Bronze Casting Procedure

 

A question we often hear is “How are bronze statues created?” The bronze sculpture process is called Lost Wax casting, and is an ancient method that dates back centuries. Today, artwork is cast at fine art casting foundries. We own and operate our own facility called Firebird Bronze Foundry in Troutdale, Oregon, and provide fine art casting services for many other artists from all over the world. While Firebird’s primary casting service is fine art sculpture in bronze, we also cast our work in precious metals. To learn more about our foundry visit www.firebirdbronze.com or call (503) 912-0400.

 

Firebird bronze foundry

 

To explain how our artwork is made, we have documented the creation process for our monumental sculpture They Bred Good Horses installed at the American Quarter Horse Association’s headquarters. To read more about the sculpture, its inspiration and story, read our Case Study.

Step 1: Inspiration for the Artwork

Every sculpture starts with an inspiration and design in our minds. For commissioned work, we will research the sculpture’s history, meaning and purpose to bring the vision for the project alive. We consider commissions to be a partnership for success, and we enter into each project with open minds and a desire to understand others’ concepts. As an artist team, we are unique in our process because we see the same vision together, and can work as one artist to approach projects. As our designs form, we create illustrated renderings that depict the composition, scale and gestures of the art piece. For commissioned projects, we will seek design approval at this stage with a rendering before moving forward in clay.

 

Concept Rendering

 

Step 2: Sculpting in Clay

We begin the creation process by working with models and reference to start the clay work. We will create a “bone structure” or armature, either made of wire or foam to support the clay. For this piece, we carved the entire monument out of foam based on real-life measurements, and layered clay over the foam. At that point, we began the arduous phase of sculpting details in clay.

In preparation for the casting process, this piece was cut into multiple sections that were individually molded and cast. The number of sections depends on the size and complexity of the piece. Each section was marked with notches to ensure proper alignment when rejoined in metal.

 

 

Step 3: Making the Mold

To create a mold for the sculpture, we applied liquid silicon rubber to each section of the clay piece. The rubber captured all of the sculpture’s details in a negative. When the silicon cured, we encased it in a plaster mother mold to hold the form in place. The molds we create are “walnut shell” concepts, so they are comprised of 2 halves separated by a seam. Molds are used to duplicate sculptures in a limited edition and will be destroyed once the full edition has been cast. For one-of-a-kind pieces such as They Bred Good Horses, the mold was only used once, and then retired.

 

 

Step 4: Creating a Wax Pattern

After we removed the original clay sculpture from the rubber mold, we heated wax to approximately 200 degrees Fahrenheit and poured it into the rubber mold, creating a thin coating. In creating wax patterns, the first “hot” layer fills in the fine details while subsequent layers of cooler wax (160-180 degrees) build up the form to be 1/8 to 3/16 inch in thickness. After we pulled the cooled wax patterns from the mold, we hand finished, or “chased,” the patterns to reveal our originally sculpted details and textures. Chasing means that we use sculpting tools and hot tools to fix any bubbles or seam lines in the wax.

 

 

Step 5: Creating the Gating System

After we chased the wax, we engineered a gating system to provide channels for molten bronze to travel. We used wax sprues, or branches, and attached them to the wax pattern. Whenever we design a gating system, we must create an efficient flow for liquid metal that also allows gases to escape. Each wax needs to have its own unique gating system.

 

 

Step 6: Creating a Ceramic Mold

Once we created the gated wax “tree,” we invested it in a ceramic shell. We dipped the tree in a vat of ceramic slurry, coating both the inside and outside of the hollow waxes. After dipping it in ceramic, we bathed it in silica sand. We repeated these steps 8-10 times, drying in between each layer, to encase the waxes in a thick ceramic shell mold.

 

 

Step 7: Burning out the Wax

Once the ceramic molds dried and cured, we placed them in our foundry’s burnout oven and melted the wax out at about 800 degrees, hence the term “Lost Wax.” By burning out the wax, we created a hollow cavity inside the shells where the 1/8” wax pattern and gating system once were. The hollow cavities and channels act as arteries to carry the molten bronze to each section within the shells.

 

 

Step 8: Pouring the Bronze

Three of our crew members are generally involved in the pour metal pour. The “lead pour” directs the hand-held crucible to the awaiting shells while the “deadman” is responsible for maintaining its balance. The third member keeps the surface of the molten bronze clear of any impurities or slag. Teamwork amongst the three is essential for the safety and success of the pour.

 

 

Step 9: Welding the Bronze

To reassemble the sculpture, we referred to photographs and measurements based on the original clay sculpture. We welded the seams using a TIG welder, and a rod of the same bronze alloy. Like the wax patter, we chased and cleaned the metal to remove any excess material and fill any pits. We sanded down the weld lines and used carabid-tipped grinders to add surface texture to the piece, blending it with the rest of the sculpture. We take pride in the fact that our seam lines cannot be detected, and we maintain the highest quality welds in strength and appearance.

 

 

Step 10: Applying the Patina

The process of applying coloration to bronze sculpture is called “patina.” It is a chemical reaction between the surface of the bronze and a variety of metal salts. We achieved this reaction by applying Ferric Nitrate, Liver of Sulfur and other chemicals, with various levels of heat. Each metal salt, usually a nitrate, reacts as a different color; different application techniques will yield various results, from a perfectly even coating, to an organically marbled look. To protect the patina, we apply wax while the bronze is still hot. As it melts, the wax seals the metal’s pores and, once cooled, several more layers are applied by hand and buffed to give the sculpture is final luster.

 

 

Step 11: Installing the Sculpture

Installing a sculpture as always an emotional experience, and a bronze will exist for generations into the future. They Bred Good Horses now sits proudly in front of the American Quarter Horse Association’s headquarters in Amarillo, TX and will forever tell the story of the Bell family’s love for one another and the American quarter horse.

 

 

To read more about the Lost Wax process, visit Wikipedia.