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The Casting Process
The Art of Lost Wax
Lost wax casting has been around for thousands of
years, yet few people understand how the process actually
works.
Although mechanization has facilitated the lost wax process of
bronze casting, the procedure is basically the same as that used by
the Chinese when they first developed the process in the 2nd
millennium BC.
The Rubber Mold
First the artist creates an original sculpture out
of any number of media, including stone, wax, clay, wood and
pottery.
This image is coated with a silicone rubber molding material that
makes two rubber mold halves (each rubber mold has a front and a
back piece). A fiberglass outer shell is added to the back of each
mold so it retains its shape and rigidity during subsequent
uses.
These molds are the only components that are ever re-used in the
casting process. All other components are re-created for each
casting.
The Wax Positive
Once the molds are done, the insides are coated
with layers of wax. The halves are then bound together and wax
poured inside to complete the wax image being created.
Once the wax has cooled, the mold is peeled away, yielding a wax
image (the wax positive") duplicating the original sculpture.
This image must then be "touched -up" to remove any seam lines,
scratches or other flaws, as well as to recreate any pattern or
texturing that was lost or damaged when the wax was made.
The quality of the finished bronze relies on a clean, high quality
mold and an impeccably recreated wax image that is as near to
perfect as possible.
Gating
The next step, "gating", is the application of a
series of tubes and funnels that allow the molten bronze to flow
through to the bottom of the ceramic shell and the hot gases to
escape at the same time.
These sprus are created by attaching wax rods to the finished wax
form at strategically spaced locations.
Ceramic Shell Casting
After the gating is completed each wax form is
dipped in a liquid ceramic silica-sand compound so it is completely
coated inside and out. Holes called "patches" have been cut into
the wax to allow an entrance to the inside of the form.
The form is subsequently dipped 6 to 12 or more times over a period
of several days until the desired shell thickness is achieved.
Lost Wax
Once these ceramic shells have dried thoroughly the pieces are
placed into an autoclave and the wax is melted out (hence the term
"lost wax"), to be reclaimed and used again. The shells are then
cured in a kiln so they will withstand the temperature of the
molten bronze being poured into them.
The Pour
Bronze ingots are melted to a temperature of
approximately 2000°F and poured into the cured ceramic
shells.
As the sculpture cools the ceramic shell begins to pop away from
the bronze.
This shell will be completely broken away, using a hammer and
chisel, before the superfluous metal materials are cut away.
Sandblasting
The casting is then sandblasted in preparation for metal
finishing.
Any pieces of a sculpture that were cast separately are welded back
onto the sculpture and any seam lines or other imperfections are
removed or "chased".
Texturing
Finally, any texturing that was lost or damaged in
the casting or welding process is recreated.
The sculpture is then polished in preparation for application of
the patina.
Patinas
The different colored finishes that are possible on
cast bronze sculptures are called patina's.
The various colors, patterns and textures obtained in the patina
process are achieved through a combined application of chemicals
and heat, augmented by hand stippling, or spraying with an air
brush, and sealed with lacquer and waxes.
Limited Editions
Most bronzes are part of a "limited
edition" containing a fixed number of castings.
This edition number is decided by the artist, usually after the
first piece has been cast, and individually stamped on each piece
(i.e. 1/100) thus concluding the process of bronze sculpture
production.
Enlargements and Reductions
Bronzesmith is able to facilitate the artist's creation on any
size up to 20 feet tall. We have done many enlargements, both for
public commissions and private collections.
