Red Gold - Purple and Blue Golds

Purple and Blue Golds

Purple gold (also called amethyst gold and violet gold) is an alloy of gold and aluminium rich in gold-aluminium intermetallic (AuAl₂). Gold content in AuAl₂ is around 79% and can therefore be referred to as 18 karat gold. Purple gold is more brittle than other gold alloys, as it is an intermetallic compound instead of a malleable alloy, and a sharp blow may cause it to shatter. It is therefore usually machined and faceted to be used as a "gem" in conventional jewelry rather than by itself. At a lower content of gold, the material is composed of the intermetallic and an aluminium-rich solid solution phase. At a higher content of gold, the gold-richer intermetallic AuAl forms; the purple color is preserved to about 15% of aluminium. At 88% of gold the material is composed of AuAl and changes color. (The actual composition of AuAl₂ is closer to Al₁₁Au₆ as the sublattice is incompletely occupied.)

Blue gold is an alloy of gold and indium. It contains 46% gold (about 12 karat) and 54% indium, forming an intermetallic compound AuIn₂. While several sources remark this intermetallic to have "a clear blue color", in fact the effect is slight: AuIn2 has CIE LAB color coordinates of 79, -3.7, -4.2 which appears roughly as a greyish color. With gallium, gold forms an intermetallic AuGa₂ (58.5% Au, 14ct) which has slighter bluish hue. The melting point of AuIn₂ is 541 °C, for AuGa₂ it is 492 °C. AuIn₂ is less brittle than AuGa₂, which itself is less brittle than AuAl₂.

All the AuX₂ intermetallics have crystal structure of CaF₂ and therefore are brittle. Deviation from the stoichiometry results in loss of color. Slightly nonstoichiometric compositions are however used, to achieve a fine-grained two- or three-phase microstructure with reduced brittleness. A small amount of palladium, copper or silver can be added to achieve a less brittle microstructure.

The intermetallic compounds tend to have poor corrosion resistance. The less noble elements are leached to the environment, and a gold-rich surface layer is formed. Direct contact of blue and purple gold elements with skin should be avoided as exposure to sweat may result in metal leaching and discoloration of the metal surface.

A surface plating of blue gold on karat gold or sterling silver can be achieved by a gold plating of the surface, followed by indium plating, with layer thickness matching the 1:2 atomic ratio. A heat treatment then causes interdiffusion of the metals and formation of the required intermetallic compound.