clinical techniques use tissue conditioning resins in a denture base as the master impression for relining or rebasing an old denture or fabricating a new one. Because these materials are very delicate and do not adhere well to the denture base, tissue conditioner impressions must be handled with great care in the laboratory.
¶ 2.50. Vinyl Resins and Polystyrenes
Vinyl and polystyrene are plastics which are chemically similar to acrylic resin but differ somewhat in their physical properties. They are formed into a denture using a method of molding known as injection molding. Advocates of these materials say they have superior dimensional stability over acrylic resin dentures.
¶ 2.51. Handling Acrylic Resin
Acrylic resin, with all its good qualities of excellent appearance, am ple strength, lightweight, and ease of cleaning, is far from foolproof in its manipulation. Even when properly handled throughout the processing procedure, acrylic resin is subject to dimensional changes. These changes appear as minor faults in the occlusion and loss of contact of the processed resin with the master cast. The changes are more noticeable across the posterior palatal seal area of a maxillary denture. However, changes can be kept to a minimum when the technician understands the behavior and working characteristics of the material, and takes precautionary steps to avoid certain pitfalls. The errors that are most apt to occur in processing acrylic resin dentures are distortion, contamination, warpage, breakage, and porosity, as follows:
2.51.1. Wax Distortion. An error can be introduced during the wax-up that may alter the occlusion on the finished dentures. To minimize the effects of baseplate wax distortion, wax up one denture at a time. After the wax-up of this single unit is completed, it should be returned to the articulator and the occlusion examined. Correct any discrepancy in the occlusion before removing the other cast from the articulator for wax-up. After completing the wax-up of both upper and lower dentures, examine the occlusion and correct any changes in it before beginning flasking procedures.
2.51.2. Acrylic Resin Contamination. Acrylic resin is especially susceptible to contamination while it is being mixed and packed in the mold. Meticulous cleanliness must be practiced, and clean measuring containers are essential. Using a clean, stainless steel spatula, the mixing should be done in a clean jar. Hands should be gloved and kept very clean. Acrylic resin liquid is an excellent solvent capable of dissolving grease and dirt from hands. A standard precaution is to handle the resin dough with plastic gloves or sheets rather than with bare hands. The mold must be absolutely clean and dry before the resin is packed.
2.51.3. Acrylic Resin Warpage. During the curing phase, several dimensional changes occur in the acrylic resin. The net effect is shrinkage. A ch aracteristic of acrylic resin is that it shrinks toward its greatest bulk. In a denture, this bulk is in the area over the alveolar ridge. From waxup to finishing, this distortion can be kept within acceptable limits if each step in the processing procedure is carefully performed as follows:
2.51.3.1. Take great care to ensure the mold has cooled to room temperature before starting to deflask a denture. Rapid cooling may create uneven internal stresses. The ideal way to cool the flask is to allow the water in the curing bath to cool down to room temperature before removing the flask from the carrier press. If faster cooling of the flask is necessary, bench-cool the flask for 1 hour and then cool it for 15 minutes in cold water.
2.51.3.2. Warpage may also result from excessive heat generated during polishing operations. Avoid excessive pressure against brushes and ragwheels because heavy pressure during polishing generates heat.
2.51.3.3. Warpage occurs from allowing the denture to dry out after it is processed. Completed acrylic resin prostheses of any kind must be stored in a container of water. If it must be mailed to another location, the denture should be sent in a sealed plastic envelope containing a small amount of water.
2.51.4. Resin Breakage. Most breakage of acrylic resin occurs during recovery of the denture from the mold. The breakage is often the result of careless deflasking. Deflasking cannot be hurried; take the time to do it right.
2.51.5. Acrylic Resin Porosity. Porosity may be due to one of the following handling errors:
2.51.5.1. An Improper Liquid-to-Powder Ratio.
2.51.5.1.1. The ratio of powder to liquid is important when mixing acrylic resin. A high percentage of powder in a mix speeds up the set and tends to reduce shrinkage during the cure. However, sufficient liquid must be used to wet the powder thoroughly if the chemical reaction between the two is to be completed.
2.51.5.1.2. The usual ratio is three parts of powder to one part of liquid by volume. Ten cm3 of liquid to 30 cm3 of powder is considered an adequate amount of material for the average denture. Measure the liquid and pour it into a clean jar. Then measure and sift enough powder to absorb all the liquid. Tap the jar on the bench top to bring any excess liquid to the surface, and then add the remaining powder. Thoroughly mix the powder and liquid with a stainless instrument. Unless the mixture is well stirred, the color tends to float to the surface of some brands of acrylic resin.
2.51.5.2. Packing the Dough Before it is Ready.
2.51.5.2.1. After mixing the acrylic resin, place a lid on the mixing jar and allow the resin to set for several minutes. Then remove the lid and test the mix by placing the blade of the spatula between the mix and the side of the mixing jar. When the mix no longer sticks to the side of the mixing jar, most heat-curing resins are ready to pack. First, the mixture appears sandy, then stringy, and finally doughy. When the doughy stage is reached, it is ready for the mold.
2.51.5.2.2. A further test involves making a roll of some of the material and pulling it apart. When it snaps apart cleanly, it has reached packing consistency. The test for proper packing consistency does not apply to all denture resins. Be sure to read the manufacturer’s directions to ensure proper procedures are followed.
2.51.5.3. Underpacking the Mold.
2.51.5.3.1. The acrylic resin dough must be packed into room temperature molds. Too warm a mold may cause the dough to become too stiff too fast. When trial packing, overfill the mold slightly and apply the pressure from the flask press very slowly until the two halves of the flask are as near as possible to metal-to-metal contact. Then release the pressure, open the flask, and remove the resin flash from the land area.
2.51.5.3.2. This procedure must be repeated at least three times to ensure the mold is full and the halves of the flask meet in metal-to-metal contact. To prevent opening of the occlusal vertical dimension, additional material must never be placed in the mold before final closure.
2.51.5.4. Curing the Acrylic Resin Too Quickly. The resin dough starts to polymerize at around 160 oF. As this chemical reaction takes place, heat is given off. The internal temperature of the flask tends to rise above the external heat being used to make the resin polymerize. The faster the resin dough reaches curing temperature, the more rapid the polymerization reaction and the higher the internal temperature of the flask. Depending on how fast polymerization progresses, a flask’s internal temperature can reach 300 oF.
2.51.5.4.1. The boiling point of the monomer component of the resin dough is about 212 oF. If the dough reaches curing temperature too quickly, the internal flask temperature exceeds 212 oF and causes the monomer to boil. A porous resin results. The thick sections of a denture base are especially susceptible to this problem.
2.51.5.4.2. A packed flask must be brought to a curing temperature at a rate that does not induce rapid polymerization. A temperature rise of 2 F per minute is recommended. Make sure the curing bath contains enough water to dissipate the excess heat that might be generated by the polymerization reaction. The flask press should never contact the bottom of the container; it should rest on a rack.
¶ Section 2F — Metals in Dentistry
¶ 2.52. Introduction
2.52.1. Metals are alike in certain aspects. There is no all-inclusive definition for a metal that is entirely satisfactory. However, metals do have certain properties that distinguish them from nonmetals. They possess a metallic luster; are good conductors of heat and electricity; and, with the exception of mercury (and one other rare metal), are solids at ordinary temperatures.
2.52.2. As compared to nonmetals, some metals are malleable (can be pounded or rolled into sheets), others are ductile (can be drawn into wire ), and most of them have a fairly high specif ic gravity (are dense and heavy as a result)
2.52.3. Metals are also different in certain other aspects. For example, each metal possesses physical properties peculiar to it alone, which disti nguishes it from all other metals. It has a fixed melting point, a definite specific gravity, and a certain degree of hardness, malleability, ductility, etc. By knowing these physical properties, you can predict with a fair degree of accuracy the way a metal will behave under different conditions. In the same way, you can also predict a metal’s degree of usefulness as a dental restoration or structural part of a prosthesis.
¶ 2.53. Structure of Metals
2.53.1. General Properties.
2.53.1.1. Metals are crystalline in structure, and many of their physical properties depend to a large extent on the size and arrangement of the crystals. The word grain is a very popular name for a metallic crystal. As molten metal cools and solidifies, clusters of molecules come together from the liquid to form solid crystal nuclei. These crystallites grow into grains. The faster molten metal cools to the solid state, the smaller will be the grain size and vice versa.
2.53.1.2. Generally speaking, small grains arranged in an orderly fashion give the most desirable properties. The size and arrangement of grains can be changed markedly by the way the metal is handled in the laboratory. The amount of heat a metal is subjected to, the method by which it is heated, the rate by which it is cooled, and the way it is worked (for example, bending or swaging) all have a pronounced effect on its physical properties.
2.53.2. Cast Metal. A cast metal is a piece of metal formed by pouring or forcing molten metal into a mold and allowing it to cool and harden. As previously stated, the size of the grains in a casting depends on the rate of cooling during solidification. The shape and arrangement of the grains are also established at the same time.
2.53.3. Wrought Metal. When rolling, pounding, bending, or twisting changes the shape of a casting, it becomes a wrought metal. Producing changes in the shape of a metal at normal room temperature is called cold-working. Working a metal changes its grain structure and has a marked (and sometimes detrimental) influence on the physical properties of the material. You must have a thorough understanding of the changes taking place in the worked metal to control and, if necessary, correct the changes (paragraph 2.56.2).
2.53.4. Metal Alloys.
2.53.4.1. Nature of Alloys. Some of the properties of a given metal might be ideal for a specific use while other properties of the same metal might be less desirable or even detrimental. By combining several metals in the correct proportions, it is possible to produce a compound in which the desirable properties of each metal are retained, while the less desirable ones are nullified or entirely eliminated. This is known as alloying, and the combination of metals thus formed is a metal alloy. The physical properties of an alloy cannot be accurately predicted solely by knowing the properties of the constituent metals. For example, two metals of extreme hardness, when combined, might yield an alloy of only moderate hardness, rather than one as hard as (or harder than) the individual component metals.
2.53.4.2. Knowledge Requirement. With few exceptions, metals used in dentistry are alloys. You should have an understanding of the structure and physical properties of dental alloys. This will enable you to accomplish the following:
2.53.4.2.1. Determine the combination of physical properties required in an alloy to be used for a prosthesis.
2.53.4.2.2. Understand the proper manipulation and heat-handling procedures to be followed with the selected alloy in order to retain and make the most of its desirable properties.
¶ 2.54. Physical Properties of Metals
The physical properties of metals are described in definite, precise terms. A familiarity with the meaning of these terms is basic to an understanding of the characteristic traits or the way a metal behaves under different conditions. Moreover, the suitability of a particular metal for a specific purpose can be determined only by someone who fully understands the terms used to describe its qualities. These qualities are explained in paragraphs 2.54.1 through 2.54.12.
2.54.1. Hardness.
2.54.1.1. This is a measure of the resistance of a metal to an indentation or scratch. It is an indication of the strength and wearability of the metal. Due to the varied functions the different types of dental prostheses must perform , hardness is a highly significant property of dental alloys.
2.54.1.2. For example, different types of restorations call for varying degrees of hardness. An onlay casting, which is to be subjected to heavy occlusal wear, should be harder than a casting made for the facial surface of a tooth. On the other hand, a metal might be too hard. The amount an inlay or crown can be burnished (adapted) to a tooth is directly dependent on the hardness of the metal. Harder metals are more difficult to burnish.
2.54.1.3. Several methods are used for measuring the hardness of metals. When an alloy has been tested for hardness, it is given an index number. Depending on the method used to test the alloy, it is then said to have a certain Brinell, Vickers, or Rockwell hardness (or another index). Regardless of the scale used, the higher the index number, the harder the metal. Figure 2.1 lists the comparative hardness of some common metals.
Figure 2.1. Comparative Hardness of Selected Metals.
| Very Hard | Medium Hard | Soft |
|---|---|---|
| Chromium Manganese |
Cobalt Nickel Copper Iron Platinum Silver Magnesium |
Gold Aluminum Cadmium Tin Lead |
2.54.1.4. Brinell hardness is determined by pressing a steel ball into a dental gold alloy under a measured load. The amount of surface indentation is computed according to the load.
2.54.1.5. The Vickers test uses a diamond in the shape of a square-based pyramid. The test is more suitable for determ ining the hardness of a wider variety of materials. Recently, the Vickers test replaced the Brinell test for testing dental gold alloys.
2.54.1.6. The Rockwell hardness is a standard measure of the hardness of an alloy. It is similar to the other tests, but with a different range of numbers. It is often used to measure the hardness of chrome alloys.
2.54.2. Ductility. Ductility is the property of a metal that permits it to be drawn into a thin wire without breaking. A study of the tables of hardness and ductility indicates that ductility decreases as hardness increases. Figure 2.2 lists the relative ductility of several metals.
2.54.3. Malleability. Malleability is an indication of the amount of extension the metal can sustain in all directions without breaking. The pressure might be applied by hammering, rolling, or burnishing. Malleability makes it possible to burnish the margin of a gold restoration to the tooth’s surface and minimize the chance of leakage between the two. Gold is the most malleable of all metals. One grain of gold can be rolled and beaten into a leaf that is 6 square feet. A more brittle metal is less malleable. Figure 2.3 compares the malleability of several metals.
Figure 2.2. Comparative Ductility of Selected Metals.
| High Ductility | Medium Ductility | Low Ductility |
| Gold | Palladium | Manganese |
| Silver | Cadmium | Beryllium |
| Platinum | Zinc | Antimony |
| Copper | Tin | Chromium |
| Aluminum | Lead | |
| Nickel | ||
| Cobalt |
Figure 2.3. Comparative Malleability of Selected Metals.
| High Malleability | Medium Malleability | Low Malleability |
| Gold | Zinc | Chromium |
| Silver | Iron | Manganese |
| Copper | Nickel | Antimony |
| Tin | Cobalt | Bismuth |
| Platinum | Molybdenum | |
| Lead |
2.54.4. Specific Gravity and Density.
2.54.4.1. Specific gravity is the weight of a unit of metal compared with an equal volume of water at the same temperature. Specific gravity is sometimes a factor in planning the design of a cast partial denture. The design selected for a dental prosthesis in which one of the heavier alloys is to be used might differ from one employing an alloy of a lighter weight.
2.54.4.2. The specific gravity of water is one, which is the standard of comparison. Thus, a metal that has a specific gravity of two is exactly twice the weight of an equal volume of water. Table 2.6 lists the specific gravity of some of the metals.
2.54.5. Elasticity, Flexibility, and Resiliency. Complete and technically accurate definitions of the terms elasticity, flexibility, and resiliency ar e quite complex. For laboratory purposes, they refer to the characteristic of an alloy that enables it to bend under pressure and then return to its former shape when the pressure is removed. This is an important property in a RPD clasp because the clasp must spring on and off an abutm ent tooth without exerting harmful pressure on the supporting structures of a tooth.
2.54.6. Elastic Limit, Proportional Limit, and Yield Strength. These three terms have subtly different definitions. However, for practical purpos es, the terms will be used interchangeably in this pamphlet. A gross definition for all three would be the maximum amount of stress that can be applied to a metal without permanently deforming the metal.
Table 2.6. Relative Specific Gravity of Metals.
| Item | Metal | Specific Gravity |
|---|---|---|
| 1 | Calcium | 1.54 |
| 2 | Magnesium | 1.70 |
| 3 | Beryllium | 1.84 |
| 4 | Aluminum | 2.70 |
| 5 | Antimony | 6.68 |
| 6 | Chromium | 6.92 |
| 7 | Zinc | 7.19 |
| 8 | Tin | 7.30 |
| 9 | Manganese | 7.42 |
| 10 | Iron | 7.85 |
| 11 | Nickel | 8.60 |
| 12 | Cobalt | 8.70 |
| 13 | Copper | 8.90 |
| 14 | Bismuth | 9.78 |
| 15 | Molybdenum | 10.20 |
| 16 | Silver | 10.50 |
| 17 | Lead | 11.34 |
| 18 | Palladium | 11.90 |
| 19 | Mercury | 13.59 |
| 20 | Gold | 19.32 |
| 21 | Platinum | 21.37 |
| 22 | Osmium | 22.48 |
2.54.7. Percentage Elongation. Elongation is a measure of the amount an alloy can be deformed without breaking. The percentage of elongation of an alloy has much to do with its suitability for making appliances that must be bent or burnished into shape. The elongation should be as high as possible, consistent with strength requirements.
2.54.8. Grain Size.
2.54.8.1. When metal is heated and allowed to cool, the rate of cooling affects the grain size. Slow cooling results in a comparatively large grain size. Fast cooling produces a finer grain structure. A metal with a fine grain structure is stronger than a coarse-grained one.
2.54.8.2. The metal rod at the top of Figure 2.4 was cast and solidified rapidly, which resulted in a fine grain structure. The rod in the lower part of the figure cooled slowly, which resulted in a larger grain size.
2.54.9. Grain Growth. Prolonged heating below a metal’s melting temperature may cause grain growth; that is, small grains merging to form larger ones. This grain growth causes the metal to be brittle. Malleability and ductility can sometimes be restored in a metal that has become brittle by heat-treating it. It is far better, however, to handle the metal in such a way that it never becomes brittle.
Figure 2.4. Differences in Grain Size.
natural_image
Two identical cylindrical objects with textured surfaces, no text or symbols present
2.54.10. Color of Heated Metals.
2.54.10.1. When gold alloys are heated, definite color changes occur. The temperature of the metal can be estim ated by the color it radiates as listed in Howe’s Color Scale (Table 2.7). As metal is heated, colors are observed in the fo llowing sequence: dull red, brighter red, orange, and finally, white as the temperature progressively increases.
Table 2.7. Howe’s Color Scale.
| Item | Color | Temperature |
|---|---|---|
| 1 | Dull red | 1020 - 1150 °F |
| 2 | Cherry red | 1300 °F |
| 3 | Light red | 1560 °F |
| 4 | Orange | 1650 °F |
| 5 | Yellow | 1740 - 1920 °F |
| 6 | White | 2100 °F or above |
2.54.10.2. Tem peratures associated with the colors are only approximations because color determinations differ from person to person. Another variable in appraising the color of a heated metal is the light under which it is examined. The metal may appear black in bright sunlight, but may look red when viewed in a shadow. When the color of a heated metal is evaluated, it should be viewed in as near normal light as possible.
2.54.11. Melting Range.
2.54.11.1. Pure metals melt suddenly at definite places or points on a temperature scale (Table 2.8). Dental alloys do not melt abruptly at precise temperatures because they contain a number of metals with different melting points. When a high enough temperature is reached, an alloy first softens and becomes mush. As the heat is increased, the alloy gradually becomes more fluid until it finally behaves much like a thick liquid.
Table 2.8. Melting Points of Pure Metals.
| Item | Metal | Melting Point |
|---|---|---|
| 1 | Aluminum | 1218 °F |
| 2 | Beryllium | 2332 °F |
| 3 | Bismuth | 520 °F |
| 4 | Cadmium | 610 °F |
| 5 | Chromium | 3434 °F |
| 6 | Cobalt | 2723 °F |
| 7 | Copper | 1981 °F |
| 8 | Gold | 1945 °F |
| 9 | Iron | 2795 °F |
| 10 | Lead | 621 °F |
| 11 | Manganese | 2246 °F |
| 12 | Molybdenum | 4748 °F |
| 13 | Nickel | 2651 °F |
| 14 | Palladium | 2820 °F |
| 15 | Platinum | 3190 °F |
| 16 | Silver | 1761 °F |
| 17 | Tin | 450 °F |
| 18 | Tungsten | 6098 °F |
| 19 | Zinc | 787 °F |
2.54.11.2. This gradual softening takes place over a spread of temperature known as the melting range. The lower limit of this range, known as the solidus, is the temperature at which the metal first begins to soften. The higher limit, called the liquidus, is the temperature at which the metal is completely molten. The spread of the melting range for most dental gold casting alloys varies from 75 to 150 oF.
2.54.12. Fusion Temperature. The manufacturer does not provide the melting range of a casting alloy. Very often the fusion temperature is provided instead. The fusion temperature is slightly above the lower limit of the melting range. It should never be exceeded when a metal is being soldered. The fusion temperature is provided to aid in selecting solder that has a melting range safely below the fusion temperature of the parent metal. This minimizes the possibility of overheating the parent metal during a soldering operation.
¶ 2.55. Deformation of Metal
2.55.1. When an external force (load) is applied to a metal, it is opposed and resisted by the internal force of the material’s otherwise regularly spaced atoms (stress). If the load is great enough, a change results in the distance between the atoms in the areas where the load is applied (strain) and a degree of distortion (deformation) occurs which is directly related to the amount of the load and the direction in which it is applied.
2.55.2. A distortion that disappears when the load is removed is called an elastic deformation. When the stresses are removed, the atoms return to their original position. A distortion that does not disappear when the load is removed is called a permanent deformation. (The metal is said to have exceeded its elastic limit or proportional limit or yield strength.) The stresses are not relieved and the affected groups of atoms slip along a plane to new positions within the boundaries of their particular grains.
2.55.3. If a piece of metal is flattened by pounding or rolling, the individual grains are also flattened. As a pulling force is exerted on the metal, like drawing it through a die plate to form a wire, each grain is elongated to assume a fiber-like appearance.
¶ 2.56. Strain-Hardening of Metal
2.56.1. Strain-Hardening or Cold-Working. When a metal is permanently deformed repeatedly, the metal becomes stiffer and harder. This process is called strain-hardening or cold-working. Continued application of a deforming load results in more and more atoms or grains slipping within the metal until the metal fractures. This is what happens when a paper clip is bent back and forth. Up to a point, it becomes stiffer and harder. Then it breaks.
2.56.2. Annealing or Heat Treatment.
2.56.2.1. Annealing is the process of heat-treating a metal to remove the stresses introduced by cold working and to prevent the metal’s fracture. For example, there is a considerable amount of cold-working to bend or contour a wire to form a clasp to fit on the surface of a tooth.
2.56.2.2. The strain hardness built up in the metal can be removed and the original properties restored by heating it to the proper temperature (for example, cherry red) and then cooling it rapidly by quenching in cold water. The cold-working can then proceed because the regular arrangement of the slipped atoms has been rest ored and the stresses and strains have been relieved. The grains retain the changed shape caused by the cold-working. Thus, ductility and malleability increase, but all other physical properties decrease. The metal is in its softest state so a crown or inlay is most burnishable and a partial denture clasp is most adjustable.
¶ 2.57. Effect of Constituent Metals
The exact role of a metal varies with the particular alloy system the metal is added to. For example, copper is included in many of the high palladium alloys to help form an oxide layer for porcelain bonding. However, copper is added to the medium silver-palladium alloys to effectively lower their melting range and permit the use of gypsum-bonded investments. The following elements are frequently used in the traditional gold-base alloys: (NOTE: Their descriptions are generalized.)
2.57.1. Aluminum (Al)
Aluminum is added to lower the melting range of the alloy. It is also a hardening agent and influences oxide formation.
2.57.2. Beryllium (Be)
Like aluminum, beryllium lowers the melting range, improves castability, serves as a hardener, and influences oxide formation. Reportedly, it improves polishability by acting as a lubricant for polishing agents, thus permitting them to work more effectively. Electrolytic “etching” of nickel-chromium-beryllium alloys removes a nickel-beryllium phase to create microretention for the etched-metal resin-bonded retainers (Maryland Bridges).
2.57.3. Boron (B)
Boron is a deoxidizer, hardening agent, and element that reduces the surface tension of an alloy and thereby improves castability. In the nickel chromium alloys, boron acts to reduce ductility and to increase hardness.
2.57.4. Chromium (Cr)
Chromium acts as a solid solution hardening agent and ensures corrosion resistance by its passivating nature.
2.57.5. Cobalt (Co)
Cobalt-base alloys are an alternate to the nickel-base types, but are more difficult to cast.
2.57.6. Copper (Cu)
Copper serves as a hardening and strengthening agent, lowers the melting range, and interacts with platinum , palladium, and silver (if present) to provide a heat-treating capability. It helps form oxides for porcelain bonding, lowers the density slightly, and can also enhance passivity.
2.57.7. Gold (Au)
Gold provides a high level of resistance to corrosion and tarnish (no associated passivity) and slightly increases the melting range as well as workability and burnishability. Gold imparts an esthetically pleasing color to the alloy while markedly increasing density.
2.57.8. Indium (In)
Indium serves as a less volatile scavenging agent, tends to lower the melting range (gold-base alloys), helps form an oxide layer for ceramic alloys, and lowers the density. Reportedly, an indium content of 20 percent can adversely affect the corrosion resistance of silver-base alloys.
2.57.9. Iridium (Ir) and Ruthenium (Ru)
These two elements serve as grain refiners to improve the mechanical properties and tarnish resistance.
2.57.10. Iron (Fe)
Iron is usually added to gold-base ceramic alloys to harden the alloy and aid in the production of oxides for porcelain bonding.
2.57.11. Manganese (Mn)
Like silicon, manganese acts as an oxide scavenger to prevent the oxidation of other elements when the alloy is melted. It is also a hardening agent.
2.57.12. Molybdenum (Mo)
Molybdenum is added to adjust the coefficient of thermal expansion and improve corrosion resistance. It also influences the oxides produced for porcelain bonding.
2.57.13. Nickel (Ni)
Nickel has been selected as the base for alloys because its coefficient of thermal expansion is close to that of gold and because it possesses a resistance to corrosion. It is easier to cast than the cobalt-base alloys.
2.57.14. Palladium (Pd)
Palladium is added to increase the strength, hardness (with copper), corrosion, and tarnish resistance of an alloy. It increases the melting range and improves the sag-resistance of a ceramic alloy. Palladium has a strong whitening effect, which renders metals as white alloys. It has a high affinity for hydrogen, and it lowers the density of the alloy slightly.
2.57.15. Platinum (Pt)
Platinum increases the strength, melting range, and hardness while it improves the corrosion, tarnish, and sag-resistance of an alloy. It whitens the alloy and increases its density.
2.57.16. Silicon (Si)
Silicon serves as an oxide scavenger to prevent the oxidation of other elements during the melt. It is also a hardening agent.
2.57.17. Silver (Ag)
Silver imparts a moderate increase in the strength and hardness of an alloy (with copper), tends to tarnish in the presence of sulfur, possesses a rather high affinity for oxygen absorption, and lowers the density of the alloy. In ceramic alloys, silver lowers the melting range by counteracting the influence of palladium . Ceramic alloys with a high silver content may produce discoloration (green or brown) in many porcelains.
2.57.18. Tin (Sn)
Tin serves as a hardening agent, tends to decrease the melting range of the alloy, and helps produce an oxide layer in ceramic systems.
2.57.19. Titanium (Ti)
Titanium is added to lower the melting range and improve castability. It also acts as a hardener and influences oxide formation.
2.57.20. Zinc (Zn)
Zinc helps lower the melting range and acts as a deoxidizer or scavenger to combine with any oxides present. It improves the castability of an alloy and, when combined with palladium, contributes to its hardness. Zinc is commonly included in gold alloy solders.
¶ 2.58. Classification System
2.58.1. The ADA classifies alloys according to the percentage of noble metals present. Noble metals include gold and the six members of the platinum-palladium group; ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os) , iridium (Ir), and platinum (Pt). The ADA classification system is identified in Table 2.9.
Table 2.9. ADA Classification System.
| ITEM | A | B |
| Classification | Au and Pt Group | |
| 1 | High Noble | Greater than or equal to 90 percent |
| 2 | Medium Noble | Less than 90 percent; greater than or equal to 70 percent |
| 3 | Low Noble | Less than 70 percent |
| 4 | Base Metal | 0 percent |
2.58.2. Table 2.10 contains specifications of dental alloys.
Table 2.10. Essential Specifications of Dental Alloys.
| Item | Alloy | Purpose | ADA Specification | Brinell Number | Melting Range | Heat Treatment |
|---|---|---|---|---|---|---|
| 1 | Soft Inlays | Subject to little stress | A minimum of 83 percent of the gold-platinum group of metals | 40 to 75 | 1740 to 1920 °F | Not subject to heat treatment |
| 2 | Medium | Inlays of any kind | A minimum of 78 percent of the gold-platinum group of metals | 70 to 100 | 1650 to 1780 °F | Bench cool for 5 minutes after casting and quenching |
| 3 | Hard | Single onlays, crowns, and fixed partial retainers | A minimum of 78 percent of the gold-platinum group of metals | 90 to 140 | 1650 to 1760 °F | Softening heat treatment: heat to 1292 °F and quench; hardening heat treatment: heat to 840 °F; drop temperature to 480 °F; quench |
| 4 | Extra Hard | RPD frameworks | A minimum of 75 percent noble metal content | 130 + | 1600 to 1800 °F | NA |
| 5 | Ceramic Gold Alloys | Porcelain veneer fusion | Enough noble metal to provide corrosion resistance | 160 to 200 | 2200 to 2400 °F | NA |
| 6 | RPD Alloys | RPD frameworks | No less than 85 percent chromium, cobalt, and nickel | 431 | 2300 to 2500 °F | NA |
¶ 2.59. Dental Alloy Terminology
There are so many different metal-ceramic alloys for porcelain bonding, each with its own peculiar handling characteristics. For that reason, it is often difficult to accurately communicate with others about these alloys. For example, Olympia is the brand name of a metal-ceramic alloy that can be called either a precious alloy, a high noble alloy, or a gold-palladium alloy. It is easy to see why there is so much confusion associated with describing a particular alloy. The most accurate description of an alloy can only be made by listing its major constituents and any minor metal considered important for its functional use.
¶ 2.60. Conventional Dental Gold Alloys (Nonceramic)
Conventional gold alloys are used to fabricate and repair inlays, onlays, crowns, and fixed partial dentures. These gold-base alloys can be organized into three categories; casting gold, gold solder, and gold foil. Each is formulated and marketed in a form best suited to its intended use.
2.60.1. Dental Casting Golds.
2.60.1.1. Casting golds are alloyed and made into ingots suitable for melting and casting into a mold. Different types of restorations require alloys with slightly different physical properties. An alloy used for an inlay must have somewhat different capabilities than an alloy used for a crown or fixed partial denture.
2.60.1.2. The physical property hardness is regarded as a useful indi cator of the strength of an alloy. Therefore, restorations such as inlays are made using a soft or medium gold alloy (formerly Type I and II). Sim ilarly, a crown or fixed partial denture requires a much harder casting alloy. On occasion, even RPD frameworks are made of extra hard gold for patients who may be allergic to chrome alloys. The dentist se lects the type of casting alloy according to the needs of the particular case, personal preference, and availability.
2.60.1.3. Elements most commonly found in gold-base casting alloys are gold, silver, copper, platinum, palladium, and zinc.
Table 2.11 shows typical compositions of conventional gold casting alloys according to hardness.
2.60.1.4. The increased use of silver in medium and hard alloys is intended to lighten the color of the alloy, not affect the hardness. As the copper content of gold alloys is increased, the color of the alloy is darkened unless it is offset by silver. Most people consider this darker color less attractive.
Table 2.11. Typical Composition of Conventional Gold-Casting Alloys.
| Item | Alloy | Gold | Platinum | Palladium | Silver | Copper | Zinc |
|---|---|---|---|---|---|---|---|
| 1 | Soft | 83 | 6 | 4 | 7 | NA | NA |
| 2 | Medium | 76 | 7 | 5 | 7 | 5 | NA |
| 3 | Hard | 74 | 6 | 5 | 9 | 5 | 1 |
| 4 | Extra Hard | 70 | 7 | 4 | 13 | 5 | 1 |
2.60.1.5. The zinc in these alloys is present to prevent the loss of copper by oxidizing the molten metal. As a result, when the copper is increased, the zinc content is also increased. Zinc lowers the melting range of the finished alloy and makes the material more fluid and easier to cast.
2.60.1.6. Heat treatment of wrought and cast golds is an integral part of all fabrication procedures in which they are used. Careless heat handling is responsible for much of the breakage of cast and wrought wire prosthetic appliances. When gold alloys are properly heat treated, their desirable properties are appreciably increased.
2.60.1.7. In general, when gold alloys are heated to 1292 oF (cherry red) for 10 minutes and then quenched in water, the metal is in its softest state (annealing). Ductility and malleability increase, all other physical properties show decreased values. Wrought gold is most bendable and cast gold is more burnishable (adaptable) after being subjected to softening heat treatment.
2.60.1.8. Gold restorations that are to be subjected to heavy chewing loads or other mechanical pressures in the mouth cannot be used in a heat-softened condition because they would deform too easily. These restorations must be treated by a process of controlled heating and cooling to restore hardness called tempering. For example, certain gold alloys are hardened by heating to 840 oF, allowed to cool slowly over a 15-minute period to 480 oF, and then quenched in water. Gold alloy destined for heat hardening should first be heat softened. Any strain hardening is relieved, and the heat hardening process yields more predictable results.
2.60.1.9. There are two ways of expressing the gold content of an alloy. One is in terms of its carat and the other is its fineness. By definition, a carat is a unit of measure that indicates purity of formulation. A 24-carat rating means that a metal is solid gold with nothing else added. A metal having an 18-carat rating is automatically an alloy, 18 parts of which are gold. The six remaining parts are other metals. The concept of fineness is similar to carat only it deals with 1,000 parts instead of 24. Pure gold is said to be 1,000 fine. Therefore, if an alloy is 75 percent gold, it is 750 fine. An easy way to convert fineness to carat and vice versa is to insert the known number into the following formula and solve for the unknown:
2.60.2. Gold Solders. Gold solders are primarily used to join units of fixed partial dentures. They are also used in repairs of all kinds--every thing from plugging holes in castings to adding proximal contacts on restorations. Solders are supplied in many forms. Wires and strips are the most common.
2.60.2.1. Physical Properties of Gold Solders.
2.60.2.1.1. Melting Range. Dental gold solders are designed to melt and flow within precise temperatures called a melting range. A melting range is the temperature range from the time an alloy begins to melt until it is completely molten. A precise melting range is necessary to prevent overheating and melting the parts to be joined (that is, the parent metal). Solders should melt at a temperature 100 oF below the fusion temperature of the parent metal. Tin and zinc are added to solder formulas to provide a fusion temperature lower than the fusion temperature of the parent metal. The melting range of a typical, conventional gold solder is 1375 to 1445 F.
2.60.2.1.2. Tarnish Resistance. After a soldered prosthesis has been in the patient’s mouth for a period of time, the solder should show a degree of tarnish resistance that is very close to the parent metal.
2.60.2.1.3. Color Compatibility. The color of the solder should match the parent metal as closely as possible.
2.60.2.1.4. Strength Requirement. As the fineness of solder increases, its strength decreases. Solders with fineness ratings in excess of 650 should not be used to unite fixed partial denture units that are subject to a lot of stress.
2.60.2.2. Choosing a Solder.
2.60.2.2.1. When the Properties of the Parent Metal are Known. Several rules of thumb can be used in selecting a solder if either the carat, fineness, or the fusion temperature of the parent metal or metals is known. The primary objective is to pick a solder with a fusion temperature about 100 to 150 oF lower. Solder with carat ratings that are two values lower or fineness ratings that are 100 units lower than the parent metal are assumed to have fusion temperatures at least 100 oF lower than the parts to be joined.
2.60.2.2.2. When the Properties of the Parent Metal can be Guessed. Fixed prosthesis castings that are not porcelain veneered are made with conventional Type II or Type III golds. The minimum fusion temperature for these kinds of golds is around 1650 oF; therefore, a solder should be selected accordingly. A problem arises when the mass of gold alloy used to make a casting already contains some solder (for example, a reclaimed fixed partial denture). The presence of the solder acts to lower the fusion temperature of the mass. If a technician tries to solder castings with this kind of unknown composition, success is doubtful. If old, previously used gold is recycled, the solder must be ground off. Ceramic golds have melting ranges in excess of 2100 oF.
2.60.2.3. Supplemental Remarks. The 650-fine solder is widely used for soldering fixed prosthetic restorations made out of conventional casting golds. As fineness increases above
650, tarnish resistance increases, but strength drops off. Below 650 fine, the solder gets stronger but may descend below acceptable tarnish resistance limits. If ceramic casting golds are soldered before porcelain fusion, use special solders that melt at around 2000 oF. After porcelain fusion, they can be soldered with materials used on conventional casting golds (for example, a 650-fine solder).
2.60.3. Gold Foil. Foil is used in repairs such as repairing a hole in a gold crown with solder. It is fabricated at the refinery by rolling gold into sheets of various thickness.
¶ 2.61. Alternatives to Gold-Base Alloys (Nonceramic)
There are precious nongold-base alloys and nonprecious chrome alloys which are offered as alternatives to conventional gold-casting alloys. The nonprecious alloys, nickel-chromium and cobalt-chromium, are less desirable alternatives and, as such, are not discussed here. The remaining precious alloy group can be subdivided into the following three subgroups:
2.61.1. High Silver-Palladium Group. Alloys in this group contain roughly 70 to 71 percent silver and 25 percent palladium . They require the use of phosphate-bonded casting investments and generally possess gold-base alloy handling characteristics. These alloys cost less than gold-base alloys.
2.61.2. Medium Silver-Palladium Group. This second group of silver-palladium alloys was developed to provide a less expensive casting alloy that could be used with conventional gold techniques. These alloys contain between 58 to 68 percent silver, 25 to 26 percent palladium, and 10 to 15 percent copper. Adding copper to this system lowers the melting range and permits the use of gypsum-bonded investments. Several of these alloys possess properties similar to extra hard casting golds, while others are marketed as hard casting gold alternatives. Some alloy brands can be cast into gypsum-bonded investments, while others require phosphate-bonded casting investments. Consult the manufacturer’s directions regarding information on their use.
2.61.3. Silver-Palladium-Gold Group. By lowering the silver content of the silver-palladium system and replacing copper with gold, manufacturers produce a more tarnish-resistant alloy. The increase in noble metal content of this system makes this alloy more expensive, but it is still cheaper than conventional gold-base alloys. Phosphate-bonded investments are indicated with these alloys and processing is similar to the high silver-palladium group.
¶ 2.62. Metal-Ceramic Alloys
These high-melting range alloys are blended for porcelain application. They retain both form and physical properties when porcelain is applied and fused to them. The alloy’s coefficient of thermal expansion is slightly more than porcelain, placing the porcelain veneer under compression when the alloy cools. The porcelain is most likely to remain attached to the alloy in this condition. The titles of the following subparagraphs show the classification of the metal-ceramic alloy in parenthesis beside each alloy topic:
2.62.1. Gold-Platinum-Palladium (High Noble)
2.62.1.1. Alloy Contents. Gold-platinum-palladium alloys contain mostly gold. Platinum and palladium are added to raise the melting temperature, reduce the coefficient of thermal expansion, and strengthen the alloy. Small portions of base metals, such as indium, zinc, and tin, are included to produce a thin oxide film on the surface of the gold alloy which provides the chemical means for bonding between the metal and porcelain.
2.62.1.2. Advantages. The bond strength of gold-platinum-palladium alloys is excellent. They also cast easily, finish and polish easily, and produce fine, burnishable margins.
2.62.1.3. Disadvantages. The only reasons for replacing gold-platinum-palladium alloys are cost and mechanical strength. To maintain its strength, this alloy must be used in fairly thick sections in areas such as connectors. One of the other alloys might be a better choice for a long span fixed partial denture subject to increased occlusal loading.
2.62.2. Gold-Palladium-Silver (Medium Noble)
2.62.2.1. Alloy Contents. The elimination of platinum and the addition of silver are the principal differences between this alloy and the gold-platinum-palladium alloy described above. Silver tarnishes and is, therefore, not a noble metal.
2.62.2.2. Advantages. These newer “white golds” or “sem iprecious” alloys have gained in popularity, probably due to their lower cost and increased mechanical strength. Because these alloys are higher in yield strength than the high gold content alloys, they are useful for longspan restorations. They are easy to cast, finish, and polish.
2.62.2.3. Disadvantages. The silver present in these alloys may cause greening of the fired porcelain and contamination of the muffle within the porcelain furnace. Also, the high palladium content can increase the risk of hydrogen gas absorption during casting. During the porcelain processing step, the release of hydrogen gas from the metal can create gas bubbles in the porcelain veneer. Although, the porcelain bond is not as good as the gold-platinum palladium alloy, it is adequate.
2.62.3. Palladium-Silver (Low Noble)
More recently, an effort has been made to eliminate gold from metal-ceramic alloys completely by substituting palladium and silver.
2.62.3.1. Alloy Contents. The manufacturer must achieve a careful balance between the amount of palladium and silver in the alloy because increasing the palladium content raises the melting range and lowers the coefficient of expansion, whereas silver has the opposite effect.
2.62.3.2. Advantages. Due to the lower density of palladium-silver alloys combined with their low intrinsic cost, these alloys have a substantia l decrease intotal metal cost compared to the gold-containing alloy systems. A further advantag e is that their handling characteristics and physical properties are comparable.
2.62.3.3. Disadvantages. The disadvantages associated with the use of palladium and silver are the same as in paragraph 2.62.2.3. Additionally, the increase in palladium makes casting more difficult because of the hydrogen gas absorption. The gas and oxygen pressure of casting torches requires critical adjustment to produce the correct flame for melting. Using carbon-free investments is also a must because residual carbon can affect the grain structure of the alloy.
2.62.4. Palladium (High Noble)
Although palladium-silver alloys were in great demand because of the low intrinsic cost, the dental profession wanted alloys that did not cause greening problems. Research led to alloy systems which reverted back to silver-free systems. In place of silver, more palladium was added.
2.62.4.1. Advantages. Increasing the palladium content imparts greater hardness, toughness, and strength to the alloys. With silver completely eliminated from the alloys, greening of the porcelain veneer is not a problem.
2.62.4.2. Disadvantages. High palladium alloys are more difficult to cast. For example, more casting pressure is needed because of the lower density, and care must be taken not to induce excessive amounts of hydrogen, oxygen, or carbon into the melt. The torch flame must have just the right balance of gas and oxygen. Carbon contamination can appear from such sources as carbon-containing investments, waxes, plastic patterns, carbon crucibles, or liners.
2.62.5. Base Metal. Base metal alloys (or nonprecious alloys, as they have been called) are similar in composition to the alloys used in RPDs. NOTE: The other elements contained in base metal alloys could be molybdenum, manganese, magnesium, aluminum, silicon, beryllium, carbon, iron, titanium, or copper. Although beryllium is often omitted from some alloys because of its toxicity, it hardens the alloy and improves castability. Cobalt-chromium alloys are rarely used in porcelain bonding.
2.62.5.1. Advantages. The properties of base metal alloys are considerably different from the noble metal alloy systems. In general, the base metal alloys are much stiffer and harder, requiring high-speed equipment to finish and polish the alloy. Because of this strength, base metal alloys for porcelain bonding are useful where long-span, thin substructures are necessary. They are also the only metal alloys that can be used in the acid etch technique of making resin-bonded FPDs. Low cost has probably been the single most important consideration in selecting base metal alloys over noble alloy materials.
2.62.5.2. Disadvantages. Specialized casting and finishing equipment is recommended when working with base metal alloys. Because these alloys do not contain noble metals, they readily oxidize when heated. In addition, they may develop too many oxides, causing the bond between the porcelain and metal to fail. Because of the high oxidizing tendency, the techniques for metal preparation and porcelain additions are different from those used with noble alloys. There has also been controversy over the allergenic and carcinogenic potentials of nickel used in nickel-chromium alloys (mostly with RPDs). In addition, alloys containing beryllium are becoming more suspect because of beryllium’s toxic and unstable character.
2.62.6. Metal Ceramic Alloy Composition.
Table 2.12 shows typical compositions of metal-ceramic alloys.
Table 2.12. Typical Compositions of Noble Metal Alloys for Metal-Ceramic Restorations.
| Item | Composition | Gold (Au) | Platinum (Pt) | Palladium (Pd) | Silver (Ag) | Indium and Tin (In-Sn) | Galium (Ga) | Copper (Cu) |
|---|---|---|---|---|---|---|---|---|
| 1 | Au-Pt-Pd | 84 | 10 | 2 | 3 | 1 | NA | NA |
| 2 | Au-Pd-Ag | 50 | NA | 30 | 12 | 8 | NA | NA |
| 3 | Au-Pd | 52 | NA | 38 | NA | 8.5 | 1.5 | NA |
| 4 | Pd-Ag | NA | NA | 55 to 60 | 25 to 30 | 10 to 20 | NA | NA |
| 5 | Pd | NA | NA | 74 | NA | 5 | NA | 14.5 |
NOTE: All figures are percentages.
¶ 2.63. RPD Casting Alloys
2.63.1. Basic Content.
2.63.1.1. The ADA specification for cast chromium containing alloys says “. . . the alloy contains a total of no less than 85 percent by weight of chromium, cobalt, and nickel.” This allows a lot of latitude for putting an alloy formula together.
2.63.1.2. When the formulas for the different brands of chrome alloy are reviewed for constituents common to all of them , they contain roughly 20 to 30 percent chromium and highly variable amounts of nickel and cobalt. As an example, Ticonium Premium 100 alloy contains about 25 percent chrome and 60 percent nickel, with no cobalt present.
2.63.1.3. Other brands of alloys have much less nickel and proportionally more cobalt. According to the ADA’s Guide to Dental Materials and Devices, a chrome alloy with a high nickel content is reported to have a lower melting range and less thermal contraction than a chrome alloy with a high percentage of cobalt. In contrast to nickel-chrome alloy, the cobaltchrome family of alloys cast at temperatures that require a high-heat investment with a silicate or phosphate binder. Smaller amounts of several other metals such as molybdenum, tungsten, beryllium, iron, manganese, and aluminum are fre quently a part of chrome alloy formulas. Table 2.13 gives a general idea of chrome alloy composition. CAUTION: Prolonged exposure to beryllium is harmful. Proper dust-collecting apparatus must be used when finishing alloys that contain this metal.
Table 2.13. Approximate Proportions of Metals in Various Chrome Alloys.
| ITEM | A | B |
| Metal | Proportions (Percentages) | |
| 1 | Aluminum | 0 to 0.7 |
| 2 | Beryllium | 0 to 1.8 |
| 3 | Carbon | 0.2 to 0.4 |
| 4 | Chromium | 20 to 30 |
| 5 | Cobalt | 0 to 60 |
| 6 | Iron | 0 to 5 |
| 7 | Manganese | 0 to 0.5 |
| 8 | Molybdenum | 4.6 to 18.5 |
| 9 | Nickel | 5 to 60 |
| 10 | Silicon | 0.7 to 0.4 |
| 11 | Tungsten | 0 to 4 |
2.63.2. Properties of Chrome Alloys. These alloys are considerably harder than the gold alloys and have much higher melting ranges. The specific gravity of these alloys is only about half that of the gold alloys. This factor makes possible a reduction in weight of the RPD casting over one made from a gold alloy. Special equipment is required for casting and finishing the chrome alloys. Due to the differences in hardness and melting temperatures of chrome alloys, equipment used for gold alloys is not suitable.
¶ 2.64. Wrought Alloys
Wrought alloys are made through a pro cess of rolling, annealing, and drawing into the various forms. This processing gives the alloys certain physical properties that are of interest to the dentist. The elastic and flexible nature of the alloy is of particular im portance. It allows a wrought wire clasp to spring into an undercut area. Due to their increased tensile and yield strength, wrought alloys do not deform as easily.
2.64.1. Base Metal Alloys.
2.64.1.1. Forms of Wrought Base Metal Alloys. Wrought base metal alloys are available in the form of wires and bond materials. Nickel-chromium and cobalt-chromium are the two main types of wrought alloys presently used. These alloys are commonly referred to as stainless steels.
2.64.1.2. Rules for Handling. Certain procedures must be followed if the desired properties of wrought alloys are to be maintained. When working with wrought wires, avoid making bends too quickly. Do not make bends that are too large or too sharp and do not nick or dent wires because it causes them to break when stress is applied. Also, if repeated bending and shaping is necessary, heat-soften the wire to relieve the stress effects of cold-working caused by bending and shaping. For best results, follow the manufacturer’s instructions for heat-softening. When soldering stainless steel wires, take special care to prevent overheating the wire. A prolonged exposure to temperature in excess of 1300 oF softens the wire and reduces corrosion resistance.
2.64.2. Gold Alloys.
2.64.2.1. Forms of Wrought Gold Alloys. Wrought gold is made in the form of wires and bars that can be used to make individual, wrought gold clasps or an entire RPD framework. The gold is alloyed to give it the properties needed so it can be bent and shaped into a desired form and so several different units can be joined by dental solder.
2.64.2.2. Grain Structure. Because of its method of manufacture, dental wrought gold has a different grain structure than cast gold. Because of its grain structure, it is considerably tougher than cast gold and has a higher yield strength and proportional limit.
2.64.2.3. Sizes of Wrought Gold Wire. The manufacturer makes wrought gold from gold alloy that is rolled, swaged, and drawn through the die plates into the desired shape and gauge. The gauge is a measure of the thickness or diameter of the wire. The numbers used are the standard Brown and Sharpe gauge numbers used by machinists. The larger the gauge number, the smaller the diameter. A comparison of the gauge number and the equivalent measurement in inches and millimeters is shown in Table 2.14.
Table 2.14. Brown and Sharpe Wrought Wire Gauge Conversion.
| Item | Gauge Number | Inches | Millimeters |
|---|---|---|---|
| 1 | 12 | 0.0808 | 2.052 |
| 2 | 14 | 0.0641 | 1.628 |
| 3 | 16 | 0.0508 | 1.290 |
| 4 | 18 | 0.0403 | 1.024 |
| 5 | 20 | 0.0320 | 0.813 |
| 6 | 22 | 0.0253 | 0.643 |
| 7 | 24 | 0.0201 | 0.511 |
2.64.2.4. Composition of Wrought Gold Wire. Dental wrought gold wires must have high fusion temperatures so the different parts of the clasp can be assembled by soldering without the danger of overheating the wire. For this reason, platinum and palladium are added in much greater amounts than those used in the casting alloys. The following formula is typical although some wrought wires presently marketed might contain considerably more platinum and palladium, and correspondingly less gold. Wrought wires are usually silver-colored, due to their high percentages of platinum and palladium. A typical formula includes zinc (1 percent), palladium (5 percent), silver (8 percent), copper (13 percent), platinum (15 percent), and gold (58 percent).
2.64.3. Gold Wire. Wrought gold wire is used to make RPD clasps. Wrought gold bars are used as lingual and palatal bars. Clasps and bars can be assembled into a complete RPD framework by using dental gold solder.
¶ 2.65. Technique Alloys
Technique alloys are used mainly for making demonstration models and for student practice. They resemble gold in appearance although they differ markedly in physical properties. They have an extremely low tarnish resistance and oxidize rapidly at ordinary temperatures. A typical formula for one such alloy is 12 parts silver, 68 parts copper, and 20 parts zinc.
¶ 2.66. Low-Fusing Alloys
2.66.1. Several low-fusing alloys are marketed for dental laboratory use. They are often used to pour the tooth portions of an opposing cast in the construction of a complete denture against natural teeth. They are also used to remount fixed prosthodontic castings in an articulator for occlusal adjustments. (See AFPAM 47-103, Volume 2, Dental Laboratory Technology, Fixed and Special Prosthodontics, Chapter 1.)
2.66.2. Low-fusing alloys melt at such low temperatures they can be poured into any elastomeric impression material as well as into alginate or agar impressions without damaging them. These alloys are marketed with a variety of trade names. All of the formulas are made using different proportions of the following low-fusing metals: bismuth (520 oF), cadmium (610 oF), indium (314 oF), lead (621 oF), and tin (450 oF), (Table 2.15). The manufacturer strives to produce an alloy with a minimum of dimensional change as it is heated and cooled. NOTE: Cadmium fumes are toxic. A good way to beat the cadmium vapor problem is to melt cadmium-containing alloys under hot water.
Table 2.15. Typical Low-Fusing Alloys.
| Item | Brand Name | Melting Point |
|---|---|---|
| 1 | Oatey 95.5 | 450 °F |
| 2 | Oatey 50/50 | 421 °F |
| 3 | Oatey 60/40 | 374 °F |
| 4 | Cerrelow® 136 | 136 °F |
| 5 | Cerrelow® 147 | 147 °F |
| 6 | Melotte's® 50.0 | 158 °F |
2.66.3. For convenient use, all of the low-fusing alloys mentioned can be melted and poured off into disposable plastic, 60 cm syringes. When metal is needed, heat it in the syringe under hot water and dispense it directly from the syringe as required.
¶ 2.67. Platinum Foil
2.67.1. Platinum foil is manufactured by rolling platinum metal into thin sheets, the appropriate gauge of tinfoil. Pure platinum has a strong affinity for molten gold which makes it a very useful metal in the dental laboratory. Platinum is used in gold, wrought-wire clasp construction as a matrix on which to flow the solder for the occlusal rest. It can be used for all types of gold and chrome alloy repair procedures.
2.67.2. Platinum foil is used in porcelain jacket work as a matrix upon which the porcelain is formed and fired. Because of its high melting point (3190 oF) and low thermal expansion, platinum foil is not affected appreciably by the heat of the furnace.
¶ Section 2G — Dental Porcelains
¶ 2.68. Introduction
2.68.1. Highly glazed porcelain is one of the materials most compatible with oral tissues and one of the most esthetically pleasing of the dental materials. It is used for denture teeth, facings, complete crowns, and veneered fixed prosthodontic units.
2.68.2. Porcelain does not have the crus hing or shear strength of cast metal, but when it is used in the proper bulk and with adequate support, it is ve ry satisfactory for dental restorations. Dental porcelains are classified according to fusion te mperature. High-fusing temperatures (2350 to 2500 F) are used for denture teeth; medium-fusing temperatures (2000 to 2300 oF) are used for porcelain facings; and low-fusing temperatures (1200 to 1950 oF) are used for crowns and veneers.
2.68.3. All of the dental porcelains used to fabricate porcelain veneers or complete porcelain crowns fall into the low temperature range and are thus classified as low-fusing porcelain.
¶ 2.69. Manufacturing Dental Porcelain
2.69.1. Glass Properties. Dental porcelain is basically glass. When heated, it can be shaped and molded into a variety of things. Glass is physically a supercooled liquid rather than a solid. However, ordinary glass must be modified and carefully compounded before it can be called dental porcelain. The composition of porcelain is carefully controlled to modify the physical properties of the glass both in the molten and solid form. These properties include viscosity, melting temperature, chemical durability, thermal expansion, and resistance to devitrification.
2.69.2. Vitrification and Devitrification. Vitrification is the development of porcelain that resembles glass. It is important to know what vitreous (mature) porcelain looks like. Mature porcelain exhibits maximum shrinkage. The surface is completely sealed, and the surface detail is slightly rounded. Devitrification occurs when the firing sequence is interrupted. When this occurs, the porcelain tends to crystallize, making it difficult to form a glazed surface.
2.69.3. Fritting.
2.69.3.1. Before this process begins, the natural feld spar and glass fluxes are mixed together in powdered form. Then the raw minerals are mixed together in a refractory crucible and heated to a temperature well above the firing temperatures used in the laboratory. The minerals all melt together to form a molten glass which is quenched in water. The mass cracks and fractures, and it is from this frit that the porcelain powders are made.
2.69.3.2. The process of blending, melting, and quenching the glass components is called fritting. Each time this is done, more of the undissolved particles are converted to glass. The particles become so small they merely fuse together when they are heated. By prefiring in this manner, the manufacturer can control the maturing temperature and translucency of the porcelain.
2.69.4. Sintering. This term more accurately describes the firing process. As the porcelain powders are heated, they partially fuse together to form a compact, noncrystalline solid. Unlike metals with crystalline structures, glass is comparatively weak because the atoms are arranged in an irregular pattern.
¶ 2.70. Basic Composition
Low-fusing dental porcelains can be divided into two groups, feldspathic and aluminous porcelain (paragraph 2.71). Although different in their specific makeup, they share a common feldspathic glass frit. Paragraph 2.69.3 describes the fritting process and its effect on the porcelain. Specific ingredients comprise dental porcelain. The following ingredients and their use provide valuable insight into the physical properties of dental porcelain:
2.70.1. Glass Formation. The principle element in all glasses is oxygen (O 2), which forms a stable bond with silicon (Si) to produce SiO 4 tetrahedra. SiO 4 is called a silicate, commonly known by another name--sand. Silicon is the major glass-forming oxide in dental porcelain, but boron and alumina may also be used.
2.70.2. Fluxes or Alkali.
2.70.2.1. Fluxes are added to the basic silicon- oxygen network to lower the softening temperature and increase the thermal expansion of a glass. Potassium, sodium, and calcium oxide are the glass modifiers used as fluxes. The manufacturer has to control the use of these fluxes because they have a drastic effect on the vi scosity (fluidness) of the glass and its thermal expansion. For example, the soda content is increased in metal-bonding porcelains to raise the thermal expansion of the porcelainto that of the metal alloys. This addition has an adverse effect on the porcelain because it is now more susceptible to devitrification (a problem associated with metal porcelains).
2.70.2.2. Lower viscosity is another problem caused by adding fluxes, but it can be corrected using intermediate oxides.
2.70.3. Intermediate Oxides. Aluminum oxide is the most common interm ediate oxide used to increase the hardness and viscosity of porcelain. De ntal porcelains must maintain their shape and not slump when heated. Fluxes used to be added to lower the softening temperature, but they also lowered the viscosity. Now, interm ediate oxides are added to produce glasses with high viscosity as well as low-firing temperatures.
2.70.4. Coloring and Opacifying Agents. Presently, the porcelain frit lacks the color to simulate the denture and enamel shades of teeth. It may appear opalescent or assume a gray-blue translucency similar to natural incisal enamel. Another problem is the slightly greenish hue exhibited by all glasses. In order to dampen down this effect and to produce life-like dentin and enamel colors, the basic dental porcelain frit must be colored as follows:
2.70.4.1. Color Pigments. The dental porcelain frit is usually colored by adding concentrated glasses. These glasses are metallic oxides fritted with the basic glass used to modify the uncolored porcelain powder. The metallic oxides used to color dental porcelain appear in Table 2.16. The dental porcelain now has color, but is far too translucent. Therefore, certain oxides must be added to opacify the porcelain, particularly the dentine shade.
2.70.4.2. Opacifying Agents. An opacifying agent generally consists of a metallic oxide ground to a very fine particle size. Common oxides are cerium oxide, titanium oxide, and zirconium oxide (Table 2.16). In the enamel porcelains, very little opacifier is used because this porcelain requires more translucency. The formation of dental porcelain is now complete.
Table 2.16. Metallic Oxides Used in Coloring and Opacifying Dental Porcelains.
| ITEM | A | B |
| Color/Effect | Metallic Oxide Responsible | |
| 1 | Pink | Chromium-tin or chrome alumina are useful in eliminating the greenish hue in the glass and adding a warm tone to the porcelain. |
| 2 | Yellow | Indium or praesodymium are the most stable for producing the ivory shades. |
| 3 | Green | Chromium oxide is green and the characteristic color of glass. This color should be avoided. |
| 4 | Gray | Iron oxide (black) or platinum (gray) are useful in making enamels or for dentines in the gray section of the shade guide. They can also give an effect of translucency. |
| 5 | Opacity | Cerium oxide, titanium oxide, and zirconium oxide. |
¶ 2.71. Low-Fusing Porcelain Systems
The basic types of porcelain systems are feldspathic porcelain (used in veneering metal substructures), aluminous porcelain (used in making porcelain jacket crowns), and leucite porcelain (used with pressable porcelains).
2.71.1. Feldspathic Porcelain. Natural feldspar is any group of minerals, principally alumina silicates of potassium, sodium, and calcium . Natural feldspar contains most of the elements needed for glassmaking. Glass fluxes such as boric oxide are added to lower the softening temperature of the glass. This mixture can be fritted at a specific temperature to obtain the desired porcelain frit. Feldspathic porcelains are used in metal ceramic crowns. They have a firing range of 900 to 960 C.
2.71.2. Aluminous Porcelain.
2.71.2.1. The addition of pure alumina Al2O3 to the feldspar-flux mass greatly strengthens the porcelain. Alumina is the only true crystalline ceramic used in dentistry. It is the hardest and probably strongest oxide known.
2.71.2.2. Aluminous porcelain is three times stronger than feldspathic porcelain and it has six times its crushing strength. Unfortunately, added alumina also decreases translucency.
2.71.2.3. The three main types of aluminous porcelain include a high-strength core material containing as much as 50 percent pure alumina crystals, dentine (containing 5 to 10 percent alumina crystals), and enamel veneer powders. The core buildup strengthens the all ceramic crown much the same as a metal substructure, but to a lesser degree.
2.71.3. Leucite Porcelain. Leucite-reinforced ceramic powders are pressed to form ingots which are the basis for an all-ceramic restoration. This system uses the lost-wax technique associated with conventional metal-ceramic restorations. Af ter burnout, the ceramic ingot is heated to a softened state and pressed into the mold. Leucite-reinforced restorations give a very esthetically pleasing result with necessary strength for veneers, single crowns, and onlays.
¶ 2.72. Porcelain Components
2.72.1. Porcelain Powders. The manufacturer supplies feldspathic porcelain powders in the following four basic forms:
2.72.1.1. Opaque Porcelain. This porcelain contains a larger percentage of opacifying agent (zirconium and tin oxide) and is quite opaque. It is used to mask out the color of the underlying metal.
2.72.1.2. Dentin or Body Porcelain. This porcelain matches the gingival two-thirds of a tooth.
2.72.1.3. Enamel or Incisal Porcelain. This porcelain is very translucent and is used to overlay the dentin porcelain and match the incisal shade of a tooth. In addition to the different porcelain powders, each manufacturer supplies a multitude of tooth shades. Even then, it is extremely difficult to make a complete match without the aid of stains or color modifiers.
2.72.1.4. Shoulder Porcelain. This porcelain is used in the facial margin area to provide an esthetically pleasing alternative to the metal collar. It is available in different porcelain shades to match the dentin and enamel components.
2.72.2. Stains and Color Modifiers. The stains and color modifiers supplied in a kit of dental porcelain are made in the same way as the concentrated color frits used to color the porcelain powders. A color modifier is used intrinsically (internally) to obtain gingival effects or to highlight body colors. A stain is more concentrated than a color modifier and is used for extrinsic (external) coloration. Because stains are applied to the surface of fired porcelain, they are usually mixed with low-fusing, air-fired porcelain. This mixing allows the stainto fuse to the porcelain at a lower temperature. Stains are used to color correct (alter shades) and characterize porcelain restorations.
2.72.3. Glazes and Add-On Porcelains. Technicians face many challenges in making porcelain restorations. Either they cannot get a crown to glaze (self-glazing) or they may need to make a simple correction to a contact area. For just this purpose, the manufacturer supplies various correction powders as follows:
2.72.3.1. Dental Glazes. Dental glazes are clear, low-fusing porcelains which can be applied to the surface of a fired crown to produce a glossy surface. Glaze powders are difficult to apply evenly and are often used to seal off a poorly baked restoration. An autogeneous (self-produced glaze) is preferred over the glass powders. Applied glazes must have a coefficient of thermal expansion that matches porcelain. Otherwise, the glass (glaze) will craze on the surface of the veneer.
2.72.3.2. Add-on Porcelains. Except for the addition of opacifiers and coloring pigm ent, add-on porcelains are similar to glaze porcelains. Th ey may be marketed as correction powders which are normally air-fired porcelains. Add-on porcelains enable minor corrections to be made without the risk of high temperatures and the vacuum cycle.
¶ 2.73. Condensation
When porcelain is fired, the particles fu se, replacing some of the formerly water-filled spaces with viscous glass and leaving some sp aces filled with air. With fewer air spaces, the porcelain is stronger and more translucent. Various methods are used to condense the particles and reduce the number of air spaces before firing as follows:
2.73.1. Manufacturing the powders with various sizes of particles.
2.73.2. Closely packing the particles when making the restorations.
2.73.3. Controlling the atmosphere in which the porcelain is sintered (vacuum firing)
2.73.4. Eliminating the vehicles used to suspend the particles.
¶ 2.74. Various Methods of Condensation
2.74.1. Condensation is the process of removing water and air from the powder-water mixture as it is applied to a matrix or frame. This is desirable because water and air can produce voids in the fired porcelain. Various methods such as vibration, capillary action, pressure-packing, and whipping are used:
2.74.1.1. Vibration is applied by serrating or tapping with an instrum ent. It will eliminate large air bubbles or spaces, but is hard to control and may unintentionally create minute cracks in the buildup.
2.74.1.2. Capillary action occurs when you blot, usually from the lingual surface. In this way, the flow of moisture from the facial to the lingual draws the particles close together.
2.74.1.3. Pressure packing occurs when you smooth with a spatula or press with a clean tissue.
2.74.1.4. Whipping or brushing the surface with a large soft brush fills in surface voids and removes loose particles.
2.74.2. The net effect of these four methods increas es the amount of surface tension within the porcelain buildup (Figure 2.5)
Surface tension is the actual driving force that tightly binds the mass together. Therefore, never allow the porcelain mass to dry out during application. If the porcelain is dry, it cannot be condensed. Also, it is difficult to rewet the buildup once it has been allowed to dry out.
Figure 2.5. Effect of Surface Tension on Condensing Porcelain.
text_image
WATER WITHDRAWN
ON PAPER TISSUE
AS WATER IS WITHDRAWN, PARTICLES PACK
MORE CLOSELY DUE TO SURFACE TENSION
¶ Section 2H — Separating Materials
¶ 2.75. Introduction
Two materials are brought together in many laboratory procedures, but they must be prevented from sticking to each other. Therefore, a separating medium is applied to the surface of one of the materials before the two are brought into contact.
¶ 2.76. Separating One Gypsum Product From Another
When one gypsum product is poured on another, the materials tend to stick or unite. Use the following separators to prevent union of gypsum products:
2.76.1. Commercial Separators. Commercially available separators are formulated to provide effective separation. However, what is equally important is that the film thicknesses they create are almost nonexistent. If these separators are not available, liquid soap or floor wax should be used.
2.76.2. Liquid Soap. Ordinary soap is an effective separator and is often used in flasking operations. Brush it on evenly to avoid creating foam or bubbles.
2.76.3. Liquid Floor Wax. Ordinary liquid floor wax is an effective separator for both plaster and stone. Apply it to a gypsum surface as a thin layer with a brush or a cotton pellet.
2.76.4. Petrolatum (Petroleum Jelly)
The film left by petroleum jelly is too thick. Petroleum jelly shouldn’t be used where maximum accuracy is essential (for example, cast mounting procedures, index fabrication, etc.). Dentists and technicians dedicated to accuracy as a work standard contend that petrolatum has no place in the dental laboratory. However, petrolatum is sometimes used in denture-flasking procedures. If petrolatum must be used as a separator, spread the material in the thinnest film possible.
¶ 2.77. Separating Gypsum Products From Acrylic Resins
Both plaster and dental stone are quite porous. They are not suitable surfaces to cure acrylic resins against. A substance is needed to line the mold that seals off the pores. Tinfoil and alginate separating mediums are the best materials for this purpose.
2.77.1. Tinfoil.
2.77.1.1. Tin is a soft, white metal which is mined as an ore. When refined and processed into foil, it is extremely malleable. It is manufactured by rolling it into thin sheets for dental laboratory use.
2.77.1.2. A thickness of 0.001 inch is used to cover the stone cast in denture flasking. A thickness of 0.003 inch is recommended for covering the waxed-up denture. The foil should be cut into pieces of suitable size and shape and the pieces burnished to the wax or on the stone cast with a blunt instrument and cotton roll.
2.77.1.3. A thin layer of petrolatum applied to the foil helps hold it in place on the wax denture or stone cast as it is being burnished.
2.77.2. Alginate-Separating Mediums (Tinfoil Substitute)
2.77.2.1. Alginate-separating mediums are liquids consisting essentially of sodium or potassium alginate in distilled water. Glycerin and coloring matter are often added. Because they are affected by moisture, ensure surfaces coated with alginate separating medium are not brought into contact with water.
2.77.2.2. Use a soft brush to paint the liquid in one or two layers on the cast and in the mold. Ensure the first coat is dry before applying the next one. Once applied, the film is quite fragile and easily scuffed. If a part of the film lifts off the stone, remove the entire film and paint the stone again. Pack the resin in the mold within an hour after applying the alginate because the film tends to deteriorate if allowed to stand for a longer period.
2.77.2.3. Care must be exercised when a tinfoil substitute is used. If gypsum particles get into the bottle of liquid, it is ruined as a separator. Do not work directly from a bulk bottle; instead, pour what is needed into a smaller container. Traces of tinfoil substitute allowed to remain on the necks of plastic teeth prevent bonding of the teeth to the resin of a denture base.
¶ 2.78. Lubricant Separators
When waxing a pattern on a die, use a separator to prevent the wax from sticking to the die material. A lubricant must neither block out the fine details on the die nor affect the physical properties of the wax. The lubricants most frequently used are commercial preparations or substances like glycerol or mineral oil. If possible, use commercial separators to meet the high demands for accuracy in waxing and casting patterns.
¶ 2.79. Miscellaneous Separating Materials
2.79.1. Talc (Talcum)
Talc is a very fine, powdered soapstone. When sprinkled onto a cast and rubbed into its pores, talc is a very effective separator against heated shellac baseplate material.
2.79.2. Plastic Sheets (Film)
Place plastic sheets between the halves of the flask so the resin does not stick to the bottom of the flask during trail packing.
¶ Section 2I — Fluxes and Antifluxes
¶ 2.80. Fluxes
Fluxes are substances that are applied to a metal to prevent the formation of an oxide film or to remove an already formed oxide film. Borax (sodium tetraborate), combined with charcoal and silica, are the principal constituents of most borax fluxes. Except for containing fluoride salts, fluoride fluxes are similar in composition to borax fluxes. Fluorides dissolve chromium oxide and are excellent fluxes for soldering base metal alloys. Fluxes are used in the dental laboratory in three different forms; paste, powder, and liquid:
2.80.1. Paste Flux. Paste flux is powdered flux that has been combined with petrolatum. In this form, it is especially useful in soldering procedures. You must cut the solder into small pieces and dip them in the paste before you place them on the joint. If required, additional flux can be placed in the joint with a small instrument.
2.80.2. Powdered Flux. Powdered flux is the flux best suited for casting procedures. Keep it in a container with a perforated lid, such as a salt shaker, and apply it to the metal in the casting crucible as needed.
2.80.3. Liquid Flux. To form a liquid flux, mix powdered flux with either water or alcohol. This form is particularly suited for soldering with an electric soldering unit. Dip the solder in the liquid flux before placing it on the joint. When soldering, more liquid may be added by picking up a few drops between the beaks of the soldering tweezers and placing the drops on the joint as needed.
¶ 2.81. Antifluxes
Some soldering operations require the solder to be confined to a very definite area. An example of this type of soldering is in building up the contact on the proximal surface of an onlay. It could be disastrous if the solder is permitted to flow onto the margin. Substances used for this purpose are called antifluxes. Ordinary pencil lead (graphite or carbon) is a good antiflux. Another antiflux can be made with chloroform or alcohol and rouge. Shave a small amount of rouge into a dappen dish and then add enough solvent to make a thin paint. Apply the paint to the metal in the desired area with a camel’s hair brush.
¶ Section 2J — Alcohols
¶ 2.82. Types of Alcohol
Alcohol is used in the dental laboratory as a solvent and as a fuel for the alcohol lamp and hand torch. There are several types of alcohol. Some are suitable for laboratory use and some are not. The physical characteristics of the more commonly used alcohols are indicated in paragraphs 2.83 through 2.86.
¶ 2.83. Grain Alcohol (Ethyl Alcohol, Ethanol)
This is a colorless liquid with a highly distinctive odor. It burns with a bluish flame and is a very satisfactory fuel for an alcohol torch.
¶ 2.84. Wood Alcohol (Methyl Alcohol, Methanol)
This is a colorless liquid with a pleasant odor. It is made either synthetically from carbon monoxide and hydrogen or by the distillation of wood. It is highly poisonous and burns in an alcohol lamp with a reddish-yellow, flickering flame. When air is applied from the bellows in the hand torch, the flame becomes slightly purple. The yellow flame has the
advantage of being easy to see in ordinary daylight. Although the flame is not as hot as the one produced by ethyl alcohol, it is satisfactory as a fuel for either the torch or lamp.
¶ 2.85. Denatured Alcohol
Denatured alcohol is a mixture of ethyl alcohol and certain poisonous materials which are added to prevent its use as a beverage. Methyl alcohol, acetone, benzene, and ether are some of the common denaturing agents used. As a fuel in the alcohol torch, it may burn easily, poorly, or not at all, depending on the volume and type of denaturing agent used. Because of the uncertainty of its behavior, ethyl or methyl alcohol is a better fuel for the alcohol torch.
¶ 2.86. Isopropyl Alcohol
Isopropyl alcohol resembles ethyl alcohol very closely. It burns in an alcohol torch with a slightly more yellow and a somewhat more vigorous flame than either ethyl or methyl alcohol. Under pressure from the bellows of the hand torch, isopropyl alcohol produces a blue flame, but tends to smoke badly when applied to wax. For this reason, it is not recommended for most laboratory or clinical uses.
¶ Section 2K — Acids (Pickling Solutions)
¶ 2.87. Introduction
2.87.1. Acids are of interest to the laboratory techni cian because they are used in procedures to remove surface oxidation from the metal immediately after the castings have been recovered from the mold. When they are used for this purpose, acids are called pickling solutions.
2.87.2. Acids must be handled with great care since they produce blisters and burns on the skin, ruin clothing, and corrode equipment. Baking soda is the antidote for acid burns. It should be applied to the affected area immediately after contact. If an antidote is not available, the affected area must be flushed with a lot of water.
2.87.3. Generally, acids are not used full strength in the dental laboratory, but are diluted with water. When making up a pickling solution, always pour the acid into the water-- never pour the water into the acid. Failure to observe this rule may result in severe burns.
2.87.4. All pickling solutions except hydrofluoric acid should be kept in glass containers with glass stoppers and should be clearly labeled. One acid must never be mixed with another.
¶ 2.88. Hydrochloric Acid
2.88.1. Hydrochloric acid is a colorless, very corrosive acid. The fumes attack and corrode equipment, instruments, and fixtures. As a pickling solution, it should be diluted with an equal part of water. On rare occasions, hydrochloric acid may be used full strength to remove sulfur deposits caused by an overheated mold.
2.88.2. Hydrochloric acid slowly dissolves both platinum and palladium. For this reason, gold alloys should never be allowed to remain for more than a few minutes in acid. United States Pharmacopeia (U.S.P.) hydrochloric acid is 37 percent strength by weight.
¶ 2.89. Muriatic Acid
Muriatic acid is another name for commercial hydrochloric acid. It may often contain impurities and may be slightly yellow. Like the U.S.P. grade, muriatic acid is supplied in a 37 percent solution by weight. It makes a very satisfactory pickling solution when diluted to half strength with water.
¶ 2.90. Sulfuric Acid
Sulfuric acid is a dense, oily liquid. It is an excellent pickling solution for gold in a solution of one part water to one part acid. It has an advantage over hydrochloric acid--it produces no objectionable fumes. It is a more effective pickling agent when it is warm.
¶ 2.91. Nitric Acid
2.91.1. Nitric acid is a colorless liquid that may turn brown if it is stored for a long period of time. The discoloration does not change the properties of the acid. It is seldom used in the laboratory because it dissolves gold alloys of high palladium content.
2.91.2. Nitric acid is sometimes used when a casting contaminated with copper deposits from an unclean pickling solution cannot be cleaned with either hydrochloric or sulfuric acid. The solution used for this purpose is one part nitric acid to two parts water.
2.91.3. Never leave gold in nitric acid for more than a few minutes.
¶ 2.92. Phosphoric Acid
This acid rapidly dissolves dental cement and is very useful for removing facings or tube teeth from metal. It does not attack any of the commonly used dental alloys.
¶ 2.93. Hydrofluoric Acid
This acid should not be used except for dissolving porcelain veneers off metals. The fumes are dangerous if inhaled, and it is difficult to neutralize when it comes in contact with the skin. Magnesium oxide ointment should be kept close by for burns if the acid is used. There are commercial hydrofluoric acid substitutes on the market that are much less hazardous (for example NO-SAN®, Triodent, Inc., Union NJ).
¶ 2.94. Aqua Regia
This is a concentrated solution made of three parts hydrochloric acid to one part nitric acid. Aqua regia dissolves both gold and platinum . It is infrequently used to etch the inner surface of a gold inlay or crown to control the fit of the casting.
¶ Section 2L — Wetting Agents
¶ 2.95. Overview
2.95.1. When water balls up on a wax surface, it is exhibiting a property called surface tension. If a wax pattern is invested, the surface tension of the water in the investment must be broken down in some manner. Otherwise, the casting will very likely have nodules on its surface because the investment has failed to adhere closely to the wax.
2.95.2. A wetting agent (debubblizer) is a liquid with a soapy feel used to lower the investment’s surface tension. When a wax pattern is properly prepared with a wetting agent, the investment flows evenly across the surface and into the small crevices of the wax. The resultant casting is free from nodules. In a similar manner, a wetting agent added to the water to flush out a denture mold lowers the surface tension of the solution and enables it to clean the mold more effectively.
¶ 2.96. Types of Wetting Agents
2.96.1. Commercial Brand Name Preparations. Commercially produced wetting agents should be used whenever possible. Their performance characteristics are very reliable. A technician has to have particular confidence in debubblizers for wax patterns.
2.96.2. Hydrogen Peroxide and Green Soap. An especially good wetting agent for inlay and crown wax patterns is a mixture of equal parts of hydrogen peroxide and green soap. The hydrogen peroxide, as it is received from the pharmacy, is diluted with an equal part of tincture of green soap. This solution can be stored in a dental cement bottle and used repeatedly. The sprued pattern can be placed on the crucible former and the crucible former inverted onto the mouth of the bottle so the pattern is immersed in the solution.
2.96.3. Household Detergent. Research done by dental investigators at the Bureau of Standards has established that ordinary household detergent is highly effective when it is added to the water used to clean denture molds. The mold should be thoroughly rinsed with clean, hot water after cleansing because detergent residue may contaminate the denture resin.
¶ Section 2M — Wax Solvents
¶ 2.97. Introduction
Even though a substance may be able to dissolve wax, it may not be used for that purpose in dental laboratory technology.
¶ 2.98. Wax-Dissolving Substances
2.98.1. Acetone. This is a colorless liquid with a particular odor. In the dental laboratory, it is used in making tacky liquid, which is used to hold partial denture patterns to the investment cast. To prepare tacky liquid, dissolve plastic forms in acetone, making a liquid just slightly more viscous than water. Apply this liquid with a small brush to the exact area of the cast to which the pattern is to be applied.
2.98.2. Commercial Wax Solvents. These are commercially available preparations formulated to dissolve wax. They are not toxic, flammable, or harmful to acrylic resin. They are very effective and much safer to use than many chemicals. Their use is strongly recommended.
¶ Section 2N — Abrasives (Polishing Agents)
¶ 2.99. Introduction
2.99.1. Abrasives are substances that wear away the surfaces of softer objects. The speed of their action depends on the relative hardness of the two materials.
2.99.2. Abrasives are made into powders by crushi ng and sifting them to produce the desired particle size. In dentistry, they are used as powders, cemented to the surface of paper and cloth in the form of discs, and bonded with binders to form grinding stones of various shapes.
2.99.3. Abrasive materials can be classified according to their hardness using a scale known as the Mohs scale. The Mohs scale is a comparative scale and a good indicator of the relative abrasive power of several materials used to smooth and polish in the dental laboratory
2.99.4. See Table 2.17 for a comparison of several commonly used dental abrasives with the Mohs number of those classified on the scale. This table also shows the relative hardness and uses of abrasives.
Table 2.17. Types of Abrasives (Hardness and Use).
| Item | Material | Mohs Number | Use | Manner |
|---|---|---|---|---|
| 1 | Chalk | Unknown, extremely fine | Used to give luster to gold alloys, amalgams, plastic base materials, and porcelain | Mixed with water, alcohol, or glycerin |
| 2 | Rouge | Unknown; very fine | Used to polish gold, gold alloys, chrome alloys, platinum alloys, and stainless steel alloys | Mixed with petrolatum or used dry in the cake form |
| 3 | Tin Oxide | Relatively fine, 4 | Used to polish teeth and metallic restorations | Mixed with water, alcohol, or glycerin |
| 4 | Cuttle | Relatively fine, 3.5 | Used to polish gold and amalgam | Mixed with water |
| 5 | Tripoli | Relatively fine, 5 | Used to polish gold, amalgam, acrylic, and chrome alloys | Mixed with petrolatum or used dry in the cake form |
| 6 | Garnet | 6.5 to 7 | Used to polish base metals and plastics | Used in dry disc and paper form |
| 7 | Pumice | 6 | Used to polish gold foil, amalgam, tooth enamel, denture base materials, and dental castings | Mixed with water |
| 8 | Zirconium Silicate | 7.5 | Used to polish tooth structures and synthetic restorative materials | Mixed with water |
| 9 | Quartz | 7 | Used to polish porcelain and gold alloys | Used in dry paper disc form |
| 10 | Aluminum Oxide | 9.25 | Used to polish all metal alloys, ceramic materials, and tooth enamel | Used dry or in rubber matrix wheels |
| 11 | Silicon Carbide | 9.5 | Used to polish gold, base metal, ceramic materials, and acrylics | Used dry or in rubber matrix wheels |
| 12 | Diamond | 10 | Used to polish ceramic materials and tooth enamel | Used in rotary instrument form |
¶ 2.100. Types of Abrasives (Polishing Agents)
2.100.1. Chalk. Chalk is a soft, nongritty form of calcium carbonate. A small quantity, made into a paste with water, is an effective high-shine compound for both gold alloys and acrylic resin.
2.100.2. Rouge. Rouge is a polishing agent used to impart a high luster to gold. It can be used to polish an acrylic resin denture, but it tends to collect in the crevices around the denture teeth. It is not recommended for acrylic resin. Rouge consists of finely ground particles of iron oxide incorporated in an inert binder. Mixed with alcohol or chloroform, it is a very good antiflux.
2.100.3. Cuttlefish. Cuttlefish is an abrasive used to coat discs used for finishing gold. It is finely ground cuttlebone that comes from the internal shell of the cuttlefish.
2.100.4. Emery. Emery is an impure form of aluminum oxide found in nature as corundum. It is cemented to the surface of heavy paper, and the paper is cut into discs which are used in the laboratory for smoothing and polishing. Emery is also used as the coating on the arbor bands used for trimming baseplates.
2.100.5. Tripoli. Tripoli material is obtained from a porous rock ground to a fine particle size and incorporated in a binder. It is supplied in stick form and is excellent for smoothing both metal and acrylic resin. Tripoli is applied with muslin buffing or brush wheels mounted on a polishing lathe.
2.100.6. Pumice.
2.100.6.1. Pumice is a form of sand or silica which is used in the form of a finely ground powder. Pumice is supplied by the manufacturer in several grades. Flour of pumice is extremely fine in particle size. Coarse, medium, and fine grits are routinely used for a wide variety of smoothing and polishing tasks.
2.100.6.2. Pumice is usually mixed with water to form a thick paste. The paste is then applied to the work as it is held against a rapidly revolving muslin buffing or brush wheel.
2.100.7. Quartz. Quartz is a crystallized form of silica. It is used in a variety of grits for many types of abrasives. A compact variety of quartz is made into whetstone (or Arkansas stone) which is used to sharpen dental cutting instruments. In the form of powdered glass, quartz is glued to cloth and paper and used as a sandpaper disc.
2.100.8. Garnet. Garnet is a crystalline mineral abrasive used to coat discs for smoothing and finishing operations.
2.100.9. Carborundum. Carborundum is a trade name for silicon carbide. Many of the stones, mounted points, and discs used in the laboratory are made with silicon carbide. It is composed of extremely hard, blue and black crystals that closely resemble diamonds in shape. These particles are pressed with a plastic binder to formstones and points, or they are cemented to the surface of heavy paper to make discs. Stones and discs are used for smoothing and polishing.
2.100.10. Diamond. Many times harder than silicon carbide, the diamond is the hardest abrasive known. Diamond particles are bonded together and used in dentistry to make mounted stones of various shapes. They are also cemented to the surface of metal to make discs, wheels, and points.
¶ Section 2O — Laboratory Gases
¶ 2.101. Introduction
Several kinds of gases are used for heating and melting operations. Some, such as oxygen and acetylene, are stored in highly pressurized containers with safety caps over the outlets. When the containers are moved or handled, the caps must be securely attached. If the outlets are broken away from the tanks, the high pressures propel the tank like a high speed missile. Therefore, the tanks must be secured to prevent their movement while they are in use.
¶ 2.102. Types of Gases
2.102.1. Natural Gas (City Gas)
When organic matter decomposes, it forms natural gas which is found in the ground in regions that produce oil. Natural gas is used in Bunsen burners and blowpipes for operations requiring heat. When it is mixed with compressed air, natural gas produces a flame of approximately 2200 oF. This temperature is sufficient to melt most dental gold alloys, but not hot enough to melt chrome alloys. If an unusually large amount (more than 1 ounce) of gold must be melted, a hotter flame might be necessary (for example, natural gas-oxygen).
2.102.2. Acetylene. Acetylene is a colorless gas with a garlic-like odor. Manufactured by the action of water on calcium carbide, it is marketed in pressurized tanks that provide the torch operator the assurance of constant line pressure. Acetylene gas is used with specially constructed blowpipes for casting and soldering. When acetylene burns in air, it produces a flame of approximately 3000 oF. This is hotter than the temperature produced by a mixture of natural gas and air.
2.102.3. Propane. Propane occurs naturally in petroleum . Huge quantities of propane are produced when crude oil is refined. Propane stored in pressurized tanks assumes a liquid state. As the pressure is reduced, the propane converts to a gas. Propane and air produce a flame greater in temperature than the flame produced by natural gas and air, but less than the flame produced by acetylene and air. Propane is a cleaner fuel than acetylene, and its use is recommended over acetylene.
2.102.4. Oxygen. The oxygen used in the laboratory is a pure form of the oxygen that is found in the atmosphere. It is used to support the combustion of other gases. Oxygen comes in highly pressurized containers, and proper safety precautions must be taken. If a leak occurs when oxygen is stored in the laboratory, the concentrated oxygen intensifies the burning process. Therefore, the oxygen container must be stored outside the laboratory in a specially prepared, isolated area. When pressurized oxygen is mixed with natural gas or with acetylene, it produces a flame of a much higher temperature than the flame produced in combustion of those gases supported by pressurized air.
2.102.4.1. Natural Gas-Oxygen. Natural gas-oxygen flames are used to melt large volumes of conventional gold or to melt ceramometals. Metals suitable for porcelain fusion have higher casting temperatures than conventional golds.
2.102.4.2. Acetylene-Oxygen. Acetylene-compressed air or an oxyacetylene flame may be used to melt a chrome alloy. Most chrome alloys have melting ranges in excess of 2600 oF. An oxyacetylene flame reaches temperatures of approximately 6000 oF. Special torches are used to burn acetylene. Acetylene flames must not be used on ceramic alloy because the flames might change the alloy’s makeup which is precisely balanced for creating porcelain bonds.
2.102.4.3. Propane-Oxygen. Propane-oxygen can be used to melt large volumes of conventional golds, ceramometals, or chrome alloys. Propane is cleaner and less hazardous than acetylene. Due to the risk of carbonizing the alloy, propane is recommended over acetylene, even when melting a base metal alloy. A propane and oxygen mixture produces a flame temperature of approximately 5500 oF.
¶ Section 2P — Miscellaneous Laboratory Materials
¶ 2.103. Articulating Paper and Articulating Film
Articulating paper and articulating film are impregnated with a colored dye that is easily transferred upon contact. Both are used for marking interocclusal contacts when adjusting the occlusion of a fixed or removable prosthesis.
¶ 2.104. Modeling Clay
Modeling clay is pure kaolin (aluminum silicate) that has been mixed with glycerin to form moldable dough. In the laboratory, modeling clay is used to block out large tissue undercuts before a master cast is duplicated. It is also used to hold casts in position when they are mounted in an articulator. Because it shapes and molds easily, modeling clay is suitable for many other uses.
¶ 2.105. Cast Spray
Cast spray is used for coating the refractory cast to make a sealed surface to place wax or plastic patterns against. (Although the exact constituents are a trade secret, these sprays probably contain polystyrene plastic in a solution.)
¶ 2.106. Mouth Protectors (Mouth Guards)
Custom mouth protectors (mouth guards) are made from polyvinyl acetate-ethylene blanks and preforms. This thermoplastic resin is molded over a cast, using a vacuum-forming machine. Mouth protectors are worn during sports participation to reduce injuries to the oral tissues, head, and neck.
¶ 2.107. Plastic Patterns
Plastic patterns are plastic resin forms shaped as clasp arms, lingual bars, retention forms, etc. They are used to make patterns for cast RPDs. Because they are soft and pliable at room temperature, plastic patterns can be easily adapted to the designed outline on the refractory cast. They are made of ethyl and methyl methacrylate with added plasticizers. (The exact composition of the plastics is a trade secret.) They must be stored in a cool place to prevent deterioration.
¶ 2.108. Ceramic Fiber Paper and Ring Liner (Formerly Asbestos Strip)
2.108.1. Except under highly controlled conditions, the use of asbestos is being discontinued in many industries. Ceramic fiber paper is a commercially available substitute for conventional asbestos stripping. Terms such as Nobestos ®, Kaoliner ® (both commercial brand names), and “asbestos substitute” are used to describe this material. Because this material is used to line the investment ring in fixed denture and RPD investing procedures, the term ring liner is befitting.
2.108.2. Asbestos substitutes are stiffer materials and generally nonabsorbent, compared to asbestos. Therefore, their use in the dental laboratory is limited to investing procedures only. Other materials are now used to replace asbestos for blocking out undercuts, lining crucibles, and insulating acrylic parts during soldering.
¶ 2.109. Quick-Setting Epoxy and Cyanoacrylate Adhesives
2.109.1. The strengths of these products are so high and their film thickness is so low that they are gaining increasing acceptance in dental laboratory technology. Epoxy and cyanoacrylate glues are used to reunite gypsum cast fragments.
2.109.2. Epoxy glue contains enough body to be used as a blockout substance on fixed prosthesis dies. Painted on in a thin film, cyanoacrylate cement is an excellent die hardener and wax pattern spacer.
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