By the time we get to this stage, I have about 50 hours invested in the work before the waxes are made. The wax work on this piece took about 40 more hours to complete because I did an extensive rework and refinement of the surface.
The type of casting we are doing is called "lost wax casting". It is also called investment casting. Either term is applicable to the process. We make a wax impression of the original, make a mold of a refractory material (investment) around the wax, melt out the wax (lost wax) and replace the space vacated by the wax with molten bronze. This is a rudimentary explanation of the process, but we will learn more about that later.
Wax working is a very important part of the casting process. The bronze casting will be exactly like the wax, so the wax must be finished the way you want it. The first thing I consider in making a wax impression is how it will be when I transform it into bronze. I think about the thickness of the wax because this will be the thickness of the bronze. I think about surface quality because I want to minimize surface defects like bubbles and striations so I can reduce the amount of work I have to do on the wax to refine the surface. On a larger bronze like this one I put in channels to improve metal flow and increase the strength of the wax and bronze without adding weight.
There are many types of wax used in foundry work. The waxes I use are termed "microcrystaline" waxes. They are petroleum based products. I use a custom blend of waxes and vary the formula depending on the work I am going to cast.
This is a shot of
some of the tools I use for wax work. In the front row, from left to right are dull knife,
surforms and a variety of Italian wax spatulas and scrapers. In the back row are
rasps, hoops and to the right dental tools for wax working. I use a propane torch
and heat gun for warming tools and surfaces.
Starting the wax. First a face coat is
applied. The wax is very hot so it will flow over the surface of the mold and pick
up all the detail. I always paint on the face coat, even if I slush the mold.
If I am using a flexible mold, I heat the surface of the mold with a heatgun
to further improve the surface quality.
I continue to apply the face coat. This is
done quickly so the wax stays hot and will adhere to itself. This is especially
critical on plaster molds. On flexible molds you have a little more leeway.
After
applying the face coat, I immediately put on another coat of wax so there will be no
delamination between coats. I use a heat gun if necessary to warm the face coat for
good adhesion. This is a shot of molded wax after about 3 coats of wax.
In this photo, I have placed 1/8" thick
by 1" wide wax strips in the mold. These strips serve 3 purposes; to strengthen
the wax form; to serve as channels for improved molten metal flow when casting and to
strengthen the bronze after casting without adding weight.
I paint on more wax to smooth the transitions
between the strips and the wax body and I paint more wax into the spaces between the
strips to add volume.
After both sides of a mold section are
complete, I trim the edges and put the two halves together. Molten wax is poured
along the seams and the parts are now joined. This shot shows the bottom section
after one mold half has been removed.
This is the top half of the wax form.
It was made the same way as the bottom. The wax form has been
removed from the mold and is ready for joining to the bottom. In the background is
the bottom half, still in the mold.
Here is
the final form in wax. Because the wax is not strong structurally, I have to
prop it up and support it. The wax weighs about 20 pounds. To finish
this work I have smoothed down the "feathers", burnished the wax, added texture
and worked the line where the halves were joined. At this stage it is ready for
gating. I like working wax and find I can add subtle texturing effects not possible
in any other medium.
To
me the this is really the bottom part of the sculpture and not a real base. It is an
integral part of the work. We'll call it a base for lack of a better term. It
will be made in three sections. This is the bottom section. The mold is to the back.
This was made the same way as the Birdform. The mold was taken from a big
ball I "borrowed" from my sons.
Here you see the bottom section with the top
removed. This part is covered by the top two forms and I cut it off to reduce weight
and make gating and investing simpler. In the back are the rest of the base. They were
made in two other molds not shown. The original forms were balloons.
The final base wax. Because of the
nature of wax, I will not be able to join the two main parts to see what it will look
like. I added the "claws" because I thought the sculpture needed a smoother
transition at this point. I am not a complete slave to the concept drawings.
Many things changed from the concept to the final wax. Most notably were the
"feathers" for the wings and the claws at the foot of the sculpture.
I also did extensive textural work on the main form that was not envisioned
at concept. The computer is a tool after all and not a master.