Casting in Bronze
To begin the process of making a bronze sculpture, most sculptors choose to make an original out of clay. There are multitudes of different clays available for the sculptor. The three main categories of clay used for sculpture are water-based clay, oil-based clay, and self-hardening/low-fire clay. Choosing the proper clay for a particular project may simply be a matter of preference. Experience, however, is perhaps the best teacher when deciding what type of clay to use.
Most of the artists with Sculptureworks prefer to work with oil-based clay (also known as plasticene, plastilene, or plastilina). The main reason for working with oil-based clay is because it never dries or hardens. It can always be softened and reworked if a change needs to be made. The main concern with using oil-based clay is how to have the clay soft enough to build the sculpture rapidly and yet at the same time have the clay hard enough to produce good detail. The usual solution is to choose a clay that is hard enough for detail work and then just use heat to soften the clay for buildup.
Once the final touches have been made to the original clay sculpture, the final procedure before molding is touching up delicate details and smoothing the surface of the clay to perfection. This is quite important because any error or imperfection in the clay would be copied in the molding process and would appear in every subsequent stage. Thus, the smoother the clay, the less work will be needed on the wax replica and the finished bronze casting itself.
Once the desired smoothness has been achieved, the piece should be allowed to dry thoroughly before continuing to the molding process.
When the clay sculpture is completely smooth and finalized, a mold must be made of the original clay. Excellent surface replication of the original can be achieved with a polyurethane mold compound or a high-quality silicon rubber.
Once the first coat of rubber has been applied to the surface, it must be allowed to thoroughly dry (usually for 24 hours) before the next coat is applied. All of the details from the original clay are now picked up within the rubber material that has been painted on. A mold most generally consists of three to five coats of rubber (applied over the course of several days).
Once the final coat of rubber has dried, a firm outer "jacket" is made to help retain the shape of the more flexible rubber mold for pouring the wax replica. This “jacket” is usually made out of plaster, Hydrocal, resin, or epoxy, and is called the “mother mold”.
When all of this is complete and it is all dry, the outer "jacket" is removed, and the rubber is then cut away from the clay, on what is called a parting line (simply the division line for the mold). The sculpture has now gone from a positive form to a negative form. If the sculpture is large or complex, it will usually have to be divided into smaller pieces, with each piece needing its own individual mold.
The finished wax is a positive replica of the original clay. A network of wax rods, called sprues and gates, are next attached to the positive wax model. These sprues will serve as a type of channel system, which will feed the molten metal to all of the areas of the sculpture, as well as allow gases and air to escape. Also, a wax funnel (called a pouring cup) is attached to the gates for use during pouring.
The ceramic shell process requires a series of dipping the wax positive into a mixture called slurry to create a hard shell. This ceramic shell, once dry, becomes a hard, durable shell around the entire sculpture that is going to receive, hold, and shape the molten metal to produce the bronze figure.
The wax is first dipped into a solvent, which cleans any loose particles or debris from the surface of the wax. The shell process is about to begin. The clean wax is dipped into a solution called prewet, followed by two coats of a very fine grained slurry. This is known as the primary coating. This is where all of the fine detailing in the piece is picked up. It is almost like grained silicon flour.
The shell then progresses through the slurry process, into different slurry mixtures, which are various grades, gradually becoming courser with each coating. Once all of the coats have dried, the ceramic shell is then sent to be “dewaxed” and placed in a high pressure sealed oven, known as an autoclave. High temperatures (1500 to 1800 degrees Fahrenheit) and pressure force the wax from the shell and the wax melts out, thus becoming "lost". This is where the name “Lost-Wax Process” has been derived.
When the ceramic shell is empty of wax, it is then re-fired and made ready to have the molten bronze poured into it. All of the hot ceramic shells are taken to the pouring floor, where they are either placed in sand to stand by themselves, or they are wired to a support frame to be held in place.
The solid blocks of bronze, meanwhile, are heated to a temperature of approximately 2250 degrees Fahrenheit so that liquid bronze is created. The liquid bronze is stirred and prepared for the pour.
When the molten bronze is ready, the foundry workers very carefully lift the crucible, containing the liquid bronze, out of its heating furnace. The workers must wear protective face shields, clothing, gloves, and boots.
Moving quickly and very precisely, the foundry workers pour the liquid bronze into each awaiting ceramic shell. When the ceramic shells are all full with the poured bronze, they are then left to cool for several hours. The negative space within the ceramic shells have now become positive bronze castings.