Bronze Casting - Lost Wax

This is Page 2 of the bronze casting and lost wax explanation. On Page 1, we looked at bronze metal itself, and how it is used. Now, in the second part, we shall examine the lost wax process, which results finally in a cast bronze statue.

Part 1 Part 2

Lost Wax Casting

The Mould
We must now allow ourselves to create duplicates of the original by making a mould. It may look a destructive process, particularly with larger sculptures that need to be cut up into sections to allow moulds to be made, however through this we are able to replicate the original sculpture, and create the number of pieces the artist requires to complete the whole work.

With some versions of the process, a separate mould is made for each casting. However, because it is more involved and therefore more interesting, we will look at the more complex method that involves making a master mould.

After applying a releasing agent to the sculpture, rubber is applied to the surface of one side, in several thin layers that eventually build up to a thick layer. This side is then covered in a resin jacket. The jacket keeps the mould in place and stops it from distorting. The sculpture is then turned over and the process repeated. 'Keys' are created to make sure that the two sides of the mould meet correctly and cannot be misaligned.

The Wax
Now using this mould we can create any number of wax copies of the sculpture. Molten wax is poured into the mould and swilled about so that the whole surface of the mould is covered in wax. The excess is then poured out. This process is repeated until the wax is considered thick enough to allow casting. The wax is allowed to cool and then the mould opened. A hollow wax copy of the original sculpture will appear. Skilled craftsmen will then touch up the wax, carefully removing any blemishes, filling holes, and removing excess wax while ensuring sharp lines and a perfect surface.

Often, at this stage, the artist will then take the time to inspect the wax before it is prepared for the next stage, and make any corrections necessary.

Wax sticks are added to points of the sculpture, often referred to as sprues, gates and risers, as well as a 'pouring cup'. This, in essence, is the system that allows the wax to escape first. When the molten metal is poured in, entering through the pouring cup, down the sprue and the gates, the air can escape through the risers.

The Shell
Most foundries now use a 'ceramic shell investment' for this stage. Using a metal rod that is attached to the pouring cup, the wax is dipped into a 'slurry', a silica mixture, which sticks to the wax. The wax is then dusted with 'stucco', a powder that will stick to the slurry. At first a very fine stucco is used to ensure all the fine details are not missed. Then as the layers are built up coarser stucco is used to give more strength. At each layer the piece must dry before we move on.

Casting
The process called "lost wax" casting derives its name from what happens next. The shell is heated whilst upside down so that the wax melts and pours out of the shell through the pouring cup. This then leaves the shell with a hollow that is an exact negative of the original sculpture. The shell is then placed in a specialist oven to bake. While we wait for this the furnace is fired up and the bronze ingots placed in the cauldron. The temperature will rise to around 1250ºC to allow the bronze to melt and be fluid enough to pour.

Once the shell is ready it is removed from the oven and half buried in sand to support it and stop it falling and breaking. Molten metal is taken from the cauldron and carefully poured into the shell. The metal will fill the space vacated by the wax earlier and will rapidly cool. Taking a hammer we can break off the shell and out will appear the sculpture in bronze.

Finishing The Sculpture

Chasing
The piece will now be cleaned with great care to remove all the remnants of the shell. The sprue and gates will be cut off, and, if we had to use several moulds, the individual pieces will be welded together. However we are not finished, as skilled 'chasers' will ensure that all the detail of the original piece is there, by reference to the original. Holes will be filled, imperfections 'chased' out; and they will also make sure the fine details are not lost, by using various small files and other implements.

The Patina
We now have our bronze sculpture, though we must apply the colour that we all associate with bronze. Be it the traditional verdigris, or dark brown, or another more exotic finish, chemicals are applied under a fierce heat. This heat allows the chemicals to adhere to the metal; most finishes sit on the metal as opposed to being absorbed by the metal. A final coat of wax is applied to give it a wonderful shine, and eventually you have your bronze in all its splendour.