Lost-wax casting

The method is simple in outline and exacting in practice. A jewellery workshop follows this order:

1. Pattern. A model of the piece is made in wax, or in a resin that burns out, either carved by hand or produced from a CAD model.
2. Sprue. Wax channels attach the pattern to a base. This sprue system holds the patterns until they are invested, lets the wax drain from the mould, then carries the metal to the pattern cavities.
3. Invest. The pattern sits in a flask and liquid investment is poured around it. A typical procedure vacuums the mixed investment for one to two minutes to draw out trapped air before it sets.
4. Burnout. The flask is heated gradually in a kiln so the wax melts and runs out, leaving a hollow in the shape of the pattern. The final stage is typically held at 1,350 °F (about 730 °C) until the investment is chalky white, and should never reach 1,500 °F (about 815 °C), where the gypsum binder starts to break down.
5. Cast. Molten metal is forced into the hollow, commonly by centrifugal force or vacuum.
6. Divest and finish. The investment is broken away, the sprues are cut off, and the surface is filed and polished.

The model is "lost" because the heat destroys it, and the investment mould is broken away to free the metal piece.

The method is very old. The oldest known examples, gold objects from the Varna Necropolis in Bulgaria, date to roughly 6,500 years ago (4550 to 4450 BC), though the dating is an estimate.

It has also been practised without a kiln or a vacuum pump. A goldsmith in Kumasi, Ghana, documented in the early 1970s, used a beeswax model coated in clay slip, a mould heated so the wax could be poured out, metal melted over a charcoal fire worked with a bellows, and the cooled clay cracked open. The sequence is the same as in a modern workshop; the equipment is not.

It does, because the material has to leave the mould cleanly. Wax melts and drains. Photopolymer resin, the material of many 3D printers, does not melt when heated. Resin begins to decompose at 300 to 400 °C (572 to 752 °F) and leaves a solid char residue, and under-cured resin can leave raised blemishes on the finished piece. Printers that jet a wax-based material can help avoid the casting problems many users have with photopolymer resins.

Hand-carved wax and milled wax avoid the resin issue, but milling cannot easily produce undercuts or hollow pieces. The choice is a trade between shape freedom and casting risk, which is why it is fair to ask which material your pattern is made from. The CAD jewellery design entry compares the two routes in more detail.

Every custom piece starts as a one-off pattern, so casting is the step that turns an approved model into metal. The cast piece is a copy of the pattern, which means size and dimensions are fixed at the pattern stage. Settle the ring size and band width in millimetres before casting, not after.

Casting quality depends on more than the model. Poor sprue design can cause incomplete filling and shrinkage porosity, so how a maker checks castings is a fair question to ask. On a finished piece, look for sprue marks that have been removed, a smooth surface, crisp detail that matches the approved render, and the quality marks that state the metal. Metal choice is separate; the karat entry explains gold purity.

To start a piece of your own, sketch it in the design studio, read about custom jewellery design, or look at finished work in a showroom.