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.
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