Investment casting is the formal name for the lost-wax casting process used throughout the fine jewellery industry. The term is industrial, with origins in mid-twentieth-century engineering casting, but the process itself is far older, traceable in jewellery practice to at least the third millennium BCE.

The process

The procedure has the same outline whether the workpiece is a single hand-carved engagement ring or an automotive turbine blade. A wax pattern is made, attached to a sprue, encased in a refractory plaster called investment, and allowed to set. The flask containing the hardened mould is heated in a burnout oven that drives off the wax and leaves a hollow cavity matching the pattern exactly. Molten metal is then poured or forced into the cavity, allowed to cool and solidify, and the investment is broken away to recover the casting.

Wax patterns

Wax patterns are produced by hand carving for one-off pieces, by injecting molten wax into rubber moulds for production runs, and by 3D-printed wax or castable resin for CAD-designed work. The choice of method depends on volume and design complexity. The accuracy of the finished casting is set by the accuracy of the wax pattern, since the cavity replicates the wax exactly within the limits of investment shrinkage and metal contraction.

Casting methods

Within investment casting, several pouring methods are used. Centrifugal casting spins the flask and uses centrifugal force to push metal into the cavity. Vacuum casting evacuates the flask and admits metal under vacuum. Static casting relies on gravity alone. Each method has trade-offs in terms of mould fill, surface finish and equipment cost, and most jewellery shops keep the means matched to the volume and metal types they use.