The process by which bronze sculpture is created has changed very little in the last 1500 years. Some of today's mold materials are a little handier to use, and the bronze alloy itself is a little better quality, but the procedures and problems Michelangelo faced 600 years ago are the same ones sculptors deal with today.
I work exclusively with oil base clay. Its texture is ideal for shaping forms and adding detail to my sculpture. The clay never hardens and so will not change in consistency throughout the months it may take to complete a piece. The disadvantage of this material is, in most artistic use it will not support its own weight. A skeletal framework is necessary to strengthen most clay sculpture. This frame is called an armature. Starting with a photograph which I modeled this sculpture from, I drew a diagram or a plan for the armature; the right side legs in blue, the left in red.
The finished armature is made mostly of malleable aluminum wire. It's not pretty, but it is functional. The wood pieces were added only to fill out the body so less clay will be necessary to begin.
We're skipping ahead a couple months here and the artistic work is nearly finished in clay. A bit of the armature is still visible where a hoof is still being worked on.
A thin coat of shellac is brushed on the finished clay to harden the surface so that activity with the mold materials will not leave marks.
There are actually two molds made over the piece. The first is made by brushing on several coats of liquid rubber. This material picks up all of the detail and form of the sculpture. A hard mold, called the mother mold, goes over the flexible rubber to stabilize it. In this photo the rubber mold is complete and I am drawing in the parting line, where the mold will eventually be cut open.
Here the piece is laying on one side and I am building a clay dam along the parting line. As you will see, the dam facilitates the application of the mother mold.
When the dam is completed, clay positioning lugs are added so that the halves of the hard mold will match together perfectly. The column of clay on the side has been placed so that a hole will occur in the mold at that point, through which wax may be poured.
Here I'm smoothing on the epoxy like material of the first section of the mother mold. It hardens very quickly.
When the first section of the mother mold is set, I remove the clay dam and apply mold release fluid to all visible parts. Then the next section of the hard mold is applied to the other side. Some simple sculptures require only two halves of mother mold. Most, such as this work, have as many as four sections, each separated by a clay dam at first and then additional mold release fluid.
Here I am prying open the sections of the finished mother mold. If the previous work was done well, the sections will part neatly and will fit together precisely later on.
In the next step the rubber mold is opened by cutting along the parting line. I use a scalpel for this job, and when this mold is open the clay artwork is discarded.
All of the mold pieces are cleaned and carefully repositioned together. Holes are then bored through the outer edges of the hard mold and it is bolted together in a single unit. Then three casts of melted wax are poured into the mold about a half hour apart.
This photo shows the two layers of the open mold and the wax replica of the original artwork. This step may be repeated as many times as wished to obtain additional wax copies of the sculpture.
Of course the replicas are never perfect when they come out of the mold. The parting line is usually visible and must be removed. Some of the detail will be blurred, requiring repair.
This next step is where the process gets its name. The wax pieces are dipped in a semi adhesive liquid contained in the vat on the left side in this photo. Then they are moved to the bin containing finely ground silica where it is fluffed over the entire surface. After drying for about twelve hours the procedure is repeated, again and again until achieving a buildup of silica nearly a centimeter thick.
This photo shows a sculpture of an elephant's head covered by a completed silica mold. The tubing you see has been added to vent air out of the mold when the cast is made. The piece is then heated in a kiln and the wax is melted and poured out.
Here foundry workers are pouring molten bronze at 1900 degrees fahrenheit into some molds. When cooled, the mold material is broken away and the bronze is cleaned. This work is called chasing. At this stage the bronze surface is approximately the color of a new penny.
Patination is the process by which coloring of the surface of the sculpture is affected. Although a rich brown color has become almost customary, many other hues and patterns can be accomlished. Different chemicals applied with varying amounts of heat and layering can produce an attractive shade which compliments the artwork.
It's a labor intensive as well as artistic endeavor, but because of the hardness and durability of bronze, the result is a piece of art work which may be enjoyed and appreciated for over a thousand years.