By Trevor Gregg
In my last post, I briefly mentioned that metal is typically cast into a mold. This week I’d like to talk in more detail about the process of casting metal.
There are different ways to make a mold. Many of them are made to pull apart into two or more pieces, giving the sculptor several advantages – multiple copies of the same sculpture can be made, a wider variety of objects can be molded, and bigger works can be cast in parts and reconstructed later. My first time casting bronze, however, I used a slightly different method of mold making called investment casting.
With investment casting, a single-use mold is created around a model of the sculpture, usually crafted in wax. The details of the sculpture need to be worked out completely, because any imperfections in the wax model will show in the metal sculpture! The process of chasing, or smoothing out the surface, is usually desirable. In the case of my model of the Hindu goddess Kali-Ma, I chose to limit the amount of chasing I did on the wax form and leave some definitive roughness to her. She’s an ancient figure associated with both creation and destruction, so this seemed like an appropriate choice!
After the wax model is completed, branches of wax called sprues are added. Sprues are necessary because they provide paths for the metal to flow into, and for air to escape from. When the model is sprued, it is dipped into a mixture of silica slurry, and then large grit sand. The sand is layered until a thick shell develops around the wax model.
Once the mold is created, the wax is burned out, and metal can be poured in. Then the mold is broken to remove the sculpture, the sprues are cut away, and it goes through another round of chasing before it’s finished.
This is a much more time consuming process than what I did to create my experimental sculptures! But it is a very rewarding process because of the amount of control that the artist has over the final outcome.