Creating A Bronze
The Lost Wax Casting Process can be simplified into 7 basic departments. Within each of those steps there are an unlimited number of micro steps and craftsmanship. It would take volumes to fully understand what goes in to manufacturing a high-quality bronze artwork. Here I will attempt to explain to clearly explain each step.
Department 1: Master Mold
After an artwork has been completed in clay, a master mold is created. This is done by first making a rubber mold over the sculpture, and then by creating a plaster mold over the rubber mold. This process requires the completion of approximately twelve stages. Each stage serves a specified function to ensure functionality of the mold later on. Between each stage, the rubber needs time to stiffen up.
These are the Rubber stages:
- Thin coat: to capture every detail of the original sculpture
- Intermediary 2nd coat: This coat is slightly thicker to fill in potential air pockets and begin to give strength to the mold.
- Registration strips. There are varying ways to do this process, but basically there are rubber parting seams installed into the rubber mold at strategic locations to allow the mold to open when in use later on, and also to allow the rubber mold to be released from the original sculpture.
- Mesh coat: A medium thick coat of rubber is applied to give tackiness to the surface for the purpose of adding a jersey type of fabric over the mold which makes the mold tear.
- Butter Coat: A very thick coat of rubber is applied to fill in gaps, add strength, and to secure the seams. Before the butter coat solidifies, using soapy water and a rubber glove, the exterior surface of the rubber mold is smoothed out.
- The finished plaster mold must be removed, so it must be predetermined how it will be sectioned off. Marking lines with a Sharpie over the surface of the rubber mold helps to remind the mold maker where to create parting lines.
- Wet plaster and some version of a mesh material are mixed in a bucket and then applied to a designated section of the rubber mold.
- After the plaster section is solidified, petroleum jelly is applied to the edges of the plaster mold to prevent that next section from adhering. The process of creating mold sections and applying petroleum jelly is repeated until the entire surface of the rubber has been covered.
- After all of the plaster has set up, holes are drilled through connecting the sections along the edges to accommodate bolts that will register the plaster sections back together after the plaster has been removed.
- Using parting tools, the plaster is removed from the rubber along the seams.
- Using sharp knives, the rubber mold is parted out and removed along the rubber seams.
- Lastly, the rubber mold is placed back into the plaster mold, closed back up and secured with bolts or large rubber bands.
Department 2: Wax
The wax department in the foundry comprises wax pouring, wax chasing, and wax sprueing. These processes involve creating a wax pattern on the form of the original clay artwork, touching up any flaws in that wax pattern, and attaching wax bars, called sprues, to strategic points on and in the wax pattern to engineer a gating system for the bronze to eventually flow through to feed the bronze casting. Here is what is involved.
- Warm the rubber mold either with heat guns or heat cabinet.
- Removing the rubber from the heat source assemble the rubber into the plaster mold.
- With the rubber mold still very warm, hot wax is poured with an even flow into the interior. The hot mold improves surface quality of the wax by reducing bubbles and other defects.
- The wax is then poured back out of the mold gradually as the mold is also slowly tumbled to ensure thorough coverage.
- The wax is allowed to cool down thoroughly.
- With slightly cooler temperature wax is poured in and out and allowed to cool down two more times
- After the wax has cooled down completely, the plaster and rubber molds are peeled away to reveal the wax pattern.
- With the freshly poured wax pattern, a wax artist uses delicate wax scraping tools to touch up any flaws in the wax pattern, including seam lines, positive voids left from the rubber mold, and bubbles from wax splashes.
- Internal voids are filled in the wax pattern, and windows are strategically cut into the pattern to allow airflow for the ceramic mold department.
- Delicate parts of the sculpture may be cut away from the pattern and reattached later after they are cast in bronze.
- With a keen understanding of bronze casting techniques, a sprue system is engineered by configuring and attaching sprues. Attaching these sprues is done by cutting and melting the sprue ends to the wax pattern. Careful consideration is given to how the bronze is predicted to cool due to potential casting flaws caused by thermal contraction.
- A cone shaped wax is melted onto the bottom of the sprue system. This becomes the pattern used to make a ceramic pour spout for casting later on.
- A second wax sprue and casting engineer inspects the surface quality of the wax pattern and the efficiency of the sprue system for casting purposes.
- The casting engineer assigns a bronze casting temperature to the sprue system so that the casting crew knows what temperature the bronze needs to be when it is poured into the ceramic mold.
Department 3: Ceramic Mold
The wax pattern and sprue system are used to form the ceramic shell mold the super-heated bronze will be poured into. This is how it is made.
- The sprue system with the wax pattern is dipped into an etching solution to allow the ceramic slurry to adhere to the surface of the wax.
- The pattern is dipped into a liquid ceramic slurry and then allowed to drain off. Before drying the pattern is covered in very fine casting sand called Zircon.
- The pattern is set onto a rack and allowed to air dry for half a day or so.
- This process of dipping the pattern in ceramic slurry and then covering sand is repeated seven more times while increasing the coarseness of the sand, using a fused silica. This increases the strength of the ceramic mold being created with each layer.
- After about ½ inch of ceramic shell has been built up over several days, the final dip is done but no sand is applied.
Department 4: Wax Burn out/Bronze Casting
To have the ceramic shell ready to pour in the bronze, the wax pattern needs to be melted out. This is how it is done.
- After the ceramic mold fully dries, a large opening is cut into the base of the cone spout of the sprue system to expose the wax.
- The ceramic mold is then placed into a super-heated furnace at around 1500 degrees F., to flash melt the wax out.
- When the wax has completely melted out, the ceramic shell mold is removed from the furnace by a casting technician wearing a volcano suit and Kevlar gloves.
- The shell cools down and is inspected for strength and structural integrity.
- The ceramic shell is again heated to around 1800 degrees, this time in preparation to receive the bronze and to crystalize the ceramic structure for structural strength.
- While the shells are heating up, bronze ingots are being melted down within a crucible inside of a separate blast furnace.
- When both the shells and the bronze have reached optimal temperatures, the crucible is hoisted out from the blast furnace and placed into a pouring cradle.
- While casting workers lift the bronze into pouring position, another worker removes the super- heated shells from the burnout furnace and places them onto a pouring rack in order of hottest pouring temperature to coolest.
- The bronze is poured from the crucible into each ceramic shell to the top.
- After cooling on the rack for at least a half hour, the bronze filled shells are removed from the casting rack and brought to the chipping area.
Department 5: Metal Prep
After castings are removed from the racks, the ceramic shells must removed from the bronze and the metal needs to be prepped for metal chasing.
- Using large hammers and pneumatic chisels, the majority of the ceramic shell is chipped away from the bronze surface by a worker.
- The castings are then brought into a sandblasting room where the remaining ceramic residue is sandblasted away from the bronze.
- The casting are the brought to a station where, using a powerful plasma cutter, the remains of the, now bronze, sprue system is cut away from the casting leaving the bronze parts needed to reassemble the bronze sculpture.
Department 6:Metal Chase
The process of reassembling the bronze sculpture and touching up the bronze is metal chasing. The final result is that the bronze should look like an almost identical bronze version of the original clay sculpture.
- All the parts' connecting edges are beveled with an angle grinder or die-grinder. This is done to get the edges to fit right and to allow room for welds
- All parts are welded back together.
- All of the welds are ground flush and texture with grinding tools to match the original sculpted surface,
- The based of the sculpture is ground flat on a leveling table
- Stainless steel mounting nuts are welded to the bottom interior of the sculpture
- All casting defects are touched up
- The sculpture is sandblasted, and quality inspected
- Final touchups are down
- A final sandblast is done in preparation for patina
Department 7: Patina/ Basing
Patination of a bronze is the process of adding color to the surface of a bronze sculpture. There are too many different ways to accomplish the patination process, so I will refer you to our patina options page to learn about the varying choices for patina. The most commonly used techniques however are as follows:
- Darkening with some sort of chemical like potash or Birchwood Casey.
- Scrubbing the bronze surface to bring out highlights from the bronze
- Using a large blowtorch, heat the bronze surface until a liquid would evaporate instantaneously on the surface
- Using varying chemicals and pigments, add color to the bronze surface by using either a brush, airbrush, or spray bottle.
- After the bronze mostly cools down, the bronze is sealed with either a wax or a lacquer. This locks the patina colors into place.
- The bronze receives one more wax coat to protect the surface and tone down glare.
- A wood or marble base is drill to match the location of the mounting nuts under the bronze and a base is bolted to the bottom.
- Finally, a nameplate with the title of the sculpture and the artist's name is attached to the base.