The Bronze Process
The Lost Wax Method of Casting Bronze
Bronze Age technology and the development of metallurgy began in about 3500 BC and continued through 800 BC. The Art Foundry in Sacramento, California, is continuing a 6000 year-old tradition of bronze casting while using some 21st Century advances in technology.
- Step 1. The original work of art
The artist creates an original work of art to be cast. The work may be in any material but most artists work in clay, wood, stone, or wax.
- Step 2. Mold making process
A silicone rubber material is sprayed, poured or brushed over the original work for a perfect imprint of the original piece. Once several layers of rubber have been applied and allowed to dry, another layer of stiffer material is added on the outside of the rubber mold. This is to make sure the impression inside keeps its form once the original artwork is removed. Once this “mother mold” has hardened, it is cut apart and pulled away, leaving the finished rubber mold. A slit is then made in a section of the flexible rubber mold and it is opened up so that the original sculpture can be removed. The inside of the rubber mold will show a perfect impression of the piece.
- Step 3. Wax
The seam cut in the rubber mold is joined back together, leaving an opening for the pouring of wax that has been melted to a specific temperature. The wax is carefully poured while the rubber mold is turned to evenly coat the inside with wax.
Once the wax has hardened, the rubber mold is again pulled open and the wax – now the perfect image of the original piece – is removed.
- Step 4. The Sprue or Gating System
Wax bars, called “sprues” are attached to the work. These sprues, or gates, are then attached to a wax “pour cup.” Together, these additions to the work make a system of channels that allow the melted bronze to flow to the bottom of the shell that will be applied in the next step.
- Step 5. The Shell (dip)
The wax copy of the original along with the sprues and the cup, is dipped into a silica slurry. This step is followed my more applications of dry silica until the shell is thick and even. Each coat must thoroughly dry before the next is added. Once this shell dries to a good hardness it is ready for the burn-out.
- Step 6. Burn Out
At this point the ceramic (silica) shell mold is placed inside a kiln that has been heated to 1,600 degrees to 1,800 degrees Fahrenheit. The shell vitrifies at this temperature and the wax melts out. It is this step that gives the entire process its name – the Lost Wax Process. Once the wax has melted, the ceramic shell, that is now yet another molded impression of the original work, is ready to receive the molten bronze.
- Step 7.
The Pour
The mold is pre-heated while ingots of bronze are melting in a nearby crucible. Once the ceramic mold is hot enough, it is suspended on a rack, with its open end at the top. Once the molten metal reaches about 2,100 degrees Fahrenheit, the crucible is lifted from the furnace and molten bronze is poured into the waiting molds. This is a very important step in the process.
- Step 8. Divestment
Once the molten bronze has cooled sufficiently and returned to a solid state, the work is “divested” (broken out) from the ceramic mold. This is usually done with hammers and tools.
The gating system is cut away from the original work and the piece is then sandblasted to remove any silica that may remain.
- Step 9. Metal Work (welding, chasing, fabrication)
Very large works of art are cast in many pieces that will have to be assembled and welded together. Smaller works may also have parts that were removed and cast separately. The seams made from welding are taken off (chased) with grinding tools. Sanders and other tools are used make the bronze look like the artist intended. The work is sandblasted once again to make it ready for the patina.
- Step 10. Patina
Patination is the process of applying hot chemicals to the surface of a bronze. The piece is heated with a blowtorch as various chemicals are sprayed on.
The chemicals oxidize the copper in the bronze and create color on the piece. Patination is an art in itself, and creates the final look of the piece. Once the patina is finished, the sculpture is sealed with a clear wax and ready to enjoy.
A sculpture in clay waits for the bronze process. The work is a memorial to former Sacramento Mayor Joe Serna and his wife - much beloved, the couple passed away within months of each other. The work is by Gerald Silva and installed near City Hall
