Gold, the most noble of metals, seldom tarnishes or corrodes in the oral cavity. Because of the softness of pure gold, it is not indicated for use in the mouth except in the form of gold foil. Gold is frequently used in combination with other metals to produce alloys that can be used to fabricate various types of dental restorations where metal is indicated. The basic types of gold alloys used in dentistry are casting gold, gold solder, wrought gold, and gold plate. The principal metals used to combine with gold to form the alloys are silver, copper, platinum, palladium, and zinc.
Controlled items subject to strict accountability and safeguarding are gold solder, platinum foil, and gold, silver, and chromium alloys. In addition, scrap from these controlled items and from amalgam are also subject to the same controls. Certain delegated members of a dental facility control these items. However, all members of the dental care system must be aware of safeguarding precious metals, collecting precious/semiprecious scrap, and accounting for receipt, use, and turn-in of these items. If dental precious metal appliances, such as gold crowns and gold bridges, are retrieved from a patient, they are turned in as precious metal scrap subject to accounting controls.
FINENESS, CARAT, AND WEIGHING
The amount of gold in a gold alloy may be rated in terms of fineness or carat. Fineness is determined by the parts per thousand of pure gold contained in the alloy. In terms of fineness, pure gold is 1,000 fine, and an alloy with three-fourths pure gold is 750 fine. In the carat system of rating, the carat refers to the parts of gold determined by dividing the substance into 24 units and then counting the number of units of gold. Thus, a 24-carat substance would be pure gold and a 12-carat alloy would be one- half gold. In weighing precious metals like gold and platinum, the troy system of weight is used. In this system, the basic units of measurement of alloy quantity are grains, pennyweights, and ounces. Gold alloys are recorded and issued by the troy system as indicated in Table 2-1.
The conversion formula for carat to fineness is:
carat = fineness
Table 2-1. Troy system of weight.
ANNEALING AND TEMPERING
Through the use of controlled heat and rate of cooling, gold alloys can be annealed (softened) or tempered (hardened). Gold alloys are hardened by slow cooling. Rapid cooling from a high temperature will soften a gold alloy. Rapid cooling is done by quenching the heated gold alloy in tap water.
Gold foil is a restorative material used in the pure state. It is used most often on facial surfaces, proximal surfaces of anterior teeth, and occlusal surfaces of posterior teeth. Its chief disadvantages are color, high thermal conductivity, and difficulty in manipulation. Gold foil is available in either adhesive or non-adhesive form. To prevent pellets of adhesive foil from sticking together before use, their surfaces are treated with moisture or gas residues. When ready for use, the moisture and gas residues are vaporized by heating.
CASTING GOLD ALLOY
Restorations made with gold foil do not exhibit as much overall strength and resilience as do restorations made with gold alloys. Casting gold alloy is used in the fabrication of various types and classes of restorations. It is alloyed and made into ingots suitable for melting and casting into molds for the restorations.
Four Types of Casting Gold Alloys.
Usage. Casting gold alloys can be whitened (white gold) by adding palladium, platinum, or silver. Casting gold alloy is also used for crowns and abutments requiring great strength and hardness.
GOLD ALLOY SOLDER. Gold alloy solder is used for joining the parts of fixed partial dentures, for building up or forming restorations, and for gold repairs. Soldering is the process of joining metals by means of a solder or a lower fusing metal.
WROUGHT GOLD. Wrought gold is used for the construction of clasps and orthodontic appliances.
GOLD PLATE. Gold plate is used less often than casting gold alloy, gold alloy solder, or wrought gold. It is used in the fabrication of some types of crowns and often used in repair procedures.
NON-PRECIOUS ALLOYS. Non-precious alloys were developed as an alternative to the expensive precious metal alloys. They are used primarily in the fabrication of ceramometal restorations. These alloys are composed chiefly of chromium, nickel, and molybdenum.
|David L. Heiserman, Editor||
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Revised: June 06, 2015