In the jewellery shop the word investment refers specifically to the refractory plaster used to encase a wax pattern in lost-wax casting. It is one of the most material-specific uses of the term in the trade and has nothing to do with financial investment.

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Composition

Investment for fine jewellery casting is a calcium sulphate (gypsum) plaster bound with a silica refractory aggregate, typically a mixture of cristobalite and quartz. The plaster gives green strength when the mould sets, and the silica provides high-temperature stability so that the mould survives burnout and the pour of molten metal at temperatures up to about 1,200 degrees Celsius for gold alloys.

Function

The investment forms the negative mould around a wax pattern. After mixing under vacuum to eliminate bubbles, it is poured into a steel flask containing the wax. Once set, the flask goes into a burnout oven where the wax is driven out and the cavity emerges. Molten metal is then poured into the resulting void by gravity, vacuum or centrifugal casting, depending on the workshop. After cooling, the investment is broken away to reveal the rough casting.

Phosphate-bonded variants

For platinum and other high-melting alloys, gypsum-bonded investment cannot tolerate the temperatures involved and is replaced by phosphate-bonded investment. The chemistry is different but the function is identical. Some specialist applications use silica-bonded or ceramic shell systems, particularly in industrial precision casting outside the jewellery trade.

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