In jewellery manufacturing, investing is the step that surrounds the wax pattern with a refractory mould material in preparation for lost-wax casting. The wax pattern, attached to a sprue and centred in a steel flask, is encased in a slurry of investment plaster that hardens around it and forms the negative mould into which molten metal will eventually be poured. The word is part of a long-established craft vocabulary that traces directly to the Latin investire, to clothe, and the action it describes is exactly that.

The investment mixture

Investment for fine jewellery casting is a gypsum-bonded plaster combined with a silica refractory aggregate, typically cristobalite and quartz. The proportions are tightly controlled by the manufacturer to balance several competing requirements. The plaster must set hard enough to hold the wax pattern in place yet remain permeable enough to vent the gases produced when wax burns out. The silica must withstand temperatures of around 700 to 750 degrees Celsius without cracking, and its thermal expansion must match the metal contraction during cooling so that the casting comes out at the right size.

The investing procedure

The procedure begins by attaching the wax pattern to a wax sprue and locating it on a rubber base inside a steel flask. The investment powder is mixed with water in carefully measured proportions, vacuumed to remove air bubbles, and poured into the flask so that the wax is completely covered. The whole assembly is then vacuumed again on a vibrating table to draw any remaining bubbles to the surface. Bubbles trapped against the wax surface translate into raised pimples on the finished casting, so this stage is critical and is often repeated.

The bench-top investment mixer

For studio and bench production the slurry is mixed in a small vacuum mixer, sometimes called an investment mixer, which combines the powder and water under a vacuum to eliminate entrained air. Larger commercial casting houses use industrial mixers and automated dispensers to handle dozens of flasks per shift. The user is required to measure water-to-powder ratio precisely; commonly cited proportions are around 38 to 40 parts water to 100 parts powder by weight, but the exact ratio depends on the brand and grade.

From investment to casting

Once the investment has set, the flask is placed in a burnout oven on a programmed temperature ramp that drives off all wax and water. The wax burns out cleanly, leaving a hollow mould. The flask is then transferred hot to the casting machine, where molten metal is forced into the cavity by centrifugal or vacuum casting. After the metal has solidified, the flask is quenched in water, the investment shatters away, and the rough casting is recovered with sprue still attached. The whole sequence relies on the investment having been mixed and poured correctly; almost every casting fault visible on a finished piece traces back to one or more steps in the investing stage.