Bronze Casting as used in Sculpture
Home Up The Traditional Technique of Hag Bronze Casting as used in Sculpture

 

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Bronze Casting and the Lost Wax Method
by: Phylactis Ierides - Sculptor

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bulletIntroduction
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Procedure Used by the Sculptor

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The Ceramic Shell Process 

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Introduction

Casting is the most primitive and also the simplest means of getting a metal or metallic alloy into desired forms. Bronze appears to be the most extensively employed material of all the sculptural media i.e. wood, stone, marble, terracotta, stainless steel, iron, pewter, aluminum, silver or gold. Its great popularity is due primarily to the many physical properties, such as structural strength, which allows a lighter, freer and more open design for a statue, its great physical permanence and resistance to atmospheric corrosion, ease of casting, and fine, compact surface, that can also take an excellent finish or patina.

Bronze is basically composed of copper (around 85%) and other smaller quantities of metals such as tin (4-13%), zinc (0,05-6%), lead (0,25-23%). The addition of more tin to copper produces bronze and more zinc yields brass. Bronzes tend to have a warmer color tone than brass.

In modeling a work in clay for eventual casting in bronze, the sculptor always takes into account that the finished metal will be some what smaller than the original, due to the shrinking of the hot melded metal, after it cools down. Another property of bronze, which a sculptor should remember is that when it cools slowly becomes quite hard and brittle. The opposite happens when is cooled quickly, after casting.

Very few sculptors nowadays cast their own statues even though the contemporary use of bronze as a sculptural medium surpasses any previous use of the material. This is because additional capital is required for expensive equipment, besides the capital for regular studio facilities, the increased manpower needed for the reproduction of statues and the many technical difficulties entailed in bronze casting. As a result many artists employ professional bronze casters for the mechanical reproduction of their prototype statues.

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Procedure Used by the Sculptor

The first step is for the artist to create the prototype sculpture with clay or modeling modeling wax or any other material he is accustomed to. Then a mold is made of the work by using silicone rubber (other rubberized materials could be used). A firm external shell of plaster of Paris or fiberglass supports the rubber mould. After this process, the mold is opened, cleaned, sprayed with a mould release agent and coated with molten wax until a thickness of 4 - 10 mm of thickness (the bigger the statue the thicker the wax). After the wax is hard, the mould is removed, and the replica is repaired of any imperfections on the wax.

When this tedious work is finished, wax rods are attached on the wax replica, in such a way, so that when the melded metal is pored, they will act as suppliers of metal and as exhaust pipes for the gases produced, during the pour. At the main supply rod, we attach a cup, in order to facilitate our pour.

 

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The Ceramic Shell Process 

When the wax assembly is finished, we cover it with a fire resistant mold, which will serve as the receptor for the melted bronze. A new cost-effective method used mainly, by the industry today, for high precision castings is the ceramic shell method.

A slurry, made up of calcined zircon flour and a binder (colloidal silica) is used to dip the wax assembly. Then, it is covered with zircon sand (stucco) and let to dry. After it dries, a second slurry is used consisting of molochide flour and binder. It is covered again with stucco sand of molochide, this time. When dry, the second dipping process is repeated 8-10 times. This is left to dry completely. All ends of the attached wax rods are cut, so that when we place the mold produced in the furnace the wax melds and runs off it. The wax remaining is burned out and we are left with a ceramic shell with no traces of wax whatsoever.

The next step is the pouring of the metal through the cup of the fire resistant mould. When the metal sets, the ceramic shell is broken to free the cast bronze. The rods added prior on the wax are now made of metal and are cut off. Sandblasting can be used to free the bronze of any remaining ceramic. It is further finished with metallic brushes or sandpaper. Many artists do not use finishing products of any kind, so that the texture of the sculpture will remain unchanged.

Different chemicals produce the patina (coloration of the bronze). The bronze sculpture is coated with lacquer or preferably bees wax in order to protect the patina from getting affected by light and moisture.

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Please contact the Artist for more information.

 

Phylactis C. Ierides
20, Ioanni Tsirou St.
3021 Lemesos
Cyprus

Tel. +357-99681967, +357-25381324, Fax +357-25339219

Email: ieridis@cytanet.com.cy