Behind the Scenes: Casting Bronze Sculptures

May 30, 2016


Have you ever been awestruck by a bronze sculpture of a prominent figure and wondered what the process was that created such an inspiring piece of art? After seeing Mike Leckie’s clay sculpture of Chief Joseph the Younger I began to wonder about the process that will soon transform the phlegmatic portrait of such an honored person from maquette to bronze portrait.

I was astounded to learn that the process that many sculptors and foundries use today is thousands of years old and has been used by various cultures throughout history. In fact, before the bronze age even began around 3500 BC, the Egyptians were using the method of lost wax casting to create sculptures out of tin and copper. This ancient method is referred to as “lost wax” because an original sculpture is formed from clay and/or wax and then used to create a mold that hot metal can be poured into, resulting in a bronze replica of the clay and wax form.

However, the process itself is a little more complicated than that. It begins with the sculptor creating the original sculpture out of clay, unless it is a smaller piece than the artist may use wax. This is a very important step in the process and the artist must be sure to include any details that they want to see in the final bronze piece.  Essentially, the wax sculpture is going to be an exact representation of the final product. Pictured to the right is a detailed shot of the clay sculpture of Chief Joseph the Younger. 

Once the clay piece is completely dry, the sculptor can begin to make the silicon mold by using a brush to carefully paint on layers of silicon or latex. This process will take a couple of days as each layer of silicon must dry before applying the next layer and a proper silicon mold requires 3-5 coats.

After the rubber is dry, several layers of plaster are applied to the silicon mold to create a firm stable mold. This will help when pouring the melted wax to create the wax replica. The plaster is applied in the same fashion as the silicon mold was, brushed on in several coats to create a thicker mold that will hold the wax in place. Once the plaster is completely dry it is carefully cut into sections and the silicon mold removed. Thus resulting in a negative silicon mold of the original clay sculpture. The rubber mold, once removed from the plaster, can be saved for later use in conjunction with the plaster mold to create replicas of the same sculpture. In the indirect method of lost-wax casting, the original master model is not lost in the casting process. Therefore, it is possible to recast sections to make a series of the same statue and to piece cast a large-scale statuary

Once the master silicon mold is removed, the plaster mold is secured together so that a wax replica mold can be crafted by pouring hot wax into the mold through an open hole. The wax is poured in several layers, with each layer being poured at cooler temperatures so that the wax forms to each detail of the mold and does not bubble or ripple. A completed wax mold should be approximately 1/4” thick, or just slightly thinner. When the wax cools and the mold is removed, a wax positive of the sculpture emerges. This, however, does not mean the wax mold is ready for the foundry. At this step it is very important that the sculptor or a professional wax chaser makes corrections and adds details to the wax mold.

The next step in the process is to create a series of vents for the molten bronze to travel. This ensures that every nook and cranny of the sculpture is filled with the hot metal and that no gaseous bubbles disrupt the sculptures intended form. The sprues, or gates, are made of wax rods and are attached to the piece by heating the wax rods and fusing them to the wax positive. A cup made of wax is also fashioned to the end of one of the wax gates in which the foundry will pour the molten bronze. Once all of the wax gates and cup are secured to the sculpture, the piece is dipped into a solvent that will remove any residual particles from the wax. Thus allowing the next step of creating a ceramic investment to capture each and every detail of the wax mold.  

The ceramic investment is a hard shell created to contain the molten bronze. This surrounds the wax piece and the gates and will later be broken open to reveal the bronze sculpture. The ceramic investment is created by dipping the wax into a number of slurries, each more coarse than the previous mixture. Seven to nine coats of the slurry are required to form a solid structure for the bronze to be poured into and each layer must be completely dry before being dipped into the next slurry mixture. Once the ceramic investment has enough layers to contain the molten bronze, it is placed into an autoclave, a high-pressure sealed oven, that melts the wax and leaves behind a detailed impression of the piece. This step also hardens the ceramic investment, ensuring that it will be able to withstand the heat and pressure of the liquid bronze. Pictured below is sculptor Mike Leckie's portrait of Chief Joseph, a prominent figure from the Pacific Northwest, encased in his ceramic shell at the Calcango Foundry in Boring, OR. 


The hardened ceramic investment is then buried in a pit of sand or wired into a support frame. Once there, the foundry workers carefully pour the molten bronze into the ceramic investment until it is full. There it will wait in the sand until cool enough to be broken out of the investment, revealing the bronze sculpture in its raw from.

This is where the bronze sculpture really begins to come to life. The sprues and vents are removed and grinders are used to smooth the areas where the vents used to be. Then the foundry workers use various precision tools to remove any flaws that may have occurred during the process. If the piece is a large sculpture that was cast in several parts, this is where it would be welded together by the experts at the foundry and any seams smoothed and finished.  After any imperfections are taken care of the foundry will sandblast the piece to ready it for the patina. The process of finding and applying a patina is a careful method of using different application methods, chemicals and pigments in order to enhance the color of the bronze. Each bronze sculpture of Chief Joseph the Younger will be hand painted by sculptor Mike Leckie, bringing out the highlights and personality of this remarkable portrait of an American Icon.

After learning about all of the steps that go into creating a finished piece of bronze art, it is no wonder that sculptors such as Leckie often form a close relationship with their foundry. Leckie has been employing the talent of Calcagno Studios for many years. We will keep you updated on the stoic portrait of Chief Joseph as he undergoes the transformation from clay sculpture to bronze portrait!