BRONZE CASTING TECHNIQUE

 

Bronze, an alloy basically of tin and copper, was first used for edged weapons and utilitarian items, and later for sculptures, by the ancient Egyptians, Greeks and Romans (not to mention early Chinese, Asian Indian and African artisans and artists). In antiquity, bronze sculptures, unlike stone and ceramic artworks, were often melted down and reused for weapons or to create new sculptures, and until the 20th century the artist not only created the sculpture but was closely involved in the casting process. Today, foundries create a mold from the artist’s model and perform the multi-step casting procedure.

The technique, very broadly, consists in creating a shell from the artist’s model and then pouring molten bronze into the shell. Once cooled, the sculpture is given finishing touches, often including surfacing with a corrosive substance called a “patina” for color effects. Susan Bahary offers a wide variety of unique patinas.

The “lost-wax” casting method, utilizing either a plaster final mold or, in recent years, a ceramic final mold, starts with the artist’s clay model. The model can be built around an armature of various materials like wood and wire.

The clay model is then moved to the foundry, where artisans paint the mold with polyurethane or silicone rubber, half or more sections at a time. When the rubber dries, a reinforced plaster “mother mold” is built around the two halves or sections, and the artist’s model removed from inside. This rubber model will be reused to cast each subsequent sculpture in a limited edition. Once the edition is finished, no new sculptures of that edition will ever be cast.

The two rubber halves or sections are rejoined and melted wax poured in ever cooler layers and sloshed around inside each time, finally forming an even coating on the inside of the rubber “negative” mold of about 3/16ths of an inch thick. The rubber is then peeled away, leaving an almost perfect “positive” wax reproduction of the sculpture.

The next step is to rejoin the wax pieces together (“chasing”). Here the artist carefully checks the wax model. The next steps will involve poking channels (“sprues” and “gates”) in the wax through which, later in the casting process, bronze will run and trapped gas will escape, and then building a rock-hard shell around the wax sculpture (“investment”). Until a few years ago, this shell was formed of plaster, sand and water. Standard technique now is to dip the wax into slurry followed immediately by a bath of sand. After about nine applications, with drying time in between, a hard ceramic “negative& shell about ½-inch thick forms around the wax. The ceramic shell is then placed in a kiln to melt out the wax,the “lost wax”.

The final casting step is the pouring of molten bronze into the ceramic shell, heated beforehand to about 1100o F to prevent the 1700o F bronze from cooling too quickly. When cool, the bronze will be “devested”: the shell will be broken off, the gates and sprues removed with a high-intensity electric arc, and the metal sandblasted to remove the fine investment from the bronze. Surface finishing, much like the “chasing” of the wax, to remove imperfections and smooth down welds where sections of larger sculptures have been joined together, is the next step. Structural reinforcements to the interior may be added for larger sculptures. Final surfacing and sometimes patina applications follow.

The new bronze is then signed by the artist (and, if in a limited edition, numbered) before shipment to the patron, where it will give years if not centuries of pleasure to art lovers, whether installed indoors or outdoors.

Susan Bahary has developed her technique to incorporate water features into both indoor and outdoor sculputres, if desired.