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Questions and Answers
What defines a noble metal?
Which of the following is NOT considered a noble metal?
What is the primary influence of base metals in dental casting alloys?
What type of dental restoration involves the direct use of gold or amalgam?
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What is a key characteristic of an alloy?
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Which of the following is a method for producing metal restorations?
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What type of alloy is used for metal-ceramics restorations?
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What characterizes noble metals in terms of their use in dentistry?
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What distinguishes Type IV Extra-hard dental casting alloys?
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What is the classification of alloys that contain > 40 wt% Au and > 60 wt% noble metals?
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Which requirement is NOT essential for dental casting alloys?
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Which of the following is a characteristic of Type I Soft alloys?
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Which metal is commonly alloyed with gold to enhance its properties for dental use?
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What is the main purpose of the lost wax casting technique?
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What type of alloys contain less than 25 wt% noble metals?
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Which of the following requirements ensures that dental alloys can be worked with during fabrication?
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What is the primary function of copper in gold alloys?
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How does silver affect the color of gold alloys?
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What effect does platinum have on gold alloys?
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Which property indicates the ease of cutting and polishing gold alloys?
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What is the elongation percentage range for Type IV gold alloys?
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Which element is known to harden and whiten the gold alloy while being less expensive than platinum?
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What is the importance of the melting range of gold alloys?
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What does the modulus of elasticity of a material indicate?
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Study Notes
Metal Definition
- A solid material that is typically hard, shiny, malleable, fusible, and ductile with good electrical and thermal conductivity. Examples include iron, gold, silver, and aluminum.
Noble Metals
- Noble metals have been used for inlays, crowns, and fixed partial dentures because of their resistance to corrosion in the mouth.
- Gold, platinum, palladium, rhodium, ruthenium, iridium, osmium, and silver are the eight noble metals.
- Silver tarnishes in the oral cavity and is therefore not considered a noble metal.
Base Metals
- These are non-noble metals.
- They are important components of dental casting alloys because they influence physical properties, control the amount and type of oxidation, and strengthen the alloy.
- Examples include chromium, cobalt, nickel, iron, copper, and manganese.
Metal Restorations
- Metal restorations and prostheses are integral to dentistry.
- Metals are among the strongest materials and provide strength and durability to any structure.
- There are two ways to construct metal restorations:
- Direct: used in modern dentistry since the introduction of direct filling gold and amalgam in the 19th century.
- Indirect: introduced with the patenting of the centrifugal casting machine and the lost wax technique by William H. Taggart in 1907.
Alloy Definition
- A metal containing two or more elements, at least one of which is metal and all of which are mutually soluble in the molten state.
Noble Metal Content
- Greater noble metal content is associated with greater corrosion resistance.
Classification of Dental Casting Alloys
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According to Use:
- Alloys for all-metal and resin veneer restorations (e.g. inlays, posts, resin and composite veneered crowns, and fixed partial dentures).
- Alloys for metal-ceramics restorations (e.g. porcelain fused to metal crowns and fixed partial dentures).
- Alloys for removable dentures (e.g. removable partial denture frames and complete denture bases).
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Based on Yield Strength and Percent Elongation:
- Type I: Soft
- Type II: Medium
- Type III: Hard
- Type IV: Extra-hard
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According to Nobility (ADA 1984):
- High Noble Metal Alloys (HN): Contain > 40 wt% Au and > 60 wt% noble metals
- Noble Metal Alloys (N): Contains > 25 wt% of noble metals
- Base Metal Alloys: Contains < 25 wt% of noble metals
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According to Hardness:
- Type I Soft: Small inlays, Class III, and Class V cavities which are not subjected to great stress.
- Type II Medium: Abutments, pontics, and full crowns.
- Type III Hard: Inlays, crowns, and bridges. Situations where there may be great stresses involved. Usually can be age-hardened.
- Type IV Extra-Hard: Inlays subjected to very high stresses, partial denture frameworks, and long-span bridges. Can be age-hardened.
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According to Major Elements:
- Gold alloys
- Silver alloys
- Palladium alloys
- Nickel alloys
- Cobalt alloys
- Titanium alloys
Functions of Constituents (Gold Alloys)
- Gold: Provides tarnish and corrosion resistance with a desirable appearance. Provides ductility and malleability.
- Copper: Principal hardener. Reduces the melting point and density of gold. Gives the alloy a reddish color. Helps to age-harden gold alloys.
- Silver: Whitens the alloy and helps to counteract the reddish color of copper. Slightly increases strength and hardness.
- Platinum: Increases strength and corrosion resistance. Increases melting point and has a whitening effect on the alloy. Helps to reduce the grain size.
- Palladium: Similar to platinum in its effect. Hardens and whitens the alloy. Raises the fusion temperature and provides tarnish resistance. Less expensive than platinum.
Properties of Gold Alloys
- Color: Traditionally gold alloys were gold-colored. The color of modern gold alloys can vary from gold to white. This depends on the amount of whitening elements (silver, platinum, palladium, etc.) present in the alloy.
- Melting Range: Ranges between 920–960 °C. The melting range of an alloy is important as it indicates the type of investment required and the type of heating source needed to melt the alloy.
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Yield Strength:
- Type III — 207 MPa
- Type IV — 275 MPa
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Hardness: Indicates the ease with which these alloys can be cut, ground, or polished. Gold alloys are generally more user-friendly than base metal alloys, which are extremely hard.
- Type III — 121 MPa
- Type IV — 149 MPa
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Elongation: Indicates the ductility of the alloy. A reasonable amount is required, especially if the alloy is to be deformed during clinical use (e.g. clasp adjustment for removable partial dentures, margin adjustment, and burnishing of crowns and inlays). Type I alloys are easily furnished. Alloys with low elongation are very brittle.
- Type III — 30–40%
- Type IV — 30–35%
- Modulus of Elasticity: Indicates the stiffness/flexibility of the metal.
General Requirements of Casting Alloys
- Must not tarnish or corrode in the mouth.
- Must be sufficiently strong for the intended purpose.
- Must be biocompatible (nontoxic and nonallergenic).
- Must be easy to melt, cast, cut, and grind (easy to fabricate).
- Must flow well and duplicate fine details during casting.
- Must have minimal shrinkage on cooling after casting.
- Must be easy to solder.
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