Although I work with a wide range of materials to create my sculptures, I do utilize bronze a lot and am fortunate to own my own bronze foundry. There are two main advantages to this: I have complete quality control, and I also have control over time-lines since I don’t have to outsource any portion of my sculpture creation.
If you are a fellow sculptor, and require bronze casting services, please contact us for a quote.
People often ask me about the bronze casting process, so I have provided an explanation below for those of you who are interested in the ‘technicalities’ of creating bronze sculptures. You can also contact us for a studio and foundry tour (by appointment only).
You can also view my video of a bronze pour.
Bronze is a metal compound containing copper and other elements. The term bronze was originally applied to an alloy of copper containing tin, but the term is now used to describe a variety of copper-rich material, including aluminium bronze, manganese bronze, and silicon bronze.
The starting place for a bronze sculpture is a ‘positive’, or original sculpture made from virtually any material (I generally use plaster or wax) from which a mold is taken. The mold will duplicate exactly, and in a negative form, the original positive.
The Flexible Mold:
Most often I will use a silicone or rubber mold. The positive will be coated with a silicone or rubber which itself will then be coated with a hard ‘mother’ mold. This will help the soft flexible mold keep its shape. Then it is removed from the positive. This mold allows many waxes, which comprises ‘editions’ of bronzes to be made.
The Wax Positive:
The original sculpture is now used exclusively as a reference point. From the ‘negative’ rubber mold, a wax ‘positive’ is created. Wax is melted to about 210 degrees Fahrenheit, poured into the mold, and evenly coated or ‘slushed’ inside. Under ideal conditions, the wax wall will be about 3/16 inches thick. When the mold is opened and the rubber peeled away, an almost perfect wax reproduction is removed.
Wax Chasing – Spruing and Gating:
‘Wax chasing’ is the delicate process of joining the wax pieces, removing seams and repairing imperfections with heated customized soldering irons or tools. Dental tools are ideal. If I am casting for someone else, artists are encouraged to visit the foundry at this point to sign and check the integrity of the waxes. After the wax is chased and approved by the artist, the piece is then advanced to ‘spruing’ and ‘gating’. The gates and sprues are also made of wax. They form the channels through which the melted bronze will travel to the artwork.
‘Investment’ is the process of building a rock-hard shell around the wax sculpture. Later, when the wax has been melted out, the investment will serve as a mold for the molten bronze. The ceramic shell technique begins by dipping the gated wax into vats of slurry followed immediately by a bath of sand. This process builds a very thin wall of silica around the wax. When repeated approximately 9 times, allowing for dry times in between dips, a hard shell about ½ inches thick forms around the wax. The wax is a ‘positive’ which must disappear in order to create a cavity or ‘negative’ for the bronze to fill. The term ‘lost wax casting’ comes from the process of the wax being melted or ‘lost’ from the shell. Ceramic built shells are ‘de-waxed’ in a kiln.
A graphite crucible, fired by a furnace, is filled with bronze ingots that are melted. The metal melts to 2050 degrees Fahrenheit. Bronze ‘seizes’ (stops flowing) when confronted with cold, which might occur if molten bronze was poured into a room temperature shell; therefore at the same time the bronze is being blasted by a natural gas furnace, the ceramic shell is heated in a kiln to approximately 1100 degrees Fahrenheit. When the pour begins, the crucible is lifted out of the gas furnace. At the same time, the glowing ceramic shells are brought out of the kiln to the pour area. The entire pour is very fast and very precise. One crucible of bronze holds 200 lbs and can fill one or two large shells. The first pieces poured are those with thin walls and intricate details; requiring hot, fluid bronze to move throughout the channel system.
‘Devesting’ is the process during which the investment is removed from the metal. Approximately one hour after the pour, the piece is cool enough to handle. Skill and strength are combined with hammers and power chisels to knock the investment off the freshly solidified metal. The gates and sprues must also be removed with a plasma cutter that can cut through the bronze like butter. The final step is to sandblast the fine investment from the bronze.
Chasing the Bronze:
Bronze must also be chased or cleaned to address the slight imperfections that may result from the casting or shell building process. On large sculptures, where assembly of cast sections is required, chasing is essential to take down weld lines formed by the joining of two planes. Metal chasing usually starts with large electric or pneumatic grinders to remove the bulk of the unwanted metal. Then, more refined and smaller tools such as die and pencil grinders are used to re-create the artist’s subtle surface texture. Many monumental bronzes require an internal structure to support the bronze ‘skin’.
‘Patination’ is the coloration of bronze by the chemical and heat application of color. The last step is the application of a coat or clear wax over the bronze to enhance the preserve of the patina.
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