Gold alloys

Electronics. Owing to its relatively high electrical conductivity, low friction coefficient, and extremely high resistance to corrosion, gold metal is critically important in microelectrical circuitry, particularly in printed circuit boards, connectors, keyboard contactors, and miniaturized circuitry. 

Dentistry and medicine. Owing to its chemical inertness, its lack of toxicity, and its compatibility with all bodily fluids, gold is extensively used in dentistry in inlays, crowns, bridges, and orthodontic appliances. 

Mechanics. Because of its chemical stability, gold plated on surfaces exposed to corrosive fluids or vapors is in demand for use in corrosive atmospheres. 

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Electroforrning, which is used in the production of art objects or jewelry is a combination of electroless plating and electro deposition. A wax mold of the object to be produced is made conductive by electroless gold plating, a thick layer of gold or gold alloy is then electrodeposited and, finally, the wax is removed by melting (134). 

Sodium does not form alloys with aluminum but is used to modify the grain stmcture of aluminum—silicon alloys and aluminum—copper alloys for improved machinabiUty. Sodium—gold alloy is photoelectricaHy sensitive and may be used ia photoelectric cells. A sodium—2iac alloy, containing 2 wt % sodium and 98 wt % 2iac, is used to deoxidi2e other metals. 

The washed slime is dried and melted to produce slag and metal. The slag is usually purified by selective reduction and smelted to produce antimonial lead. The metal is treated ia the molten state by selective oxidation for the removal of arsenic, antimony, and some of the lead. It is then transferred to a cupel furnace, where the oxidation is continued until only the silver—gold alloy (dorn) remains. The bismuth-rich cupel slags are cmshed, mixed with a small amount of sulfur, and reduced with carbon to a copper matte and impure bismuth metal the latter is transferred to the bismuth refining plant. 

When freshly mixed, the carboxyHc acid groups convert to carboxjiates, which seems to signify chemical adhesion mainly via the calcium of the hydroxyapatite phase of tooth stmcture (32,34—39). The adhesion to dentin is reduced because there is less mineral available in this substrate, but bonding can be enhanced by the use of minerali2ing solutions (35—38). Polycarboxylate cement also adheres to stainless steel and clean alloys based on multivalent metals, but not to dental porcelain, resin-based materials, or gold alloys (28,40). It has been shown that basic calcium phosphate powders, eg, tetracalcium phosphate [1306-01-0], Ca4(P0 20, can be substituted for 2inc oxide to form strong, hydrolytically stable cements from aqueous solution of polyacids (41,42). 

The minimum total expansion must be 1.5% for casting of inlays and crowns and 1.3% for partial denture gold alloy casting (122). 

Gold and gold alloys serve the needs of dentistry better than any other metals or alloy systems. Gold alloys have a broad range of working characteristics and physical properties, coupled with excellent resistance to tarnish and corrosion ki the mouth. 

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