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An alloy of gold and copper is used to improve the durability of gold jewellery. Pure gold (24 carat) is too soft to stand up to the stresses of most jewelry and craftsmen discovered that mixing with other metals made it more durable. The most common additions to gold are silver and copper, which also add hardness, but many alloys include nickel, palladium, manganese and tin. The color of the finished product depends on the amount of silver and copper. An alloy with 75% gold and 16% silver yields yellow gold, while more copper shifts the color to a pinkish gold and still more copper yields reddish gold.
Gold-copper alloys tarnish on exposure to air owing to oxidation of the copper and blacken on heating from this cause. This tarnish can be removed and the colour restored to that of fine gold by plunging the alloy into dilute acids or alkaline solutions, an operation known as blanching. The oxidised coating may be removed without previous tarnish by immersing the alloy in hot, dilute nitrate of potash or alum, for example.
Gold-copper-silver alloys which have been refined to a purity level of at least 999.9 parts per thousand do not readily oxidize under most conditions and are suitable for making articles which are to remain unplated. This is an improvement over the prior art which tends to oxidize and discolor articles of 9 and 10 carat gold and which require subsequent plating for their satisfactory appearance.