Casting bronze with the lost wax method has been done for four thousand years. Though the techniques have improved or changed, the steps of the process have remained intact. Below is the journey of "Rachel Weeping" from clay to bronze. This sculpture was cast by Inferno Art Foundry.
The first step is the original sculpture.
This is usually sculpted from an oil based clay; however, I have also created bronze sculptures from fired stoneware clay originals. "Rachel Weeping" is an 8 ft figure that will be installed on a 2 ft base. The original sculpture is approved by the client through a studio visit or a video and photos.
Father John Bohn's approval visit to the studio.
The second step is creating the mold.
Carl Pelizza from Inferno Art Foundry outside of Atlanta, came to the studio to create the mold of the original sculpture. The mold is made in two parts: the rubber mold and the "mother" mold.
First the studio is prepped with plastic sheets and the left arm is removed by specific sleeves that were built into the armature of the sculpture. This was done to enable Carl to get to the face and create the mold of that area.
Carl starts by mapping out the sections he wants and separating them with plastic shims. Then he begins brushing on layers of rubber which cures over time. The initial coat is a thin "print coat" that catches all the details and surface texture of the orginal sculpture. (left)
Successive layers of rubber mold are thicker. (right)
After the rubber mold is completed, the "mother" mold is created. This is a hard shell mold that will support the flexible rubber mold during the wax casting process.
Once the mold is completed and cured. Carl removes the "mother mold" in sections and demolds the flexible rubber mold from the sculpture. (left) The rubber sections are placed within their "mother" mold, bolted together and made ready for transport to the foundry. (right)
At this point, the original sculpture has served its purpose and the oil-based clay is removed from the armature and is used for the next sculpture to be created.
The third step is the wax casting and chasing.
Once the molds are received at the foundry, they are cleaned of any clay residue. Melted wax is brushed and then poured into the molds. This creates a duplicate of the sculpture in wax sections. These sections are hollow, as bronze is cast hollow with a thickness of 1/8" -3/16". The following photos show the visit I took to the foundry to personally do a 'wax check' as the sculptor.
Each section of wax is looked over, discussing each piece with the wax department of the foundry. Below you can see bright red sections of wax square tubing. These are used to create the 'gates' and 'sprues'. Gates and sprues create pathways for the wax to melt/burn out, and for the molten bronze to be poured in, as well as for the air to escape as the molten bronze is cast into the ceramic shell.
If I find any areas of the wax that need reworking, I mark it so that the wax department can finish their 'wax chasing'. This process removes any imperfections, mold lines from the rubber, air bubbles, etc that appear in the cast wax sections. Also the wax sections are engineered and cut in the best possible way to cast each section in bronze. Notice above that the back of the head is now a seperate section.
The fourth step involves creating the ceramic mold/shell for each wax section.
Photo credit for the following steps: Carl Pelizza.
The wax sections are dipped in a ceramic slurry, and then coated by hand with silica sand, starting with the finest grit and then successively larger grits as each slurry layer is added. The sections are hung to dry between coats. The process is repeated until the desired thickness of the ceramic shell/mold is achieved; thus a shell inside and outside of each wax section is created.
The fifth step is the burn out, in which the wax is 'burned' or melted out of the ceramic shell sections: hence the term "lost wax" bronze casting. [A so called 'bronze' sculpture that is "cold cast" would refer to a resin/plastic material that has bronze powder mixed into it.]
The burn out kiln located outside the foundry will have the ceramic shell sections placed into the top kiln. The shells are heated to about 800 degrees. This process melts and burns out the wax from the shells and also hardens the shells prior to their being used for the molten bronze cast. Most of the wax is reclaimed as it melts out and can be used again. However some of the wax residue is burned out as the heat increases thereby acheiving a nice clean cavity in the ceramic shell.
The sixth step is the actual bronze cast or pour. This is the most exciting part of the entire bronze casting process.
Casting bronze with the lost wax method has been in use for four thousand years. Though the techniques have improved or changed, the steps of the process have remained intact. Bronze is an alloy of metals that is mostly copper in content. It is industry standard in the USA for sculptors and foundries to use silicon bronze (95.8% copper, 3.3% silicon, and 0.9% manganese.) Though 'bronze' sculptures can be purchased from other countries; China, India, etc. the copper content is only 60-70% with a large of amount of various alloys included from 'trash' metal (old pipes, radiators, etc.) This results in weak and inferior bronze sculptures that deteriorate quickly. For quality bronze sculptures and monuments, silicon bronze is the only option.
The heated ceramic shell sits in a vat of sand, and the silicon bronze is melted to 2100 degrees.
A three person crew casts the molten bronze into a 'gate' on the ceramic shell.
After the bronze is 'cast' or poured, the mold is left to slowly cool, usually about 24 hours before the ceramic shell is broken off of the cast bronze section.
The seventh step is preparing the sections for welding and metal chasing.
The ceramic shell is broken off of the cast bronze. Each section is then sandblasted to make sure all the ceramic shell has been removed from the sculpture.
Below are photos of the bronze sections that have been sandblasted and are in the midst of having the gates and sprues cut off.
The photo to the left shows the interior of the bronze section and the even thickness of the cast bronze. The gate has not yet been been cut from the top section of the head.
The eighth step is welding and metal chasing.
Once the gates and sprues are cut off and ground down, the bronze sections are carefully assembled and welded together.
Each section has been marked by the foundry, those marks along with detailed photos of the original sculpture help to specify where each section needs to be assembled.
Then the process of metal chasing begins. Each weld is carefully ground down with various grinding tips to replicate the original texture of the sculpture. At this point any seam lines or surface imperfections are "chased". This gives a unified appearance to the bronze sculpture that replicates the surface of the original clay sculpture.
Below are photos of the bronze sculpture reassembled and in various stages of chasing. The photo to the bottom left was sent to me for approval on the exact position of the sculpture in relation to the front side of the base.
The ninth step is the patina.
To achieve the final 'color' of the finished bronze, a patina is applied using a variety of metal salts and heat to create a chemical reaction with the bronze metal. Ferric Nitrate and Liver of Sulfur are commonly used to create the traditional 'French Brown' patina that is shown in the photo to the left, though other combinations of salts and chemicals can be used to create various colors through the knowledge of an experienced patineur. [to see a 'white' patina view my bronze sculpture Adoration.]
Details are highlighted by brushing back on the patina and allowing the lighter bronze to show through.
While the bronze is hot, several layers of wax are applied to seal the bronze. The wax can last for a year or more if the sculpture is installed outside. While a yearly maintenance of applying wax is recommended to keep the look of the patina; some clients allow their sculptures to acquire a 'natural' patina through environmental elements. This usually results in a darker patina and/or a verdi-gris which is a blue-green color - all depending on the chemicals present in the rain and air that surround the sculpture over time.
Below are photos that show the finished sculpture affixed to the base, prior to delivery and installation.
To the right, Tracy stands next to the molds used to create the bronze of "Rachel Weeping". Some bronze sculptures are duplicated by starting the process over again from step three: wax casting. However, other bronze sculptures are meant to be one of a kind orginals, as in this case. As such, the molds for "Rachel Weeping" were 'retired' and destroyed.