The Casting Process for Foundry Bronze metal
Here is photographic guide of the process of making a foundry bronze metal sculpture, from the very start, to the very finish!
10a. The wax sculpture is cut into sections and wax runners for pouring the molten bronze are fixed to the sculpture
10b. The wax sculpture is cut into sections and wax runners for pouring the molten bronze are fixed to the sculpture
15. The cast is prepared for patination, the chemical colouring that gives it the tradional bronze look
The Casting Process for Bronze Resin (often called cold cast bronze)
The process called Bronze Resin casting, cold casting or bonded casting is similar up to stage 8 above – the moulding of the original sculpture with a silicone rubber mould. But because this casting process is ‘cold’ that first rubber mould can be used to make a resin casting, removing the need to create a second, single-use, heat-resistant ceramic mould, as in foundry bronze casting.
Bronze Resin casting provides the look and feel of a hot foundry pour or other materials for far less in cost and time. It is based around the principle of mixing a liquid bonding agent with a finely ground powder of bronze metal, creating extraordinary castings that mimic foundry bronze metal.
The liquid bonding agent, typically a polyurethane resin, is mixed with the bronze powder to create a heavy slurry of resin and bronze powder, which is poured into a mould, partially filling it. The mould is then slowly rotated, usually by hand, allowing the mix to evenly coat the interior of the mould. As it hardens, the aim is to form a bubble free exterior face coat on the cast.
That ‘wall’ can be thickened with additional pours, or a solid cast can be created using a filler material, like resin and stone dust (aiding longevity of the casting when exhibited outdoors). We use a solid pour, generally recycled marble dust, as this helps protect against cracking damage from expansion and contraction when a sculpture is exposed to the weather cycle outdoors.
The resin casting is hand finished, often using grinders or dremels, and the application of additional resin slurry, to conceal seam-lines and recover any of the artists mark-making lost in casting process. The bronze on the surface of the sculpture can then be activated with a brass brush or steel pad, prior to application of a protective layer of wax. The wax, similarly to a piece of outdoor furniture, will help protect the bronze colours against the weather, and can be reapplied periodically, usually following a good clean.
The bronze resin casting process is many times more economical and faster than a traditional cast bronze above, and it is not limited to bronze. Almost any finely ground material can be used to create a bonded casting. Typical applications have included marble or iron (for a white, or a rusty orange finish respectively.