Making a Bronze Sculpture

Making a Bronze

Every bronze sculpture takes a surprising amount of human handwork to produce. That’s why even a small bronze can be quite expensive.

Though modern power tools can clearly make certain steps in the process much easier, the process itself is still labor intensive and centuries old. Most modern foundries in America use the “lost-wax process”, so named because one step in the series involves melting, or burning, the wax out of a ceramic shell (more on this later). Other techniques exist, but this site describes the process I use to make my bronze sculptures.

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.

Skin the armature and set initial position and attitude of the figure. Adjust armature as needed (in this case, I added more foam for the head).  
   Begin definition of major features. In this case, major muscles in the neck, position of eye, nostril and cheekbones, etc.
Begin adding details. Here, the eye is in place, mouth has been opened, nostrils refined.  
  Continue refinement of figure. Here, a dramatic mane has been added, and texture on the face has been smoothed.
With the addition of ears and breast collar, this is the finished original.  

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).
Unfortunately, no mold is entirely leak-proof, and no wax flows entirely free of air. So the wax parts have to be cleaned up. Evidence of seams are removed and any bubble holes filled in. This step is called “chasing the wax”.  
  Next, all the wax pieces are reassembled. More chasing is needed to restore details and textures anyplace pieces are joined.
Now you have an exact replica of the original sculpture made of hollow wax. Suzanne with Bobby Hunt, sculptor and one-time co-owner of Schaefer Art Bronze foundry in Arlington, Texas

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.

The hot shells are set open-side up in a sand-pit, and a crucible filled with liquid metal gets hoisted into position and tipped enough to pour the metal into the shell.  

 

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 cools a little bit, the ceramic shell gets smashed and broken away with hammers, chisels and other tools.. This is called “breaking the mold”. The metal then finishes cooling the rest of the way.  
  Hammers and chisels remove the big chunks of the ceramic mold, but it takes a sand-blaster to remove the mold material from detailed areas.
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.

 
  Large sculptures, even though hollow, are still very heavy. Chains, winches, and wheeled platforms all assist during the assembly phase.
Sometimes a sculpture is so large that a metal scaffold has to be built INSIDE the sculpture, just so it won’t collapse under it’s own weight!  

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.

 
  The bronze is secured to a support, and then heated with a blowtorch to enhance the chemical reactions.
Chemicals may be daubed, sprayed, brushed, or applied to the hot metal surface in any way that produces the desired pattern effect. In this example, the chemical mixture is applied in a way to mimic the veining of stone.  
  The chemicals are heated with the torch to “set” them. After all chemicals are applied, water will be dumped or sprayed over the entire surface to neutralize any remaining chemicals.
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.

 
  Once the patina is finished and the metal cooled, the sculpture can be mounted on its base.
At last! A finished bronze sculpture!  _
   

That’s what it takes to make the first bronze in a series. To make the rest of the bronze sculptures in a series, you have to repeat steps 2-5, using the mold to make another wax copy. Notice that the mold is the only thing you can re-use in making all the other sculptures in a series! Everything else – the wax, the ceramic shell, the bronze parts -is created each time, and destroyed each time, for every single bronze sculpture that can be taken home or installed in a yard.