“I’ll Never Understand Time”
Using this composite photo as
a reference, I sculpted a figure made of plasticine clay. This is a clay with oil added so it will not
dry out. It is the preferred clay for
most sculptors working in bronze. The
resultant sculpture represents a positive image of what the final art will look
like. As you follow the progression,
you’ll notice that the art begins as a positive image, then a negative, then
another positive, another negative, then finally the bronze positive.
The positive clay sculpture
The back half of the mold has been completed
The photo below shows the completion of the front of the mold. The three bumps you see, one on the forehead and two on the torso, are registration points that are used to align this mold with the “mother mold.”
The front of the urethane mold is completed
The urethane mold is flexible and would not stand up to the steps that follow. Therefore, a “mother mold” made of plaster is added over the urethane mold. When reassembling the flexible mold with the mother mold, the registration points are used to assure a perfect alignment.
Plaster mother mold added
Unfortunately, I didn’t take photos of the following steps, but I believe they are fairly easy to grasp without the need for pictures. Once the mold is completed, the four sections of the mold are separated from the clay sculpture, the clay is removed, and the mold is cleaned to remove any remaining clay fragments. At the point the original clay sculpture is essentially destroyed. The interior of the mold is now a negative image of the original sculpture. The four sections of the mold are now reassembled. A special casting wax is melted and poured into the mold. It is swished around to create a thickness of about three eights of an inch. When the wax has cooled and solidified, the mold is again taken apart, revealing a positive wax copy of the original clay sculpture.
You’ll notice that there are “fins” along parts of the wax sculpture. These are called “flashing” and occur where two sections of the mold meet. The next step is to remove the flashing. In addition to the flashing, there are typically small areas that may have bubbles or other imperfections from the wax casting. These areas are also repaired in a process called “chasing.” Chasing continues until the wax casting exactly resembles the original.
Wax positive with flashing and imperfections repaired
The next step is to add a pouring cup and pouring gates to the wax positive. To explain the purpose of these, I’ll jump ahead a bit. The wax positive will eventually be covered in a heat-proof ceramic shell. The shell will be heated to a high temperature and the wax will be melted from the ceramic shell. This again will create a negative of the original sculpture. Molten bronze will be poured into this shell to create the final bronze piece. However, in order for the bronze to flow properly to all sections of the sculpture “gates” have to be added. The red sections you see in the following photo are red wax gates. These are all connected at the bottom of the photo to a “pouring cup.” Eventually, this will be turned upside down and the bronze will be poured into the pouring cup, flow along the lines of the gates and fill the sculpture.
The burn-out furnace in action
Now the fun begins. While the burn-out furnace is running, bronze ingots are placed into a crucible and heated to 2,100 degrees in a crucible furnace. When the bronze is at temperature, the shell is removed from the burn-out furnace and placed into a pouring rack with the pouring cup facing up. In the following video clip, you can see the crucible being removed from the crucible furnace, and the molten bronze being poured into three different shells. You may notice that the shell is now white instead of yellow. This color change occurs when the shell is subjected to the high heat of the burn-out furnace.
When the bronze has cooled, most of the ceramic shell is chipped away from the bronze casting.
Bronze sculpture with the shell removed
However, many small sections of ceramic shell still remain. These are removed with a sand blaster. You’ll notice that the gates and pouring cup are still attached to the final sculpture. The next step is to cut these from the sculpture.
Sculpture with gates still attached
It is now time to “chase,” or clean up, the sculpture. I’m sorry, but I don’t have any photos of this process. However, the chasing process is as follows. First, the section removed from the forehead is welded back into place. Any bubbles or other voids are also welded and filled in at this time. The welds are then ground down as are the remaining nibs left from the removal of the gates. All these surfaces are then worked with files, grinders, and an assortment of other tools so the texture blends in with the areas surrounding it. Any areas of flashing or other imperfections are also reworked so the surface identically matches the original clay sculpture.
The final sculpture with patina applied and sitting on a cement pillow
And that’s it. Many steps and a lot of work, but the end result is a sculpture that will last much longer than a mere lifetime. If you have any questions about the process, please contact me.