9 Steps to Casting a Bronze Sculpture

I have often wondered how a bronze sculpture is made. Unlike Michelangelo's statue of David, which is made of marble, you cannot carve bronze. But you do carve or create your sculpture in other material and then cast it in bronze. This takes multiple steps, and you have to make a mold without ruining the original. Here are the basics of the process, in 9 simple to understand steps.

• A sculpture has to be made out of clay. This can be done, like David, by carving the piece out of the clay. Or it can be done, like pottery, where the clay built up and formed. Each sculptor creates work that is unique. The clay sculpture is sent to the foundry so that an exact replica can be made in bronze.
• The first thing they do at the foundry is make a mold of the outside of the sculpture. This is done by applying multiple layers of liquid rubber over the clay which creates a flexible mold. The rubber is then coated in liquid fiberglass, which will harden into a supportive shell.
• Once the fiberglass hardens, it is carefully removed from the sculpture, usually in two or four main pieces. The fiberglass mold is then put back together and turned upside down. So, you think, simple, just fill with molten bronze and you're done. But bronze is hot, so a mold out of a different material is needed. To do this, you start by filling the fiber glass mold with melted wax. This is called wax casting.
• The next step is gating. The fiberglass and rubber molds are gently removed from the wax mold, leaving an exact wax replica of the clay sculpture. Wax rods, called gates, are attached to the sculpture. These gates will create channels in the final mold, which allows for the molten bronze to be poured into the mold.
• The wax replica is then dipped into a vat of ceramic slurry. As the vat vibrates, there is a build up of ceramic that will be the final mold for the bronze. So, why don't we just dip the original clay sculpture in the ceramic slurry? One, because the original needs to be handled very carefully, you do not want it damaged. If something happens to the wax mold, you can go back to the original and start again. Second, the channels need to be created for pouring the bronze into the mold.
• The burnout step is removing the wax from the mold. Since you want the mold to have a bottom, you leave the bottom of the mold in tact and just expose the ends of the gate channels. When the ceramic mold is placed into a kiln to cure, the wax melts and runs out of the channels, this leaves the inside hollow, ready for the bronze.
• Bronze pellets are melted at 2200 degrees Fahrenheit. The molten bronze is poured into the ceramic mold.
• Once the bronze cools, the ceramic shell is broken and the gates are sawn off.
• The sculpture is then ready for the finish work. The gate stubs are removed along with any other casting marks. If desired, a patina is applied to color the bronze. A final transparent coat of wax is then applied to protect the finished sculpture.

For an original piece of art with a limited number of pieces, the process is repeated for each individual piece. The specific number of the piece, such as 5/25 (meaning the 5th piece made out of a limited edition total of 25), is often etched into the wax before it is dipped. That way the number will appear on the bottom of the finished bronze.

So the next time you see a bronze sculpture, you can appreciate it more because you know how much work went into creating that wonderful piece of art. It took a lot of time, patience and love.