Above: Wax is poured into molds. Below: the empty shells come out of heat for melting wax.
Above and below: Foundry workers manipulate the molten bronze and pour it into the empty shells.
Above and below: Skilled finishers polish and perfect the raw cooled bronze.
The Bronze Process3
If the artist wishes to make several copies, or a limited edition of the sculpture, it’s necessary to make a mold. Various materials are used for this process - silicone rubber or plaster are two popular choices. A two-part silicone mixture is often used if there is detail in the piece. The material is brushed, poured or sprayed over the original art. The first coat is very liquid, so that it seeps into the details of the work. Layer-by-layer the rubber is applied and thickened until the piece is encased in a sturdy mold. This process can take hours and some sculptors send their original work to professional mold-makers. Others make their own molds. Once dry, the rubber is carefully slit open so that the original piece inside can later be removed. The inside of the mold should be a perfect, positive impression of the art.
To ensure the integrity of the rubber mold, thinner layers of another material - such as fiberglass or plaster laced with fiberglass - is painted over the outside of the mold. This is called a “Mother Mold,” and often requires wearing a mask to avoid breathing toxic fumes present in some of the materials. Once dried and hard, the mold can be opened and the original art (wax or clay) is removed.
With the Mother Mold fit snuggly over the rubber, the artist melts a special wax and pours it into an opening - often the bottom of the piece - and manipulates the work so that wax evenly coats the inside. The first application of wax is hot and, as additional layers are poured, the wax becomes cooler. The goal is to make a perfect, hollow impression from the mold - one that is not too thick (which would require more bronze) or so thin it collapses or breaks. Once cooled, the wax is carefully removed by parting the slits on the mold. The results should be a perfect wax replica of the original art. The wax impression often requires additional detail work and smoothing. Again, foundries employ professionals who do this work for artists, although some complete the wax finishing themselves.
The Sprue / Gating System
This important step in the process us usually done at the foundry by experts who attach wax bars to specific places on the wax mold, and then affix the bars to a wax “pour cup.” These sprues (or gates) are positioned to allow the molten bronze to flow evenly to each part of the piece. This seemingly simple step actually requires a lot of experience and can make the difference between the success and failure of a bronze pour.
The wax mold with sprues in place and attached to a wax cup is then dipped into a mixture of silica and water. The mixture coats all the wax surfaces, inside and outside. Gradually, thicker layers of silica are applied and, after each coat, the piece is left to slowly dry. Eventually, the silica is thick enough and has dried to be extremely hard.
The Burn Out
The wax, sprues and cup - all encased in hardened silica shell - is then placed inside a kiln that has been heated to about 1800°F. The shell “vitrifies” (changes into a glass because of the high heat) and the wax simply melts out. On a smaller scale, this is the same method used by jewelers in the “lost wax” process. With the wax melted out of the shell, a hollow impression of the sculpture is left for the most dramatic step - the pour.
The Bronze Pour
The silica shells are heated again in the kiln. At the same time, ingots of bronze are dropped into a crucible and melted to about 2,100° F. Foundry workers wearing protective gear then transfer the shells from the kiln to a rack near the furnace and crucible. The shells are hung, open cup-side-up, on a rack. They then hoist the crucible from the furnace and maneuver it to the rack, finally pouring the bubbling, lava-like molten bronze into the cups.
After a few hours, the newly poured pieces have cooled enough and foundry workers use hammers and other tools to break the silica shell from the bronze. Once the large pieces of the shell have been removed, the piece is taken to an enclosed sandblaster where the rest of the shell is blown away.
The Chase & Finishing
Skilled finishers then take the raw piece and use a variety of air-powered tools to clean it thoroughly - and bring it ever-closer to the artist’s vision. Very large sculptures are usually cast in multiple pieces and finishers assemble the parts as they would a jigsaw puzzle, welding the pieces together and then “chasing” the welds with tools. Finally, the piece is cleaned, but still has a dull finish and needs one final step.
Good patineurs are highly regarded as artists in their own right. Using heat, water and specific mixtures of chemicals they apply tone, color and hue to the piece. Patina is not paint. The mix that is applied to the sculpture reacts with the bronze naturally, creating an endless variety of finishes for sculpture. Finally, special wax is applied to the still-warm bronze to bring out its inner luster. The bronze piece has arrived at the end of its long and complex journey, ready to be enjoyed and appreciated.
How many people does it take to make a finished bronze?
ARTISTS / WAX WORKERS / SHELL MAKERS / FOUNDRYMEN (AND WOMEN) / FINISHERS / PATINEURS and the brave individuals who dare to own bronze foundries to keep the art alive for future generations.
Above : A Pateneur creates the finish on bronze using heat, chemicals and years of practice in the art.