The Alchemy of Clay to Bronze
Some 5,600 years ago, bronze was discovered. By 1100 BC in China, during the Shang period, a high level of artistic and technical skill in casting bronze had been achieved. They were already using investment casting, or what's known as the "lost wax process," to create highly detailed designs in ceremonial utensils and knives. This is the same process that is used today, though today's techniques and materials are more sophisticated.
Bronze is created by alloying (combining) copper with tin in various proportions. Many other elements (lead, zinc, aluminum) can be added to create different kinds of bronze with characteristics for specific uses. Many fine-art foundries use silicon bronze. The silicon adds strength and corrosion resistance to the bronze, good flow characteristics when molten, to reproduce detail, and good tool-working qualities when cooled.
Nearly any object can be cast in bronze. We'll use a figurine of clay or plastilene (a non-hardening oil-based clay) for an example to take through the investment-casting process.
The Master Mold
First a mold is made of the figurine. A mold is a container used to shape material; an ice-cube tray is an example of a simple mold. For our purposes, the idea is to make a container in the shape of the figurine. To do this the figurine is divided into two sides by inserting thin flat metal strips (shims), on end, into the surface of the soft clay. Next a rubber material is poured or lightly brushed onto each side of the figurine. The rubber flows and forms to every detail of the figurine. Several coats are applied, being careful not to completely cover the metal shims. The shims keep the two rubber sides from sticking together. After the rubber has dried, wet plaster is put on top of the rubber. The plaster hardens to form a casing, or mother mold, around the flexible rubber to maintain its shape once the figurine is removed. When the plaster has hardened, the mold is opened at the shims dividing the figurine. The clay figurine is removed leaving two halves which when joined back together form a container/cavity in the shape of the figurine (a negative figurine). Larger or more complicated sculptures are divided into multi-section molds and/or cut into several pieces with molds made of each piece.
Once the master-mold is cleaned and put back together, a special melted wax is poured into it, flowing into every detail of the mold. When the wax is cool the mold is opened to reveal a positive figurine in hardened wax. The wax figurine is worked by the artist to remove seam lines, correct any missing areas and rejoin any mold pieces to restore the piece to the shape of the original clay figurine. The wax then goes to the next step in this process.
The Investment Mold
The investment (from vestment or clothing) mold is a shell covering the wax figurine, made from heat-resistant materials that will hold the 1600įF molten bronze. First the wax positive is fitted with wax sprues, or channels, which will facilitate the quick and smooth flow of molten bronze to all parts of the figurine. The prepared wax figurine is then dipped and covered with several coats of investment material. After it has dried, this investment shell is then fired at a high temperature, melting out the wax figure and leaving a hard shell, or negative figurine.
The Bronze Cast
The red hot shell is then half buried in a pit of sand to stabilize it and absorb the weight and heat of the molten silicon bronze which is poured into it. After the investment shell has cooled, it is removed, leaving a bronze cast of the figurine. This cast is then worked by the artist to remove the sprues, weld on other cast parts (if necessary), and remove blemishes or add missing areas by filing, sanding, sand blasting, grinding with small and large grinders, brazing and welding. It is also prepared at this time to mount on its base of marble or other selected material. After the cast has been finished to the artist's satisfaction, it is prepared for patina or other final finish.
The patina is the finish. Technically it is a green film caused by the oxidization of the copper in bronze. This process can be accelerated and modified by applying different solutions of chemicals, acids and colored pigments on cool or heated bronze. Many sculptures are now painted with enamels and acrylics for a desired finish. After the patination process, the sculpture is mounted to its base.
© 1998, Sam Axton. All rights reserved.