Bronze Casting Information and the process of Bronze Casting

Bronze has been the most sought-after metal for cast metal sculptures in the bygone years. Bronze alloys have a desirable and unusual property to expand slightly prior to their setting. This helps a great deal in filling every corner of the mold. Their ductility and strength causes them to be molded into any form. However, bronze has a basic disadvantage of not being durable. Hence, there are hardly any traces of ancient bronze statues found. Bronze casting generally takes place by a process called lost-wax casting. Though, centrifugal and sand castings are also employed, around 90% of bronze casting takes place through lost-wax casting.

Lost-wax method: Lost-wax casting, in the industry, is better known as investment casting. It is costlier than die and sand casting, but outshines them in terms of accuracy. It is easily possible to make complicated structures through ‘lost-wax casting’. The process can be described as follows:

Sculpting: First, the original artwork is created by the artist from clay, wax, or some other material. Mostly, clay (oil-based) and wax are used due to their property of retention of softness.

Mold making: Majority of molds consist of 2 pieces, along with a shim placed between 2 halves at the time of construction in order to put back the mold accurately. Keys are kept in the shim. The small sculpture molds generally consist of plaster. Fiberglass can also be used. To have the minute details preserved on the surface of original artwork, there exists a mold inside. It is made up of vinyl, silicone, or latex supported by plaster part of mold. Generally, the destruction of original artwork takes place during making. This is due to the solid nature of the originals. The other reason is the rigidity of the originals at the time of removal of plaster mold. That’s why; the original is cut off into thin, long pieces and separately molded. At times, a number of molds are required for recreating the original structure.

Wax: After the completion of latex-and-plaster mold, the pouring of molten wax takes place. Then, swishing is carried out till a uniform coating is obtained. The thickness of the coating is 1/8 inches. The mold’s inner surface gets covered by it. This process is repeatedly executed till the preferred thickness is attained.

Wax removal: Artwork’s ‘hallow wax copy’ is detached from mold. This mold may be reused by the artist for making more copies of wax, but its use gets restricted due to frequent ‘wear & tear’. Around 25 copies can be made for tiny bronze artworks.

Chasing: The chasing of each copy starts. Rubbing of the marks is done by a hot metal tool. The marks showing ‘flashing’ or ‘parting line’ are rubbed out.

Spruing: The spruing of copy generates paths to flow (for molten bronze) and causes air to move out.

Slurry: The dipping of sprued copy into a liquid silica slurry, and then into sand-like ‘stucco’ takes place. This process is repeatedly carried out till the coating attains thickness of at least ½ inch.

After these steps, the out-and-out processes like burnout, testing, pouring, releasing, metal chasing, and patinating are carried out.