Blue Gold

Like purple gold, blue gold can be achieved through the formation of intermetallic compounds and surface coloration.

Intermetallic compounds. The two colored gold intermetallics are gold-indium at 46 percent gold by weight, AuIn2, which has a clear blue color, and gold-gallium at 58.5 percent gold by weight, AuGa2, which has a bluish hue. The use of the blue intermetallic compounds in commercial jewelry appears to be limited.

Oxidation. Blue surface layers can be obtained by oxidation of karat golds containing special alloying additions. Jewelry with such blue color was launched in 1988 by VA Blue Gold SA of Switzerland, based on a U.S. patent by Ludwig Muller (USP 5,059,255). In this method, 18k to 23k gold containing iron and a small amount of nickel is oxidized at 840°F to 1,110°F (450°C to 600°C) for 10 to 12 minutes. For an 18 karat alloy containing 24.4 percent iron and 0.6 percent nickel, a blue-green color is obtained, while for a 20 karat alloy containing 14.4 percent iron and 0.6 percent nickel, a good blue color is achieved.

As research into colored gold continues, attention to workability and wear is essential. At present, intermetallic colors are in-trinsically brittle, and surface coatings by any technique are liable to be fragile. Pro-gress in these areas will result in special gold colors that are not only attractive, but also increasingly feasible for manufacturing.

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photo credit: Bracelet by Jarretiere.

Christopher W. Corti is managing director, international technology for the World Gold Council in London.