Purple Gold

Intermetallic compounds. There are three gold intermetallic phases known to have attractive colors. Of these, the most well known is the gold-aluminum compound AuAl2, which has an intense purple or violet color. Known as “purple gold” or “amethyst gold,” it occurs at 32.9 to 33.9 atomic percent gold, which is about 79 percent gold by weight and 21 percent aluminum by weight. Thus, it can be hallmarked as 18k gold.

If the alloy is made aluminum-rich (versus exact composition), the result is a two-phase alloy comprising dendrites of AuAl2 and aluminum solid solution, and the purple color is diluted towards that of aluminum. On the gold-rich side, the second phase is a gray intermetallic, AuAl. Some researchers have claimed that the purple color is preserved down to 15 percent of aluminum, but work by Leach & Garner in North Attleboro, Massachusetts, has shown that the purple color is quickly lost as the composition deviates from 79 percent gold, 21 percent aluminum by weight.

Solid pieces of the purple gold intermetallic can be made by vacuum melting gold and aluminum in the correct ratio and casting. As stated earlier, castings are brittle and cannot be worked in a traditional manner. However, Mintek in South Africa has shown cast purple gold that has been faceted into a pseudo-gemstone by machining (right).

Coating. Purple gold can also be applied to a substrate, such as a conventional karat gold, by thermal spraying of gold-aluminum alloy powders. The molten alloy particles adhere to the substrate surface to give a purple gold decorative coating.

Physical Vapor Deposition (PVD) techniques such as sputtering or evaporation of gold and aluminum can also be used to produce purple gold coatings. Jewelry with such purple gold coatings has been produced commercially.

Powder Metallurgy. Some techniques for making purple gold have been patented. In one Japanese patent (JP 62240729), additions of seven to 30 percent of cobalt, nickel, or palladium powders are made to gold-aluminum powders, which are pressed and sintered to give solid shapes. The patent-holders claim that this approach yields a purple gold material with satisfactory workability.

A similar world patent was granted to Singapore Polytechnic in 2000 (WO 00/46413), and purple gold jewelry made under this patent is marketed by Aspial Corp. in Singapore. In a new Japanese patent (2003183710), ornamental purple gold alloys comprising 70 to 85 percent gold (with the remainder being aluminum) are made from vacuum melted and atomized powder, which is pressed in a mold and sintered by electric discharge heating to produce solid material.

Thermal diffusion. Another approach to making purple gold material, which is defined in a Japanese patent, is through bundling aluminum-coated gold and gold-coated aluminum wires together and drawing them down to a composite wire (JP 4176829). The bundle is then subjected to a thermal diffusion treatment in a reducing atmosphere at 840°F to 1,290°F (450°C to 700°C). In this way, a fibrous wire of purple gold is produced, with some gold in a two-phase structure that is tough and flexible. Simply depositing an aluminum layer onto a gold surface and doing a diffusion treatment to produce a purple gold coating is also reportedly possible.

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photo credit: Machined purple gold intermetallic pseudo-gemstone by Mintek Co.