The Making of A Bronze Sculpture

STEP 1: Make an original sculpture.

In the beginning, an artist makes an original sculpture out of anything that will hold a form: clay, wire, wood, plaster, plastic, foam, etc.

In most cases, I use a plasteline clay - that is, a clay that never hardens when exposed to the air - built over an armature (the internal support structure).

Building an armature. An armature can be made of anything strong enough to hold the weight of the clay in the desired position. This armature is steel plumbing pipe, wood, hard insulation foam, and aluminum wire. The foam not only gives support to the clay, it acts as a "filler". The foam is both lighter in weight and less expensive than the clay.

When the sculpture is small enough, there's no need to build an armature.

STEP 2: Make a wax copy of the original sculpture.

Soft rubber layered onto clay. Notice that the
front leg and tail have been removed.

Another layer of rubber, with "fins" added, to create
sections so the mold can be taken apart.

To make the wax, the artist or the people at the foundry first have to make a mold of the original sculpture.

The rubber for the mold is very fine and soft, so it faithfully captures the surface texture of the original work. Because it is so soft, though, it needs support to hold the shape; that's why a coating of something hard, usually plaster, surrounds the inner-layer of soft rubber. This plaster-support is called "the mother mold", because it envelopes and supports the fragile rubber inside.

In order to fully reproduce the legs, ears, or other parts that bend or stick out at odd angles, the mold is created in as many sections as needed to encase the whole piece. Because of this, and because soft clay distorts when the rubber is finally pulled out of deep creases and undercuts, the original sculpture is almost always destroyed in this step.

The mold to the right has been opened to show the hollow soft rubber nestled in the hard plaster of the mother mold.

This mold is the only piece in the bronze-making process that can be used over and over.

Now the mold is used to create the wax sculpture.

The mold is assembled in sections. Hot wax is poured into the hollow shell, allowed to settle for a few minutes, and then the excess wax is poured back out. This leaves a thin skin of wax lining the rubber mold, which is then allowed to cool. This step is repeated until a layer of wax about 1/4" thick lines the walls of every section of the mold. When the mold is removed, you have a wax copy of the original leg, body, head, etc.

Sometimes the sculpture has such a finely-detailed surface that wax needs to be drizzled directly in to the critical areas before putting the mold's parts together to cast the wax piece (shown at left).

Next, all the wax pieces are reassembled. More chasing is needed to restore details and textures anyplace pieces are joined.

STEP 3: Make a mold for the bronze using the wax copy.

Now we need a mold for the bronze. We coat the wax in fine ceramic clay, and fire the clay-encased wax in a kiln which burns out the wax and leaves an extremely hard, hollow ceramic shell that can withstand the temperature of molten bronze. This is the step called "lost wax", because the wax will be melted away, or lost, during this process.

Like hot wax, molten bronze has particular ways that it flows in narrow or far-away spaces. To account for this, the wax sculpture is usually cut apart into sizes and shapes more conducive to the flow characteristics. In addition, lengths of wax to span gaps, called "sprue bars", may be added to the wax pieces to ensure that the molten metal will get to all parts of the sculpture at the right time, and at the right temperature.

Each sprued wax piece takes a bath in a slurry of ceramic-shell batter, and then gets dusted with dry ceramic powder to add strength and thickness (like rolling an egg-battered chicken breast in bread crumbs).

The first batter is very thin and fine to preserve the details and textures on the surface of the wax.

To build a shell of the necessary thickness, each piece gets dipped several times in successively coarser and thicker batters and dry powder. The batter-and-dust coating is allowed to dry between each dip.

(Wet shell is green; dry shell is cream/tan.)

Once the thickened shell is dry, it goes into a kiln to harden and burn out the wax. The fired shell is a white color. After the shell cools, it gets filled with water to test for leaks. If the shell leaks, the holes are plugged, and the shell is fired again.

In the end, we have a shell with a hollow space of the exact size, shape and thickness of the original wax.

STEP 4: Make the bronze.

Make all the parts of the sculpture in bronze, and then reassemble them.

Now that we have a mold that won't melt and won't leak, it goes back into the kiln to get hot. It has to be hot enough so that pouring molten metal into it won't make it crack.

Standing next to a kiln or the bronze crucible would be impossible without protective gear.

Since it takes a lot of time and energy to melt the bronze, the foundry fills as many pieces at one time as possible.

This process of filling the shells with molten bronze is called "the pour".

Once the metal is free of the mold, metal-workers solder, weld, grind and do whatever it takes to re-assemble the pieces into a bronze version of the original sculpture.

Once again, bubbles may need filling, and surface details and structures may have to be rebuilt wherever pieces are joined. In large sculptures with many parts, there are also always small adjustments to make to handle minor warping of the metal during the cooling process.

STEP 5: Patina and mount the bronze.

"Patina" is the term for how the surface of the bronze is finished. It is usually treated with chemicals to change the color of the metal, and then sealed with wax or varnish to protect it from fingerprints, the effects of weather, or both.

"Mounting" means securing the bronze to a base. For monument-sized works, affixing the right kinds of supports to the bottom so that it can be secured in its final destination may be enough. Some sculptures have a base made out of bronze, included as part of the sculpture itself.

The size and shape of the base can dramatically alter the impact of the work. It has to be large enough to support the weight of the bronze, and appealing so that it enhances the work instead of detracting from it.

Bases are commonly made of stone (marble, granite, etc.), woods (walnut, oak, etc), other metals, or combinations.

Before applying a patina, the bronze goes through the sandblaster again to remove skin oils from fingerprints acquired during normal handling; otherwise, the skin oils will make the patina chemicals react differently in those places.

Colors are applied in layers. Here, the solid black of the eye goes on top of the mottled green-gray stone colors.

After the patina is applied, places that should be shiny in the finished piece get burnished. Then, any clear, protective finish coatings are applied.