The "Lost Wax Casting Method" is an incredibly accurate process for reproducing sculpture in metals such as bronze and aluminum. It was developed thousands of years ago and although modern techniques have improved the process, it still remains labor intensive as skilled craftspeople are needed every step of the way. The process begins by taking the original sculpture, which might be made of materials such as clay, plaster, wood, and making a "negative" rubber impression of it. This negative rubber mold is used to produce a replica of the original sculpture, with the properties and thickness of the wax capturing all of the detail of the design. The wax pattern is removed from the negative mold and a skilled artisan retouches the seams and any imperfections. Often the artist will review and approve or decide to touch up the wax pattern further themselves.
Once the wax pattern is approved, a "circulatory system" of wax tubes, often referred to as sprues, vents, and/or gates is attached to the wax pattern. This system will allow all the wax (in the soon to be made casting mold) to melt out of the casting mold, and for the metal to be poured in, and gas to escape. once gated, the wax pattern will be encased or what's called "invested" in either a traditional or solid investment mold or ceramic shell casting mold. Both of these methods use refractory materials to encase the wax pattern and can withstand the temperature of the dewaxing process and the molten metal. The casting mold is placed in an oven or furnace and the wax is melted away leaving the negative impression in the casting mold.
For the next step, bronze is melted to approximately 2000 degress Fahrenheit (various alloys have slightly different melting points and ideal pouring temperatures), and when ready, the bronze is poured into the casting mold. This is a particularly exciting and dramatic moment at the foundry. After the metal cools, the metal finishers will clean and chase the metal surface using a number of tools, from hand held chasing tools, to air powered grinders, and welding. Work is done until the cast meets the qualities of the original model.
At this point, if it is bronze cast, the artist will often want to further the color or patina from the natural metal color achieved during the metal finishing. The patina process is the application of chemicals to the surface of the bronze that changes the chemical composition of the metal so that it takes on a different or varied color. This process is truly limited only by ones imagination and time. Many great patinas have been achieved simply by allowing the bronze to sit in nature and change with the air and moisture around it. In most cases at the foundry, we are required to make this process happen quickly and this is done by heating the metal and applying chemicals such as copper nitrate or ferric nitrate. It can also be done without heat: a cold patina. Either way, the patina process brings with it the many wonderful qualities that make bronze so luminous. Aluminum or other metal casts may be treated to achieve various results, highlighting the metal. Most casts are waxed for protection from the elements before they leave the foundry.
)
