Casting in Jewellery Manufacture
Casting in Jewellery Manufacture
Lost-wax and related methods for forming metal components
Casting is the process by which molten metal is introduced into a shaped mould to produce a jewellery component or complete piece. It is the dominant production method in the contemporary gem and jewellery trade, enabling the efficient reproduction of complex, three-dimensional forms that would be prohibitively time-consuming to achieve by hand fabrication alone. The technique is applicable to gold, silver, platinum, palladium, and their alloys, and is routinely employed for rings, pendants, earrings, brooches, and the settings that hold faceted stones or cabochons.
Lost-Wax Casting
The most widely practised variant is lost-wax casting (also known by its French designation cire perdue), a method with origins traceable to ancient Mesopotamia and pre-Columbian South America. In its modern industrial form, the process begins with a master model — typically carved or machined in hard wax, though resin and metal masters are also used — from which a rubber mould is vulcanised. Liquid injection wax is then forced into the rubber mould under pressure to produce individual wax replicas. Multiple replicas are attached to a central wax sprue, forming a tree-like assembly.
This wax tree is placed in a steel flask and surrounded with investment, a fine-grained refractory plaster formulated to withstand thermal shock. Once the investment has set, the flask is placed in a burnout oven, typically ramped to temperatures between 700 °C and 750 °C, which vaporises the wax entirely and leaves a precise negative cavity. Molten metal is then introduced into the cavity, most commonly by centrifugal force or vacuum-assisted pressure, both of which drive the metal into fine detail and reduce porosity. After cooling, the investment is broken away, the individual castings are cut from the sprue, and each piece undergoes a sequence of finishing operations: filing, grinding, polishing, and, where required, rhodium plating or other surface treatments.
Direct and Computer-Aided Casting
Contemporary workshops increasingly employ computer-aided design (CAD) software to generate three-dimensional models, which are then output as physical wax or resin patterns via computer-aided manufacturing (CAM) milling or stereolithographic (SLA) 3D printing. These digitally produced patterns are invested and cast by the same lost-wax procedure, but with tolerances and reproducibility that manual wax carving cannot consistently achieve. The approach is particularly valuable for pavé and channel settings, where stone-seat geometry must be uniform across many identical pieces.
Metal Behaviour and Alloy Considerations
The choice of alloy materially affects casting behaviour. Yellow gold alloys in the 14-karat and 18-karat ranges flow readily and are forgiving of minor temperature variation, making them the workhorses of volume production. Platinum, by contrast, requires casting temperatures above 1,700 °C and demands specialised induction or argon-arc melting equipment; its higher density also necessitates adjusted sprue design to prevent shrinkage porosity. Sterling silver, though easy to cast, is prone to surface oxidation and firescale if flux management is inadequate. Palladium-white-gold alloys occupy an intermediate position, offering good castability with a lower specific gravity than platinum.
Porosity — the presence of microscopic voids within the cast metal — is the principal quality concern. It arises from gas entrapment, premature solidification, or insufficient metal feed, and manifests as pitting after polishing or as structural weakness at prong tips. Vacuum-assisted casting significantly reduces gas porosity; careful sprue placement and metal-to-investment temperature differentials address shrinkage porosity.
Casting Versus Hand Fabrication
While casting dominates volume production, hand fabrication — in which sheet, wire, and tube are cut, formed, soldered, and assembled — remains the method of choice for bespoke commissions, antique reproductions, and work by studio jewellers. Fabricated pieces are generally denser and more homogeneous in metal structure than cast equivalents, and they command a premium in the bespoke market accordingly. Many high-end houses combine both approaches: a cast shank may be fitted with a hand-fabricated collet or bezel for the principal stone.
Finishing and Stone Setting
A casting emerges from the flask in a rough state requiring considerable post-processing before it is suitable for stone setting. Sprues are cut away with a separating disc, excess metal is filed, and the surface is progressively refined through rubber-wheel abrasion and polishing compounds. Pre-set castings — those designed to receive stones in pre-formed seats — must be checked dimensionally before setting begins, as minor casting shrinkage (typically 1–2 per cent for gold alloys) can alter seat diameter relative to the wax model. Setters adjust seats with gravers or ball burrs to achieve the precise fit required for secure prong, bezel, or pavé settings.