Bronze Casting Process

This page provides a pictorial guide to the entire bronze casting process. Although it is accurate, it does not adequately show the amount of work and time each step in the process requires.

Step 1: The Original Artwork

Our artisans begin by studying the sculpture to determine how to section, or “lay up” the original sculpture to create a mold. During this crucial first step, photographs are taken at various angles, and measurements of the original are recorded to ensure the cast sculpture is faithful in every way to the original artist’s vision. Complex or large works require the original artwork to be sectioned into smaller, more manageable pieces. This is a critical step of the procedure to capture the detail of the original. If this step is not carefully executed, critical details of the original will be difficult to replace in the future steps.

Step 2: Molds

A silicone rubber mold ready for application of the shell

The sculpture is sealed and a release agent is applied that allows the rubber mold to be easily removed. All of the molds at Stevens Art Foundry are made of 100% silicon rubber.

After the original sculpture is prepared, the mold maker applies the first coat of rubber. Once the rubber has set, additional coats are applied as required. During this process, “keys” are placed in the rubber to ensure proper alignment of the mold. When the last coat of rubber is set, a plaster shell is applied to reinforce the rubber during the wax pouring process. After the plaster shell is complete, the shell is removed and the rubber is gently pulled back to reveal and remove the original sculpture. The mold is now ready to receive the wax.

Step 3: Wax Casting

Wax casting begins by separating the two halves of the mold, and the first coat of wax is applied inside to capture all the finer details of the artist’s masterwork. The mold is put together and additional wax is poured in. The mold is rotated until an even coating of wax is achieved. Once the wax has cooled, additional coats are applied until the wax achieves a minimum thickness of 1/8 of an inch. After the wax has cooled, both the “mother mold” and the rubber mold is removed revealing the wax casting, which is now an exact duplicate of the original piece.

Step 4: Wax Chasing

During this step, seam lines are removed and the wax pieces are fitted and tested for alignment. Then register marks are inserted, followed by the final wax inspection.

Step 5: Spruing and Gating

Spruing and gating describe the process of attaching wax rods to the sculpture. A large cup is attached at one end, and it is this cup that receives the molten bronze when it’s poured. The gate system allows the molten bronze to flow while also allowing gases to escape through smaller bars called vents.

Step 6: Ceramic Shell

The ceramic shell is actually a secondary mold. Although today the materials used to create the shell are modern, the shell provides the same function fire-clay did 5,000 years ago. Today the wax is dipped in special ceramic-shell slurry, to which a very fine silica sand is applied. Each coat is allowed to completely dry before applying another coat. The number of coats required is determined by the size and weight of the piece – heavier pieces require a thicker shell to support the metal as it’s poured. The slurry coats both the inside and outside of the wax allowing the sculpture to be hollow.

Step 7: Metal Casting

Pouring Molten Bronze into a Mold

Pouring Molten Bronze into a Mold

The ceramic shell is placed into a burn-out oven, cup side down. A burn-out oven is simply a large oven that bakes the shell to remove the wax. This is where the “lost wax method” gets its name. After the shells are de-waxed, they are cooled and inspected for cracking, and then they are heated up to 1400 degrees and placed in the pouring pit. Bronze ingots are heated from 1900 to 2100 degrees. The exact temperature that bronze is poured is determined by each individual casting. Nevertheless, the molten bronze is then poured into the hollow shells and allowed to cool and solidify.

 

 

 

Step 8: Devesting

Removing the ceramic shell

Devesting, or removing the ceramic shell from a cast bronze

After the shell has cooled, the shell material is now removed, and the unfinished bronze casting is revealed. The casting is then glass beaded, water, or sand blasted to remove any remaining material from the shell that obscure the intricate details of the casting.

 

 

 

 

 

Step 9: Welding

If the original was sectioned into pieces, it’s now be carefully fitted and reassembled. A thorough visual inspection is required to check for any casting inclusions. If there are any surface defects, they are repaired at this time. Every section is welded together with paying close attention to alignment.

 

Step 10: Metal Chasing

cleaning up a bronze prior to applying the patina

"Chasing," or cleaning and removing defects from a bronze casting

Metal chasing describes the process where all signs of welding or any other casting defect is removed to prepare the bronze for the patina (coloring process).

 

 

 

 

 

Step 11:  Patination

A patina refers to the finished color of a bronze sculpture, and traditionally the patina was applied by a master patineur . Depending on the artist’s preference, a variety of different chemicals are applied to the metal to achieve a certain look.  After the patina is applied the sculpture is sealed with a wax coat and/or lacquer to protect the finish.