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alloys whose main component is gold (Au).
The alloying of gold with other metals (alloying elements) has the purpose both of increasing strength and hardness and of economizing in gold. Binary gold alloys have been studied in the greatest detail. Gold-silver alloys are continuous solid solutions. At 20–40 percent silver these alloys are greenish-yellow; at 50 percent silver, pale yellow. All are soft and malleable, and their melting points increase with an increase in the gold content. Gold-copper alloys (which also form continuous solid solutions) are reddish-yellow. They are less malleable than pure gold but harder and more elastic. Annealing (below 425°–450°C) makes alloys of composition close to CuAu hard and brittle; quenching imparts softness and plasticity. Gold has been alloyed also with platinum and palladium. Gold yields intermetallic compounds (so-called aurides) with many metals. Small admixtures of antimony, bismuth, and lead make gold brittle. Gold-mercury alloys (amalgams) are used in gilding metal articles. Gold alloys are used to make coins, articles of jewelry, dental prostheses (gold-copper alloys), and electrical contacts in instruments with critical functions (gold alloys of platinum and silver). The gold content of gold alloys is expressed as purity.