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  • Writer's pictureDan Woodard

Bronze Casting

“I’ll Never Understand Time”

Many people ask me how a bronze sculpture is made. It’s actually a very labor intensive and complex process. In this posting I’ll give a brief explanation of the various steps involved in creating a sculpture in bronze. The method used today for most castings, the “lost wax” process, was originated in Egypt around 1,500 B.C. Many improvements have naturally been made since then, but the basic process has remained unchanged for over 35 centuries.

I’ll use my sculpture “I’ll Never Understand Time” as a basis for exploring the lost wax process. Like all sculptures this one began with an idea. In this case, I wanted to capture my befuddlement over the concept of aging. How was it possible that a cute baby could morph into an old man? I started with a baby picture of myself...

I then superimposed a photo of my current face over this photo...

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 next step in the process is to produce a mold of the clay sculpture. First the clay is coated with shellac and thin metal shims are inserted to delineate the different sections of the sculpture. In the photo below, the back of the sculpture has already received four coats of a urethane molding material. You can see the metal shims sticking out of the baby’s legs. For this piece, the entire back was one section of molding material. The front, however, required, three separate sections. I am now ready to apply molding material to the three front sections.

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.

The wax positive after removal of the mold

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.

Wax positive with gates and pouring cup added

Wax positive with gates and pouring cup added

You may notice that a piece has been removed from the forehead of the baby. This is necessary so air can flow freely through the piece when the ceramic shell is applied. I’ll explain this in more detail in the next step. The section that was removed from the forehead can be seen attached to the gate on the right side of the above photo. This piece will also be cast, and ultimately welded back into place after casting.

Gated wax positive with some of the shell added

It is now time to add the ceramic shell. The shell consists of two parts: a liquid slurry and a silica sand. First, the wax positive is dipped in the slurry and then it is coated with a fine textured silica sand. This is allowed to dry, and this is where the hole in the forehead comes in. Without the hole, the drying time would be very long and it would not be possible to inspect the inside of the piece to ensure that the slurry was dry. Once the first coat has dried, the piece is again dipped in the slurry this time, however, the piece is coated with a coarser silica sand. The first coat is finer so that all of the detail of the sculpture can be captured. The ensuing coats are coarser to provide strength. Again, the second slurry coat is allowed to dry. The process is repeated until the wax is covered with six coats of the slurry, sand combination.

When the piece is completely dry, it is placed into a “burn-out” furnace at 1,600 degrees. Most of the wax melts out of the shell. However, to ensure that no wax remains the piece is kept in the furnace for half an hour to completely burn out any wax that remains. In the photo below, the flames at the top of the furnace are from the burning wax.

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.

Video Clip of Bronze Pour

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.

Finally, the sculpture is sand-blasted, and a patina is applied. The last step is to heat the sculpture and apply a coat of wax to seal and protect the surface.

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.

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