I have just come back from Siena where I was sculpting a portrait in clay, which I am taking to the foundry, tomorrow to be cast in bronze. Here is a picture of the clay and the lady you sat for me, Dominique S-J.
To make a bronze, using the lost wax process, is quite a complicated practice. Firstly a mould has to be made from the original, in silicon from which you can cast a hollow wax copy. Next one works on and re-touches the wax. In the case of this sculpture I will be paying particular attention to fixing the neck of the piece, as you can see the armature has got in the way, but this can all be corrected at the wax stage.
Once the wax has been corrected and signed by the artist you hand it back to the foundry where the lost wax process can begin.
Each hollow wax copy is then "chased": a heated metal tool is used to rub out the marks that show the parting line where the pieces of the mold came together. The wax is dressed to hide any imperfections. The wax now looks like the finished piece. Wax pieces that were molded separately can be heated and attached; foundries often use registration marks to indicate exactly where they go.
The wax copy is then sprued or gated with a treelike structure of wax that will eventually provide paths for molten casting material to flow and 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. This spruing doesn't have to be hollow, as it will be melted out later in the process. A sprued wax copy is then dipped into a slurry of silica, then into a sand-like stucco, or dry crystalline silica of a controlled grain size. The slurry and grit combination is called ceramic shell mold 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. Only the inside of the cup is not coated, and the cup's flat top serves as the base upon which the piece stands during this process. 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 often it is 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 and vent tubes and cup are also hollow. The ceramic shell is allowed to cool, then is tested to see if water will flow through the feeder and vent tubes as necessary. Cracks or leaks can be patched with thick refractory paste. To test the thickness, holes can be drilled into the shell, then patched. 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. If the shell were not hot, the temperature difference would shatter it. The filled shells are allowed to cool. The shell is hammered or sand-blasted away, releasing the rough casting. The sprues, which are also faithfully recreated in metal, are cut off, to be re-used in another casting. Just as the wax copies were chased, the casting is worked until the telltale signs of the casting process are removed, and the casting now looks like the original model. Pits left by air bubbles in the casting, and the stubs of spruing are filed down and polished. Then to change the colour of the bronze, to your preferred patina the cast is heated with an open flame until scolding hot and various chemicals are applied, which react with the metal and change the colour accordingly.