The same trick that cast 6,500-year-old gold now makes dental crowns
Sculpt a figure in wax, bury it in a heat-proof shell, melt the wax away and pour metal into the empty space. Goldsmiths at Bulgaria's Varna Necropolis did it around 6,500 years ago. Dental labs, jewellers and some car makers still rely on essentially the same idea.
Lost-wax casting, also known as investment casting or cire perdue, copies an original sculpture in metal such as bronze, silver or gold, and handles fine detail well. The oldest known examples are gold knucklebones, beads and bracelets from Varna, dated to about 4550 to 4450 BC. A wheel-shaped copper amulet from Mehrgarh in Pakistan dates to around 4000 BCE, and the Indus Valley later produced the bronze dancing girl of Mohenjo-daro. The method spread widely and held its place in Europe until piece-moulding overtook it in the 18th century.
In a modern foundry, the indirect route starts with a model in wax or oil-based clay. A flexible rubber or silicone inner mould, braced by a rigid plaster or fibreglass jacket, captures its negative. Molten wax is sloshed inside to build a hollow shell, often in layers about 3 mm thick, then removed and cleaned up with a hot tool to erase seam lines. A tree of wax channels called sprues is attached so metal can flow in and air can escape.
Next the wax is dipped repeatedly in silica slurry and grit until it wears a coat at least half an inch thick. Fired cup-down in a kiln, the coating hardens while the wax runs out, leaving a precise void. Workers check the channels with water, patch any cracks, reheat the shell so the molten metal will not shatter it, and pour. Once cool, the shell is hammered or blasted off, the metal sprues are sawn away, and the surface is filed and polished.
The principle works with anything that burns, melts or vaporises. Some carmakers form engine blocks around polystyrene foam that the hot metal simply evaporates. Glass artists melt chunks into a plaster mould at 790 to 830 degrees Celsius, then anneal for three to five days. Dentists cast gold crowns and inlays this way from alloys of roughly 60 percent gold. Ancient Siberian riders even embedded fabric in the wax, leaving textile imprints on the backs of gold plaques now in the Hermitage.
Source: Lost-wax casting
