One of the earliest metal technologies developed was the lost wax casting process, developed for copper and adapted for gold in the Middle East at least 4,000 years BC. The earliest known example in gold was made in Ur, Mesopotamia in 2,600 BC. Today, the lost wax (or investment casting, as it is also known) process is the most used process for the manufacture of gold jewellery. However, it has undergone considerable evolution, particularly since the mid-20th century, with the development of more sophisticated equipment and materials.

Investment casting tree

It is a complex process consisting of many steps:

a) A master model is made in a hard alloy such as nickel silver (a copper-nickel-zinc alloy) or often in silver.

b) A rubber mould is made by surrounding the master model with sheet rubber in a mould frame. Placing it in a heated press and vulcanising it. On cooling, it is cut with a scalpel into to halves, thus releasing the master model.

c) The rubber mould is used to make many copies of the master model in wax, by use of a wax injector which injects molten wax, often under a vacuum to remove air from the mould, into the mould cavity. On cooling, the wax is removed to give an exact copy of the original master model in wax.

d) The waxes are assembled into a 'tree' around a central feeder or sprue. This tree is cleaned of dust, and placed in a metal cylinder, known as a 'flask'. Special investment powder is mixed with water to a slurry and poured around the wax tree. It is placed under a low vacuum to remove air bubbles and allowed to set and harden to form the refractory mould.

e) The flask is then inverted and the wax removed by melting in steam or in air in a furnace (the burn-out oven. The investment mould is then carefully heated in the burn-out oven in set stages to the maximum burn-out temperature of 750°C for several hours and then cooled down to the temperature required for casting (typically in range 450 - 650°C).

f) The hot flask is placed in a casting machine. The gold metal or alloy is melted in a crucible and then cast into the investment mould. It is allowed to cool and solidify and is then quenched into water which helps to break off the investment mould material to leave the cast gold 'tree'. The castings are cut off from the tree, assembled into the jewellery pieces and polished.

A modern static vacuum assist casting machine

Defects in castings can be a major quality problem with investment casting, particularly porosity. This latter can be due to solidification shrinkage or to gas dissolved in the molten alloy. Many problems such as gas porosity, sandy surfaces and fins and watermarks can be attributed to poor investment mould practice and poor temperature control. There are two types of casting machine in use - the older technology centrifugal casting machine and the more modern static vacuum assist machines. The latter are preferred for quality and productivity reasons. Further details of investment casting and defect fromation can be found in many issues of Gold Technology magazine (e.g. Nos 28, 30 and 31) and the WGC publication Investment Casting Manual. Case studies of actual casting defects form the basis of the WGC publication, Handbook on Casting and other Defects in gold jewellery manufacture (see publications