The Lost Wax Casting Process - Cire perdue
Throughout the ages, bronze has been a medium that has captivated the imagination of peoples across all cultures and in all corners of the globe. The first historical evidence of bronze-casting work has been traced back to at least 5000 BC, in areas stretching from the Mediterranean to China.
Bronze is an alloy with a very high copper content and is revered as a sculpture-casting material because of its great versatility in this process. One of the great attributes of bronze is its particular patination qualities. The patina - or colour - of a bronze sculpture is the result of oxidation on the surface of the metal, which defines and enhances the visual impact of the artwork.
A flexible mould is a “negative image” taken of the original sculpture, on completion of the artwork. It is made from silicone rubber and has an outer casing of fibreglass and resin. The rubber lining of the mould captures all of the surface detail and textures of the original sculpture, while the casing provides support for an accurate form for the overall piece. The mould is used, essentially, for reproducing a completely accurate replica of the original sculpture, using fluid, heated wax.
Heated wax is painted into the two halves of the flexible mould. As soon as the wax layer is thick enough (3mm) the two halves of the mould are joined, facing one another, and bolted together. The mould is then washed out with hot wax and left to cool down. The two halves of the wax, which have now been joined on the seam line, are then removed from the mould to reveal a wax replica of the original sculpture. This wax is worked off on the seam line and the gating system for the actual casting is designed and attached to the wax sculpture. The gating system or feeding system includes the pouring cup, feeders (tubes) and risers, used to funnel molten bronze into the ceramic mould and release the air inside. The gated wax is checked and used to create a ceramic mould, in which the bronze will be cast.
The ceramic mould is made of a highly refractive material and is built up around the gated wax in layers. The wax is submerged into liquid slurry that creates a layer over the inner and outer surfaces of the hollow wax piece. Before the slurry is dry, it is coated in a layer of fine refractive sand that clings to the wet surface. The piece is left to dry and the process repeated several times, using courser sand on the outer layers and leaving each layer to dry in turn. Once completed the wax inside the ceramic shell is removed by melting the wax with hot water. The wax is drained from the ceramic shell, and the hollow shell is fired in a kiln. (hence the term “lost wax process”).
The bronze casting process reaches its climax with the pouring of bronze into the prepared ceramic moulds. The moulds are pre-heated in a kiln and stacked with the pouring cups upward for casting. The bronze is cast at a temperature of 1200 degrees Celsius and must be cast carefully, to ensure a quality bronze cast. The metal solidifies quickly but is left to cool down over time, to avoid cracks and draw marks. Once the ceramic shell is removed, the bronze cast can be finished off.
Fettling denotes the process of finishing off a raw bronze casting. In fettling, the gating system attached to the bronze is removed using a grinder, and the metal is recycled for future castings. The different parts of the sculpture are welded together using an argon gas welder, and the welds are worked off with pneumatic die-grinders, hammers, chisels and files. The aim is to restore any defects that may have occurred during the casting process to produce a bronze copy of the original piece.
The final step in the casting process is the patination of the bronze sculpture piece. This oxidation process naturally occurs over time, but can be speeded up by means of applying the patina onto a hot metal surface. The combination possibilities and sheer multitude of colours in patination has transformed this particular part of the process into an art form of its own - and it takes years to master. The patina defines the piece and sets the tone of the sculpture. Once the patina has been generated, the sculpture is polished with wax and mounted on a suitable base, before being transported to a gallery or art dealer.
The final artwork is photographed and the size, edition number and any relevant information documented for future reference. After the final aproval by the artis the work is made available to the public.
Last updated on Aug 29 2014 at 22:20:27 by Andre Stead - Copyright 2012 - Andre Stead