THE LOST WAX METHOD
This is where the wax sculpture is turned into a bronze. It is a lengthy highly skilled process involving the making of negative and positive versions of the animal in various different mediums - silicone, fibreglass, ceramic, secondary wax casting if it is a series of the same animal. This is the ancient ‘Lost wax method’.
MAKE A MOULD
A mould is made of the original wax sculpture. The rigid outer moulds contain the softer inner mould, which is the exact negative of the original model - often made of latex, polyeurathane rubber or silicone, which is supported by the outer mould, made from plaster, or fibreglass. Most moulds are made of at least two pieces, and a shim with keys is placed between the parts during construction so that the mould can be put back together accurately.
POUR WAX INTO THE MOULD
Once the mould is finished, molten wax is poured into it and swished around until an even coating, usually about 1⁄8 inch (3 mm) thick, covers the inner surface of the mould. This is repeated until the desired thickness is reached.
THE WAX COPY IS REMOVED FROM THE MOULD
This hollow wax copy of the original model is removed from the mould. This is how several versions of the originals can be made, and in my case, all the bronzes are limited to a series of 12.
CLEAN UP THE WAX COPY
Each hollow wax copy is then "chased": a heated metal tool is used to rub out the marks that show the parting line or flashing where the pieces of the mould came together. The wax is dressed to hide any imperfections. The wax now looks like the finished piece. Wax pieces that were moulded separately can now be heated and attached.
ADD WAX FLUTES TO THE WAX COPY
The wax copy is sprued with a treelike structure of wax that will eventually provide paths or flutes for the molten casting material to flow and for air to escape. The carefully planned spruing usually begins at the top with a wax "cup," which is attached by wax cylinders to various points on the wax copy. The spruing does not have to be hollow, as it will be melted out later in the process.
COAT THE WAX COPY WITH A HARD SHELL
A sprued wax copy is dipped into a slurry of silica, then into a dry crystalline silica of a specific grain size. The result of this combination is called ceramic shell mould material, although it is not literally made of ceramic. This shell is allowed to dry, and the process is repeated until at least a half-inch coating covers the entire piece. The bigger the piece, the thicker the shell needs to be.
MELT OUT THE WAX
The ceramic shell-coated piece is placed cup-down in a kiln, whose heat hardens the silica coatings into a shell, and the wax melts and runs out. The melted wax can be recovered and reused, although it is often simply burned up. Now all that remains of the original artwork is the negative space formerly occupied by the wax, inside the hardened ceramic shell. The feeder, vent tubes and cup are also now hollow.
TEST THE CERAMIC SHELL
The ceramic shell is allowed to cool, then is tested to see if water will flow freely through the feeder and vent tubes. Cracks or leaks can be patched with thick refractory paste. To test the thickness, holes can be drilled into the shell, then patched.
POUR IN THE MOLTEN BRONZE
The shell is reheated in the kiln to harden the patches and remove all traces of moisture, then placed cup-upwards into a tub filled with sand. Metal is melted in a crucible in a furnace, then poured carefully into the shell. The shell has to be hot because otherwise the temperature difference would shatter it. The filled shells are then allowed to cool.
REMOVE THE SHELL
The shell is hammered or sand-blasted away, releasing the rough casting. The sprues, which are also faithfully recreated in metal, are cut off, the material to be reused in another casting.
Just as the wax copies were chased, the casting is worked until the telltale signs of the casting process are removed, so that the casting now looks like the original model. Pits left by air bubbles in the casting and the stubs of the spruing are filed down and polished.
Now comes the process of applying a patina:
A wide range of chemicals, can give a variety of patinas. They are often used as surface embellishments either for color, texture, or both. Patination composition varies with the reacted elements and these will determine the color of the patina. For copper alloys, such as bronze, exposure to chlorides leads to green, while sulfur compounds (such as "liver of sulphur") tend to brown. The basic palette for patinas on copper alloys includes chemicals like ammonium sulphide (blue-black), liver of sulfur (brown-black), cupric nitrate (blue-green) and ferric nitrate (yellow-brown). With bronzes, patination is often deliberately accelerated by applying chemicals with heat. Colors range from matte sandstone yellow to deep blues, greens, whites, reds and various blacks.
For my bronzes, I have always liked a patina that shows age or antiquity - which is a very traditional ‘look’ but I feel is synonamous with bronze work. A well polished brown black is also very powerful as a ‘look’, but having said that, each should have it’s own unique treatment. They can also be given a patina to suit a client’s preference.