An investigation was carried out by Mintek to examine the influence of a range of alloying additions (titanium, vanadium, chromium, molybdenum, cobalt, zirconium, boron, aluminium, silicon, zinc, platinum, palladium, rhodium, iridium and ruthenium) on the formability, hardenability and colour of 18 carat gold alloys. This paper provides chemical formulae for yellow 18 carat gold alloys that have hardnesses which are higher than the hardnesses of the corresponding conventional 18 carat alloys, in particular in excess of 300 HV, but with colours similar to conventional 18 carat gold. A preparation method for the alloys will also be described, which includes the steps of melting, annealing, quenching, cold working and age hardening.

The mechanical properties and processing of a newly developed ternary Au-Sb-Co alloy with 995 fineness are discussed. The high gold content of this alloy (99.5 per cent) facilitates marking as 24 carat gold (or Chuk Kam) in many countries. The alloy has a rich yellow colour, virtually identical to that of pure gold, and can be hardened to a peak hardness of 130 to 140 points on the Vickers hardness scale using a combination of cold work and a low temperature (250 to 300°C) ageing heat treatment. This hardness is comparable to that of cold-worked 22 carat yellow gold, and almost double the maximum hardness that can be obtained in pure gold. The 995 alloy is highly formable, and is amenable to casting, granulation, remelting, wire drawing, forming and rolling.