In the beginning there is a model created by the artist, which can consist of different materials and is to be cast in bronze. Small sculptures are usually modeled from wax, plaster, clay or air-hardening material, depending on the desired surface texture. Larger and more complicated sculptures must be stabilized with a solid skeleton of wire and an “inner life” of fillers such as paper and Styrofoam. Once the model is complete, a silicone mold must then be made.
First, the entire sculpture is surrounded with one or more layers of silicone. Due to the viscous consistency of the silicone, the mass of the model surface conforms to the smallest detail. This allows the silicone mold, after it has solidified, to reproduce the model surface with pinpoint accuracy. After that, everything is covered with a thick coat of plaster. If the model has a very complicated structure, it may need to be carefully dissected beforehand so that several partial molds can be made. When the plaster jacket is folded apart, it reveals the model or model part.
The silicone negative mold is now dabbed with liquid wax and then filled with another layer of wax. No air bubbles should form during this process. After cooling, the two mold parts are rejoined and more wax is poured into the cavity. When the wax layer is approx. 4 – 5 mm thick, the remaining liquid wax is poured out. This leaves the wax model hollow on the inside. This hollow space is filled with liquid fireclay, which after cooling forms a solid core that gives the wax model the necessary stability and protects it from deformation. After everything has dried and solidified, the plaster-silicone shell is opened and the wax model is removed. If the model was initially separated into several parts to create partial molds, these parts must now be reassembled in the wax model.
After the wax model has been reworked once again, many pipes or casting channels are made of wax around the model. These are necessary to allow the liquid metal to enter and the air and wax to escape during casting.
The wax model with all its casting channels is now hung or placed in an iron cylinder, which is filled with a liquid fireclay-gypsum mixture until the wax model has completely disappeared under the fireclay. The fireclay must now dry through and the solid cylinder prevents the fireclay mantle from bursting apart in the process.
The fireclay mold with the wax model inside is now heated in the oven until the wax melts out, evaporates and burns. After this, everything in the fireclay cylinder that was previously wax is hollow, namely the model and the casting channels. This process gives the whole procedure the name “WAX MELTING OUT PROCESS”.
The molten and red-hot bronze (a metal alloy that is essentially about 90% copper and 10% tin), is poured into the hollow fireclay shell. Air escapes through the casting channels and the casting slowly cools. After the bronze solidifies, the fireclay mantle is broken and then the webs and casting channels are sawed off.
Now the chaser has the task of removing any beginnings of casting channels, flaws, air bubbles and other imperfections.
Different chemicals can be used to finish the bronze surface in different ways. The color scale of patina ranges from bronze tones to various shades of brown, green and black. The patina is burned in with the help of a Bunsen burner, but the methods and techniques are the well-kept secret of the foundry or the artist. Sometimes parts are polished and finally the sculpture may be waxed or given a protective finish.
When viewing a bronze sculpture, every art lover should consider what fine work and how many operations each piece requires until its completion, because only in this way can he realize the real value of a bronze sculpture.