August 18, 2011 Part 2: The Process of Bronze – Molding
Let’s start at the beginning (briefly). Bronze has been the metal of choice by artists for centuries for its casting and color capabilities. Originating in Asia and The Mediterranean thousands of years ago, the bronze casting process varies little from the process used in ancient times. A few facts: Bronze is made up of 95% copper with silicon, manganese and tin making up the remaining 5%. It will never rust and endures the passage of time well.
Before we get to bronze casting, our story starts in the studio where the artist puts the finishing touches and marks on an original sculpture. The original itself may be made from a variety of materials: clay, wax or plaster. Texture plays a key role in the artist choosing which material or combination of materials will be used.
In the example of Woman at the Center, Tom Corbin’s newest sculptural accessory, he’s chosen wax. The sculpture itself is complete and now we want to create a mold so that multiple castings may be made. Each casting is an “edition” in the series and all Corbin sculptures are limited editions with the exception of a few miniatures.
The first step in creating a mold is to consider how the piece will be cast at the foundry and plan accordingly to obtain a successful casting. In order for the foundry to use a mold, they need to be able to open it up, requiring a two-part mold (a top and bottom or two sides depending on how you look at it). In this case, a bed of clay is used to partition the sculpture. Shown in bright red wax, sprues (or channels) are added to the mold. The sprues will ensure that bronze flows smoothly to the figure at the center.
Registration marks are engraved in the surrounding clay bed so that both parts of the mold will register perfectly.
Multiple coats of silicone rubber are painted on the assemblage, picking up each and every detail in the original sculpture, including the finest of details like fingerprints.
Registration tabs (shown below in light lavender) are added ~ this time to make sure the rubber mold will register with the plaster “mother mold” (the next step)…
While the rubber mold retains all of the detail in the original sculpture, it does not have the structure to hold the shape or form. Plaster is applied to create a rigid “mother mold”. This will keep the positions of elements such as arms and legs in proper form.
The clay bed is removed so that the other side can be molded. A release agent has been sprayed so that the two halves may be separated later on. Below, Tom paints the first layer of rubber on the opposite side of the two-part mold. The plaster process is now repeated on the second side, which completes the mold.
The original sculpture is removed from the rubber and plaster mold. Occasionally, Tom will keep the original sculpture in wax or clay to display in the studio, but most often the materials are reused for a future sculpture. The finished mold is now delivered to the foundry where they will begin the casting process.
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