Casting Gold Alloys in Dentistry: Classification, Composition, and Uses

Casting Gold Alloys

Classify casting gold alloys. Describe the composition, role of each constituent metal, and uses of casting gold alloys.
Answer:

Pure gold is a soft and ductile metal and so is not used for casting dental restorations and appliances.

Alloying gold with other metals not only improves its physical and mechanical properties but also reduces cost.

Classification of Casting Gold Alloy:

In 1932, the National Bureau of Standards surveyed and later by ADA specification no. 5 and the American National Standard Institute (ANSI) classified Gold Based Casting Alloys.

Depending on the Hardness:

• Type 1: Soft. VHN between 50–90 low strength.
• Type 2: Medium. VHN between 90–120 medium strength.
• Type 3: Hard. VHN between 120–150 high strength.
• Type 4: Extra hard. VHN between >150 extra high strength.

Type 1:  Soft or low-strength alloys are used for castings subjected to very slightly less stress-bearing areas,

For example, Inlays. The minimum yield strength is 80 MPa and VHN 50-90 minimum percent elongation 18%.

Type 2: Medium-strength alloy is used for casting subjected to moderate stress,  inlay, onlays, and full crowns.

This has a minimum yield strength of 180 MPa and varies from 90- 120 VHN and the minimum percent elongation is 10%.

Type 3:  Hard strength for castings subjected to high stress For example, Onlays, thin copings, pontics, crowns, and saddles.

The minimum yield strength is 270 MPa, minimum percent elongation is 5%.

Type 4:  Extra hard strength for castings subjected to very high stress.

• For example, Saddles, bars, clasps, thimbles, certain single units, and partial denture frameworks.
• Type 1 and  Type 2 are called Inlay alloys while types 3 and 4 are called as crown and bridge alloys.

Depending on their Nobility:

• Noble
• Base metal

Depending on the Functions:

• Crown and bridge alloys
• Base metal alloys
• Removable partial denture alloys.

Based on the Color of the Alloy:

• Yellow gold alloy: Those with more than 60% of gold and those with low gold or economy gold with 42 to 55%.
• Gold has a yellow color, for example,  Noble, high noble, predominantly base metals, and Japanese gold
• White gold alloy: Those with gold of more than 50% but palladium give white color.

Uses of Casting Gold Alloy:

• Inlays and Onlays
• Crowns and bridges
• Partial denture frames (only the type IV)
• Post and cores.

General Composition of Gold Alloy:

Composition of Gold Alloys based on Constituents:

Function Of Each Element:

• Gold
  • It provides tarnish and corrosion resistance and has a desirable appearance.
  • Provides ductility and malleability.
• Copper:
  • It is the principal hardener.
  • It reduces the melting point and density of gold.
  • In sufficient amounts, it provides a reddish color.
  • It helps to age harden gold alloys.
  • In greater amounts, it reduces resistance to tarnish and corrosion of gold alloy.
• Silver:
  • It whitens the alloy thus helping to contrast the reddish color of copper.
  • It increases strength and hardness.
  • If large amount it may reduce tarnish and corrosion resistance.
• Platinum:
  • It increases strength and corrosion resistance.
  • It increases melting point.
  • It has a whitening effect on the alloy.
  • It helps to reduce grain size.
• Palladium:
  • It hardens and whitens the alloy.
  • It provides resistance.
  • It reduces the cost of the alloy.
  • It also raises fusion temperature.
• Zinc:
  • It acts as a scavenger for oxygen.
  • Without zinc, the silver in the alloy causes absorption of oxygen during melting and later during solidification.
  • The oxygen is rejected producing gas porosities in casting.
  • Indium, Tin, and Iron
  • They all harden the alloy.
• Calcium:
  • It compensates for the decreased coefficient of thermal expansion that results when the alloy is made silver-free.
  • Iridium, Ruthenium, and Rhenium
  • They help to decrease the grain size.