BRONZE SCULPTING PROCESS

By Marilyn and Bill Noland

The process of taking a wax or clay sculpture to a finished bronze casting is a process consisting of the following steps.1. The mold, 2. The wax , 3. The shell, 4. Pouring,

5. Finishing, and6. The patina.While some artists may do all of the casting process themselves most do only part or none of it.They take the finished sculpture to professional mold makers and art foundries where the work is done by people who are artists in their own right.This is their profession and the good ones are very good at what they do.Each of these steps are briefly described below for those who are interested in knowing more.

1. The Mold

The mold is generally made in two or more parts to facilitate removing it from the sculpture. The first step in making a mold is deciding where the part lines will be and shims or dams applied along these lines. The part lines determine where the different parts of the mold will end. Keys are added to the shim of dams so the parts of the mold may be reassembled in exact alignment.

The parts are now painted with layers of silicone based rubber.Silicone rubber is applied in a thick liquid state and will flow into ever part of the sculpture, filling event the smallest details.The first coat of rubber is allowed to partially set up and additional coats of rubber are added until a mold of sufficient thickness is built up. Because the rubber mold is flexible and will not hold it’s shape once it is removed form the sculpture a rigid mold is made over the rubber mold to support it. This mold is generally made of reinforced plaster or fiber glass and is called the mother mold.The mother mold and the rubber mold are keyed together with tabs on the rubber mold so it may be removed from the mother mold and reinserted in exactly the same position.

The mold is now removed from the sculpture and is now contains an exact negative of the original sculpture.

Because of their complexity many times a sculpture may have to cut into several pieces and a mold made of each piece, the parts will be reassembled later in the process.

Once the mold if completed it may be used over as many times as necessary to complete an edition of the sculpture.

2. The Wax

Once the mold is completed it is coated with melted wax.The wax may be painted on the pieces of the mold or if the mold is small enough the parts of the mold may be reassembled and the wax may be poured into the mold and out again.This is repeateduntil the wax is built up to the proper thickness, usually about 3/16”

The wax is allowed to set up and harden and the mold is removed, leaving and exact hollow wax copy of the original sculpture.

Many times waxes will have small flaws or imperfection in them such as small voids or air bubbles.When this happens the artists or foundry worker will correct these flaws using extra wax and various heated tools and scrapers.All of the textures and details of the original sculpture must be restored, so that the wax is a copy of the original sculpture.They may also at this time reattach some parts that were cut off for ease of molding.

A plugis cut out of the sculpture so that shell can get inside of the hollow wax in the next step.

Sprues and gates and in some cases vents are attached to the wax.The sprue or pouring cup is where the bronze is poured in and the gates attached to it will allow the molten bronze to flow to different parts of the shell.The placement of the gates on the wax is important in that it insures that all parts of the sculpture will filled with bronze. Vents may be added to allow air trapped in the shell to escape.If the air is not allowed to escape bronze will not flow into that area and a void will result.

3. The Shell

When the wax is completed it is coated inside and out with a ceramic shell by dipping it in a liquid slurry and then in to a fluidized bed of fused silica sand.The shell is allowed to nearly dry out and then it is dipped again. This process is repeated over again until a shell of proper thickness is completed. This usually takes at least six coatings .

When the shell is compete it appears to be solid and will not allow wax or molten bronze to pass through but it is porous enough to allow air to pass through it.If the wax worker did not believe enough air could pass be fore the bronze will begin to cool and will stop flowinga vent or vents will have been added to allow the air to escape more quickly and they too will have been encased in shell.

The shell is allowed to dry and results in a solid shell encasing the sculpture along with the sprue gates, and vents.

Once the shell is dry it is placed into a kiln and the temperature is raised to a temperature of 1600 degrees.This causes the wax to melt and run out through the gates and sprue and is captured for reuse.Although the wax would melt and run out at a much lower temperature the shell is heated to such a high temperature to completely dry it, to temper it against the shock, of the molten bronze that will replace the wax and to aid the molten bronze to reach all parts of the mold before cooling. The shell is now complete and contains a void where the wax was.This void will be filled with molten bronze in the next step.

4. Pouring the Bronze

The shell is removed from the oven and is placed in a container and sand is packed around it to hold it with the pouring sprue sitting at the top.The container will generally be filled with several individual molds that will be filled in one pour.

The bronze is placed in a pouring crucible and is heated to a temperature of 2100 degreesin apropane or electric furnace.Once the bronze is at the proper temperature and is now molten the crucible is removed from the oven with special tongs , by two people and poured into the molds.

The bronze is poured into the sprue and flows through the gates into the hollow of the sculpture pushing the air out through the air out through the shell or vents. The shell is now allowed to cool.

When the shells are cooled the shell is broken away with hammers and other tools and the bronze casting is revealed. After most of the shell is broken away both from the inside and out, the gates are cut away with an electric plasma torch and the casting is sandblasted to remove the last bits of shell.Although every effort is made to remove all of the shell from the inside of the sculpture some will usually remain. Some of this remaining shell may later break away and may be heard rattling inside of a finished bronze.

5. Finishing The Bronze

The bronze is now ready for the metal workers to do the finish work.If the sculpture was poured in many pieces it will now be assembled by matching the pieces and the seams closed by welding.On smaller sculptures which had parts removed will now have the parts welded back, and any imperfections in the metal will be filled with welding metal.

All of the welding marks will now be chased or touched up by metal workers using an array of various grinders, burrs and polishers.Any detail or texture that was in the original sculpture and was lost in the welding will be restored to the bronze sculpture.

The artist will now examine the finished bronze and if he or she is satisfied the piece will be given a final polish or sand blasting, and is now ready to be patinaed

6. The Patina

The patina or coloring of the bronze is usually done by using various chemicals on heated metal although some base coats such as liver of sulpher is applied to cold metal.

The chemicals react with the metal and will, depending on the chemical used, change color.Many different patinas can be achieved by the use of different chemicals either mixed together or in layers. Paint may also be used either to completely color the sculpture or in combination with chemical patinas.

The sculpture is now sealed with wax or in cases where the sculpture will be subjected to the elements it may be sealed with lacquer and then waxed. If lacquer is not used then the wax will usually be applied while the metal is hot so that the wax will penetrate deeper into the metal.

The sculpture is now completed and is ready to be attached to a base, if one is to be usedor ready to go if a base is not needed.