There are a number of different techniques for casting bronze sculptures. However, the basic overall process remains the same regardless of which specific method you prefer:
1. Apply molten bronze into a mold
2. Leave it to set
3. Refine the object
4. Apply a patina
The variations in method refer primarily to how the molds are made and how the liquid bronze is poured.
Different types of molds include high-quality silicon rubber, polyurethane mold compound, and sometimes even latex. Additionally, with today’s technology, molds can also be made from a 3D printer, skipping the need for the traditional clay portions of the project, if desired. Meanwhile, different ways to pour the liquid bronze include cold casting and lost wax casting.
Here, we further break down in detail about how to create bronze sculptures.
Creating the Clay Model
Classically, an artist would start with making an original work out of clay. Popular clay types include oil-based, water-based, or low-fire clay.
Next, the artist would create an armature in order to help support the weight of the clay. Armatures are usually made out of pipe, wire, or aluminum. The clay can then also wrap around the armature as needed to help obtain the desired effect for the sculpture.
Finalizing the Clay
Before creating a mold of the clay, it is important to touch up every single detail to get it as close to the finished product as possible. This is because any scratch in the clay, however small, will imprint on the mold itself, thereby remaining present in the finished product. It is also important to be sure to let the clay dry thoroughly before moving onto the molding process, or else you will be at risk of needing to start the clay portion over from scratch because.
Creating a Mold
A common mold created via the clay method is, as noted above, made from a high-quality silicon rubber or polyurethane mold compound that is painted over the surface of the dried clay figure over the course of a number of days. On top of this silicon or polyurethane mold, there needs to be a firmer outer layer in order to hold the shape of the more supple inner layer. The outer layer is often faceted out of materials such as epoxy, resin, plaster, or Hydrocal. Once all of the layers completely dry, the mold is then removed from the clay. If the piece is of a certain size or complexity, the mold might need to be partitioned and moved away in pieces to then be rejoined later.
Pouring a Wax Replica
The next step involves creating a positive form wax replica from the mold. This is usually completed in about four stages with varying degrees of temperature, starting at about 220 degrees Fahrenheit, moving to 200 degrees Fahrenheit, and the final two stages coming in at about 180-185 degrees Fahrenheit. Ideally, after four coats, the wax will now be about ¼” inch thick. After the wax finally cools off, you are left with a wax positive of the mold.
After you have your wax positive, it is important to perform a step called “wax chasing”. This is similar to the perfecting clay step to ensure there are no flaws in the wax. This can either be done by the sculptor themselves or by a professional wax chaser. Normal tools for this include a hot tool for filling in imperfections, as well as thin, delicate tools for scraping. Once the sculptor is content with the wax positive, they move on to spruing the wax.
Spruing a Wax
The wax positive is now a replica of the original clay form. Wax rods, known as gates and sprues, are attached to the wax positive. They serve as a network that allows the molten bronze to enter and excess gas to escape. A wax funnel is also utilized to help with the pouring of the molten metal. All of the sprues need to be equally spread out so that no part of the sculpture accidentally does not have enough bronze in order to fully fill the wax positive, which could lead to a possibly incomplete sculpture.
Ceramic Shell or Investment Casing
After this is done, it is time to move onto the ceramic shell, sometimes referred to as investment casting. First, the wax is cleaned off with a solvent to make sure there is no debris present on the outside. Next, it is dipped into a prewet solution followed by a couple of coats of a fine-grained slurry. This is continued by multiple other slurry solutions, which together progress to become more and more coarse. This is done for roughly seven to nine coats, at which point, after it has dried to a hard-outer casing, the sculpture is ready for the next step.
Melting or Burning Out the Wax
Once the hard-outer casing is complete, it is placed in an autoclave at temperatures ranging from 1500 to 1800 Fahrenheit, forcing the wax out of the shell. This in turn leaves just the shell, which forms a very accurate impression of the original clay model and further hardens the shell to be ready to receive the molten metal.
Casting/Pouring the Bronze
The hard shell is then re-fired one more time in preparation for the molten metal. Once that is complete, the hot shells are taken and put either in wire frames or sand to hold them. As soon as this is complete, solid blocks of bronze are then heated to incredibly high temperatures, around 2250 degrees Fahrenheit to be exact, in order to move the blocks to a molten state. This molten bronze is then (extremely carefully) poured into the shells and left to cool.
After the cooling process is complete, the shell is then broken off using a variety of instruments such as power tools, hammers, sand blasters, etc. It is very important to do this with extreme caution so that no damage is caused to the metal sculpture itself. The entire shell should be removed, as well as the sprues which are now metal.
Similar to the wax chasing step, it is now time to perfect the metal itself. This is completed through high frequency welders. Meanwhile, any remnants from the sprues should be perfected with different grinders such as angle grinders, pencil, grinders, etc.
Metal Welding & Assembly
Now is that time that, if your sculpture was quite large or more complex, and therefore had been molded in sections, to assemble the entire sculpture together. This is done through a high frequency welder. Afterwards, any visible lines from welding the pieces together should be flattened out so that the sculpture looks as if it were always one piece to begin with.
Sandblasting or Bead Blasting the Bronze
Once you are happy with the overall welding of the piece, it then moves on to be sand-blasted or bead-blasted for a smooth and silky finish. This is usually done in a sand-blasting cabinet that a worker would have access to through holes with protective gloves in order to monitor the progress. After this is done, it is thoroughly inspected to make sure the product is the level of desired quality.
The patina is actually the color of the bronze. It is created through a chemical reaction between the bronze, high temperatures, and acidic chemicals that oxide the surface of the sculpture. The surface is usually heated to 450 to 475 degrees Fahrenheit, at which point the chemicals are added by bristle brushes, air brushes, spatter guns, or spray bottles. The exact color of the sculpture can be customized based on the types of chemicals used and can range from golds to reds to greens to blues. Finally, a sealant is applied to seal in the patina, followed by a wax to finish it off.
The bronze sculpture is now complete. Bronze sculptures can be placed inside or outside. Inside, they should last in their original form for an extremely long time if they remain untouched. If placed outside, especially depending on how corrosive of the environment is, they may need additional protection such as a durable lacquer or metal protectant to help shield the sculpture. There is now a completely unique, new bronze sculpture!
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