Clay to Bronze - The Casting Process
Foundry bronze sculpture lasts indefinitely and is an investment, considered by many to be the premier and most authentic sculptural material.
Foundry bronze is the traditional method of bronze casting, using the ‘lost wax’ process – often employed by sculptors and virtually unchanged for thousands of years. The bronze is melted at extremely high temperatures and poured into a ceramic mould which is later destroyed to reveal the metal sculpture within.
The lost wax method of casting is a highly skilled, complex, time-consuming and labour intensive process. This, combined with the innate value of the bronze (made from copper and tin), gives rise to the comparatively high price of a foundry bronze sculpture. Maddy does not currently make cast sculptures in cold-cast 'bronze' resin.
Stage 1: Sculpting in clay, mould making
The sculpture is made in clay, often build around a steel / aluminium wire armature or frame, which is proportional or roughly equivalent to a skeleton. Once modelled, the sculpture is delivered to London Moulds & Casting in Shoreditch. There, they use silicone to create mould that picks up all the original details of the clay version. It is split into sections, and each section has its own plaster 'jacket'. The sections are bolted together.
Stage 2: Wax Casting and Chasing
The sections of plaster and silicone mould are separated and the original clay sculpture can be removed and often recycled. Maddy is hoping one day to try and fire clay that has emerged from a silicone mould into one unique ceramic edition of a sculpture. Once the clay is removed, the pieces are bolted back together and molten wax is poured in to give a thin, even coating of wax around the inside, usually 1 - 3cm thick. After the wax has cooled, the mould is separated and removed, leaving the wax cast from each section. The wax is then joined back together and seams blended out. This is the last chance to make any changes to this edition of the sculpture before casting in bronze.
The silicone mould is re-assembled and can be kept for future editions of the sculpture.
Stage 3: Runners and Risers
Sprues, also made of wax, are added to the sculpture, together with a large 'cup' at one end. This cup is there to receive the molten bronze pour. The sprues are positioned to allow the bronze to flow through these areas once the pour is made.
Stage 4: Ceramic Shell
Next, a second mould is made - taken around the wax sculpture. This is done by dipping the wax sculpture into a liquid binder solution called 'slurry'. The interior of the wax sculpture is also filled with a ceramic solution so that the final bronze cast is hollow. Next, a very fine silica sand is coated onto the slurry to create a ceramic shell around the wax that can withstand the temperature of the molten bronze.
Stage 5: Metal Casting
The ceramic shell is placed upside-down in a large oven. This melts the wax, which pours out through the runners, leaving just a thin negative space - formally occupied by the wax, and before that, the clay - between two layers of ceramic shell. This is again heated up, and placed into a pouring pit (often a large metal bucket of sand). Bronze ingots are heated to 1200 degrees Celsius and poured into the hollow space and allowed to cool and solidify.
Stage 6: Fettling
The ceramic shell is hammered away and destroyed, revealing the bronze cast underneath. It is then sand blasted to remove and remaining shell material from inside and outside the bronze.
Stage 7: Welding and Chasing
All sections of the bronze are aligned and welded together, and any defects removed to ensure that it looks the same as the original clay artwork. Finally it is chased to remove any seams or signs of welding.
Stage 8: Patination and Finishing
The desired colour for the sculpture is achieved by applying gradual layers of heat with a blowtorch and chemicals. Finally the sculpture is sealed with a wax coating and lacquer to protect the finish. All stages from 2 onwards are repeated for each edition of the sculpture, and each edition will be ever so slightly different from the last.
For information on commissioning bronze sculptures, pleased see 'Purchasing Original Artwork', particularly the page on bronze pet portrait sculptures.