How We Make A Bronze Statue:
At the Randolph Rose Collection, we cast bronze statues using the traditional “Lost Wax” casting method.
Bronze art casting using the ancient lost wax process has been around for thousands of years. This labor intensive process takes several weeks (sometimes months) from start to finish. Bronze is an alloy. It is a mixture of about 85% copper, plus small amounts of tin, zinc, and silicon.
We've broken down the casting process into some basic steps.
Important to remember: Copies of the original model are either positives – which are duplicates of the model that look just like the model itself, or negatives - molds of the model that fit the model (think of a glove on your hand).
- 1. The sculpted clay model is covered with silicone rubber. The rubber forms a mold of the original model and preserves all of the surface details of the original.
- 2. When the rubber hardens, it is covered with an outer layer of plaster. The plaster is rigid enough to support the rubber and keep the mold from changing shape when the wax is poured in.
· When the plaster hardens, the rubber and plaster are removed.The rubber mold is a "negative" version of the original.
· If we are going to make more than one copy of the finished statue, we can make additional molds at this point.
· The rubber mold allows us to make several copies of the wax pattern which we want to cast in bronze.
- 3. Artisans repeatedly pour melted wax into the rubber mold to build up layers of wax inside the mold.
· When the wax is hard, the mold is removed. This makes another "positive" like the original sculpture, except it's made out of wax instead of clay.
- 4. A crucial difference between the original model and the wax copy is that the wax version is hollow.
· The thickness depends on how many times wax was added to the rubber mold.
· Parts of the wax pattern may be cut off and cast separately in order for the ceramic mold to coat the inside of the wax pattern as well as the outside.
- 5. Wax sticks called "sprues" are attached to the wax pattern. Sprues make channels inside the ceramic mold to allow bronze to be poured into all parts of the mold.
- 6. The wax pattern is covered in another mold, called the "investment." This mold reproduces both the outside of the sculpture as well as filling the hollow interior of the wax pattern.
- 7. The wax pattern is dipped into ceramic slurry. This is repeated several times to make the investment strong enough to survive the casting process.
- 8. After drying, the ceramic is solid. The unfired ceramic and wax are put in a kiln and heated to melt the wax, which can be recovered and reused.
· The ceramic is fired at a higher temperature to harden it and burn out the remaining wax.
· The wax is now "lost" and the remaining ceramic is the second "negative" version of the original sculpture.
· It is now ready to be filled with bronze.
- 9. Next is the bronze casting.
· The ceramic mold is heated in a kiln.
· Bronze ingots are melted and poured into the mold.
- 10. After the bronze cools, the ceramic mold is broken away from the bronze casting, leaving the last "positive" version of the sculpture.
· The remains of the ceramic investment are sandblasted off.
· If the sculpture was cast in separate pieces, they are welded together.
- 11. Chemicals, acids and heat are applied to get the desired patina.
- 12. Finally, wax is applied to the bronze for a protective finish.