About Sculpting In Bronze



Inspiration

I get my inspiration form various sources. I often get inspiration from doing figure drawing. A certain model in a particular pose will start me thinking about the meaning of the pose. In the end I have come up with something, perhaps entirely different, expressing the essence of an emotion, feeling or state of being.

Preparing to Model

After I have decided what I want to do I find a fine art model to pose for my work. Since there are number of them in the San Francisco Bay Area this is not typically that difficult a problem. My biggest problem is often synchronizing my schedule with that of the model.

After I have scheduled a model I build an armature out of pipe and aluminum wire which I mount on a base that will be attached to a modeling stand. It looks like a wire "stick figure" suspended in air by pipe. I ask the model to give me her measurements (height, inseam and arm span). I use these (typically scaled down to the size of the piece) to guide me in building the armature.

If the pose the model will be taking is unusual, I build a custom stand for her to pose upon. For example, in a sculpture I plan to start in the near future this will entail building a small section of a curved bridge.

Modeling

During the model's first session I adjust, by eye, the armature to the actual pose. Since the wire can be rebent at any time it is not critical to get the armature perfect the first day. In addition at the first session I measure the position of the critical "bony" landmarks on the model. These are used to check that the sculpture is accurate to life.

During the bulk of the remaining sessions I slowly build the figure up on the wire with a plastilene modeling clay (Chavant Le Beau Toche HM). Before each session starts I bring the plastilene up to its best working temperature in a hot box. During modeling I pay close attention to the lines, planes and curves of the body as I build the figure up. I often trace the major lines of the body onto the clay and mark the spots that correspond to the bony landmarks.

In the last sessions I spend my time fine tuning the surfaces to see that they are exactly as I want them. I often readjust the lighting and rotate the model's position. Shadows are the sculptor's best friend during this process because they expose all the otherwise hidden planes and edges.

I am is not yet done after I finish with the model. I spend additional time alone with the clay figure, smoothing it with silicon tipped modeling tools and small artist's paint brushes loaded with a gentle solvent (Avon's Skin So Soft).

Off to The Foundry

My portion of the creative task is almost done at this point. After dropping off the clay figure (and a fat check) at the foundry, the rest of process is largely in the hands of the highly skilled artisans that work there. I go back several times during the process to approve the work done and give my input to the foundry staff.

Lost Wax Casting

In principle lost wax casting is done the same way it was when the ancient Greeks used it to make statues. In practice, it has greatly improved in both methods and materials. The basic steps are to make a mold of the figure. Make a wax positive of the figure in the mold. Cover the wax with a tough, heat insensitive, investment material. Burn the wax out of the investment, with heat, leaving a negative space in the shape of the figure. Pour molten bronze into the negative shape in the investment. Remove the investment from the outside of the resulting bronze figure and finish (called chasing) the surface. Apply a patina to the bronze.

Making a Mold

Before the mold is made the clay figure is often disassembled into separate sections. This allows for easier production of the wax and easier casting of the bronze.

The mold is actually two molds. The inner mold is flexible and made out of any of a number of flexible synthetic rubber like compounds. The outer "mother" mold is rigid and is typically made from a high strength plaster. The purpose of the mother mold is to provide support for the flexible inner mold when it is used to make a wax.

Making a Wax

Molten wax is used to create "the wax" with a technique called slush casting. The hot wax is poured into the mold and rolled around until it has coated the inside of the mold. The remaining hot wax is poured out and the mold is allowed to cool. This is repeated with successively cooler wax until the wax is between 3/16 and 1/4 of an inch in thickness. If the wax was any thicker, the resulting bronze would noticeably dimple as it contracted the during cooling that occurs after being cast.

After the wax is removed from the mold it is chased. Special modeling tools and soft waxes are used for this process. I go to the foundry at this point to check that the wax is to my satisfaction. This is important because small flaws are often introduced when the wax is made.

After this point a sprue system is added to the wax. It is a system of wax tubes and channels that allows, during casting, the molten bronze to enter and the hot gases to exit the cavity in the investment.

Coating with a Ceramic Shell Investment

The wax is first dipped in a colloidal silica slurry. Next it is coated with dry stucco material and allowed to dry. This process is repeated until a shell 3/8 of an inch thick is formed. The stucco used in this process is an extreme respiratory hazard, and respirators must be worn at all times during the process.

Burnout aand Casting

The investment is inverted and heated to remove the wax. This process is called burn-out. It is done in either a kiln or an autoclave (with steam). Following this the shell is fired in a kiln and, after cooling, rinsed with water to remove any potential impurities that could contaminate the metal. At this time, any cracks that have occurred in the investment are repaired with a special material.

The shell and the bronze are heated in two different kilns to two different temperatures (1800 and 1950-2050 degrees Fahrenheit respectively). The molten bronze is skimmed and immediately poured into the hot shell. After this has cooled the investment can be removed with the gentle chipping of a hammer and the abrasion of a wire brush or sandblaster.

For the ancients, bronze was simply copper and tin. A mixture of copper, tin, zinc and lead evolved for use in statuary. The much safer bronze used in a modern fine art sculpture is silicon bronze. The most common of these (used in my statues) is Everdur, which is 95% copper, 4% silicon and 1% manganese.

Chasing the Metal

At this time, there are most certainly some artifacts from casting in the surface of the bronze that must be repaired. In addition the sprue system must be removed. If the sculpture was cast in sections, the parts must now be welded back together. Any mechanical tasks are accomplished with a selection of grinders and sanders. The welding is typically done with a TIG (tungsten inert gas) welder. Lastly, the final surface finishing is done with a sandblaster or wire brushes.

Patination

Patination is the process of applying chemicals that react with the cleaned surface of the bronze. It serves the dual purpose of giving coloration and providing protection to the surface of the bronze. The typical patina chemicals are applied after heating the surface of the sculpture with a torch. The chemicals in this state are highly dangerous and are applied under a fume hood by a technician wearing a respirator. I am here at this time (standing back out of danger area) advising the technician about the coloration that I wish the technician to achieve. After the patina is applied a protective coating of wax (often applied while the statue is still hot) or some other material (e.g., acrylic lacquer) is also applied.


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