The Casting Process

Greenshield drawing for Chrissie RestingTom Greenshields created all his original sculptures in clay, always working with a life model, and using working sketches and drawings to refine and focus his ideas. Clay is a very sensitive and versatile medium, but needs to be kept moist to prevent cracking and distortion. When the original sculpture is finished a silicone rubber mould, usually with a fibreglass jacket, is made from the clay which usually results in the fragile original being destroyed.

This first mould is then used to cast a more robust and stable resin marble copy of the original which is further worked with hand tools by the artist to become the master copy.

The bolted fibreglass jacket ready to be opened
The bolted fibreglass jacket ready to be opened

A further silicone rubber mould, with fibreglass jacket, is then made from the master copy. This becomes the production mould from which all subsequent sculptures in an edition are cast. The process from clay to production mould is essentially the same for all casting mediums, although exact mould design ie. position of seams and type of rubber will differ for resin and lost wax casting.

Resin Bronze and Resin Copper Casting

Greenshield sculpture: EmmaTom Greenshields was one of the first artists to use the resin bronze process for the production of serious sculpture for the fine art market. The process produces very faithful copies of the original master sculpture and the surface can be patinated to produce a finish very similar to lost wax process bronzes.

The casting method is exactly the same for resin bronze and resin copper, the only difference being the metal powder used. Finely ground bronze (or copper) powder is mixed with a marine grade liquid polyester resin, the aim being to load the resin with as much metal as possible whilst still remaining pourable. The resin itself is just a carrier for the bronze powder and holds the metal in a rigid matrix when cured.

The bronze/resin mixture, with added catalyst, is carefully painted into each silicone rubber sculpture mould. The mould is then bolted together with its fibreglass jacket and more bronze/resin mixture added. The mould is turned by hand every few minutes, for up to two hours, to ensure a thick and even distribution of bronze within.

When the resin bronze layer has gelled ie. is no longer flowing the mould is filled with the core material which is a mixture of resin and fine stone/marble powder and pigment.

The jacket opened revealing the silicon mould and casting
The jacket opened revealing the silicone mould and casting

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The filled moulds take 4 hours to cure and produce heat in an exothermic reaction. Sculptures are de-moulded and transferred to the finishing workshop where the seams are fettled (worked down with hand tools) and the surface worked, patinated and polished with great attention to detail. All sculptures, whether limited edition or unlimited, are hand cast and finished in exactly the same way to a standard we believe is second to none.

'Birdbath' figure removed from silicon mould
'Birdbath' figure removed from silicone mould

Resin Marble Casting

Greenshield sculpture: Kneeling Mother Resin marble sculptures are cast in exactly the same way as resin bronze/copper pieces, but with the omission of metal powder. They are cast solid with a mixture of resin and ground stone/marble powder. The subsequent cast is again fettled by hand and then sand blasted to produce an even, durable and non-shiny surface.

Lost Wax Bronze Casting

Greenshield sculpture: Dancer StretchingThe techniques for lost wax bronze casting have changed little over the centuries. Bronze is an alloy of copper (86%), tin, zinc and lead.

Hot wax is painted and swilled into the production mould (see above) to create a wax copy of the original sculpture. For larger sculptures the wax is cut into sections for separate casting. Wax runners and risers are attached to the wax copy to allow for the flow of molten bronze and air during the pouring process.

The wax copy, with its runners and risers attached, is dipped into a liquid ceramic slurry several times to build up a ceramic shell. The core of the sculpture is filled solid with more ceramic material, effectively sandwiching the wax layer between the ceramic outer shell and the inner ceramic core.

The whole structure is then baked in an oven causing the wax to melt and be lost - 'lost wax' - leaving a gap which is now filled by pouring molten bronze at a temperature of 1,250 oC into the mould. The bronze hardens as it cools and the ceramic shell is beaten off to reveal the cast scupture.

The runners and risers are cut off and the bronze sculpture is fettled, patinated, polished and finished by hand. Initial patinas are not always stable and often need to be heated and cooled several times before they become stable to the varying temperatures and moisture in the environment.