To truly understand the bronze casting process, one must study the ancient methods of "lost wax" casting.
In search of perfecting this process, some foundrys add or modify
a step or two. Proven over time, this overall method clearly
captures the essence, true to the detail of the original
sculpture. Having someone skilled in carrying out every step
dramatically affects the finished product.
Step 1. Making
the Mold from an original work of art:
This step is by far the most
critical. All the detail which appears on the original sculpture
must be captured in this mold because details can not be
recaptured later in the process. The mold (depending on the size
of the sculpture) is cut into sections for casting.
Step 2. Making the Wax Casting:
Only the best of art casting waxes is poured into the
mold created in multiple layers to cover all interior surfaces.
After cooling, the mold is removed revealing an exact duplicate of
the original bronze.
Step 3. Chasing and Gating the Wax:
The wax is then pulled from the mold and hand
chased (re-detailed) by a skilled artisan to make sure that they
are free of seam lines and imperfections. Upon completion, a
sereies of channels are added - called "gating" - to allow gasses
to escape and also molten metal to flow in evenly during the
bronze pour later in the process
Step 4. Casting the Ceramic Mold (Investment Casting):
In a temperature controlled climate of 72 degreees
Fahrenheit, the wax casting is dipped into Investment liquid
several times. On the first dip a fine powder is applied. On the
next dip a course ceramic sand will be applied. This step is
repeated several times, each increasing the coarseness of the
material to create the ceramic mold. Between each dip, the ceramic
layer must cure (dry) before another layer can be applied.
Step 5. Burning Out the wax:
The ceramic shell is placed in a kiln and fired.
The shell bakes and the wax is melted (lost) from the shell. This
creates a hollow ceramic shell mold where the bronze will be
poured. Thus the term "Lost Wax".
Step 6. Preheating the shells:
Once the wax is melted out, the now-empty ceramic
shells are placed in another oven that bakes them a temperature of
1800 degrees F. This preheating process strengthens the shells
and helps the molten bronze flow quickly throughout all areas of
the shells
Step 7. Casting:
The ceramic shell (mold) is removed from the kiln and
immediately the molten bronze liquid is poured into the shell. At
the time of pouring, the bronze is 2100 degrees Fahrenheit.
Step 8. Break-Out the Shell:
After the casting has cooled several hours, the
shell is carefully broken away leaving the unfinished raw cast
bronze.
Step 9.
Sandblasting:
Any fragments of the ceramic shell are removed by
sandblasting. This also helps give an even texture to the surface
of the metal. The sculpture is
also carefully inspected at this point.
Step 10. Assembly:
At this time the various pieces making up a single
statue are meticulaously welded together into an assebled whole
using an argon gas welding system.
Step 11. Chasing:
All the weld marks are chased and re-detailed .
Step 12. Glass Beading or Grinding:
This is similar to sandblasting, the glass is extremely fine
to ensure an even bronze finish. Some foundrys using precision die
grinders and other specialized tools, remove any imperfections
from the metal surface.
Step 13. Polish:
The raw bronze sculpture is now hand polished in
preparation for the patina.
Step 14. Patina -"Color":
This step, in particular, has distinguished the
true superiority in creativity and skill in the artisans from one
foundry to the next. The bronze surface is first heated with a
blowtorch. The patina chemical is hand applied with multiple coats
to react with the heat to create the gorgeous finish which is
ultimately fused into the surface of the bronze. Afterwards, thin
coats of wax may be applied to give the sculpture a lustrous
glow.
Step 15. Waxing:
After colling, wax is hand-polished and buffed to
a sheen.
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