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The Bronze Process

Ancient and Elemental

Bronze reaches more than 6000 years back into human history. Although modern foundries
use some new technologies, much of the process remains unchanged over the eons.
Foundries are noisy spaces in which the sounds of metal-on-metal bounces off concrete
walls; where the loud whir and buzz of air-powered tools reverberates and iron chains rattle -
a cacophony of sound that summons ancient spirits of foundry workers, patineurs, artisans
and artists. The hub of this wheel of work is a 2200°F crucible bubbling with glowing molten
bronze that is poured into molds, cooled and hammered free to become art works that will
endure the test of time. Here is how it happens:

The Art
Artists work in many mediums to create their sculptures. Some use an oil-based clay, others
use wet (ceramic) clay, some sculpt in wax and others in wood and even stone. The medium
is a very personal choice.

The Mold
If the artist wishes to make several copies, or a limited edition of the sculpture, it’s necessary
to make a mold. Various materials are used for this process - silicone rubber or plaster are
two popular choices. A two-part silicone mixture is often used if there is detail in the piece.
The material is brushed, poured or sprayed over the original art. The first coat is very liquid,
so that it seeps into the details of the work. Layer-by-layer the rubber is applied and
thickened until the piece is encased in a sturdy mold. This process can take hours and some
sculptors send their original work to professional mold-makers. Others make their own molds.
Once dry, the rubber is carefully slit open so that the original piece inside can later be
removed. The inside of the mold should be a perfect, positive impression of the art.

To ensure the integrity of the rubber mold, thinner layers of another material - such as
fiberglass or plaster laced with fiberglass - is painted over the outside of the mold. This is
called a “Mother Mold,” and often requires wearing a mask to avoid breathing toxic fumes
present in some of the materials. Once dried and hard, the mold can be opened and the
original art (wax or clay) is removed

The Wax
With the Mother Mold fit snuggly over the rubber, the artist melts a special wax and pours it
into an opening - often the bottom of the piece - and manipulates the work so that wax evenly
coats the inside. The first application of wax is hot and, as additional layers are poured, the
wax becomes cooler. The goal is to make a perfect, hollow impression from the mold - one
that is not too thick (which would require more bronze) or so thin it collapses or breaks. Once
cooled, the wax is carefully removed by parting the slits on the mold.  The results should be a
perfect wax replica of the original art. The wax impression often requires additional detail work
and smoothing.  Again, foundries employ professionals who do this work for artists, although
some complete the wax finishing themselves.

The Sprue / Gating System
This important step in the process us usually done at the foundry by experts who attach wax
bars to specific places on the wax mold, and then affix the bars to a wax “pour cup.” These
sprues (or gates) are positioned to allow the molten bronze to flow evenly to each part of the
piece. This seemingly simple step actually requires a lot of experience and can make the
difference between the success and failure of a bronze pour.

The Shell
The wax mold with sprues in place and attached to a wax cup is then dipped into a mixture of
silica and water. The mixture coats all the wax surfaces, inside and outside. Gradually, thicker
layers of silica are applied and, after each coat, the piece is left to slowly dry. Eventually, the
silica is thick enough and has dried to be extremely hard.

The Burn Out
The wax, sprues and cup - all encased in hardened silica shell - is then placed inside a kiln
that has been heated to about 1800°F. The shell “vitrifies” (changes into a glass because of
the high heat) and the wax simply melts out. On a smaller scale, this is the same method used
by jewelers in the “lost wax” process. With the wax melted out of the shell, a hollow impression
of the sculpture is left for the most dramatic step - the pour.

The Bronze Pour
The silica shells are heated again in the kiln. At the same time, ingots of bronze are dropped
into a crucible and melted to about 2,100° F. Foundry workers wearing protective gear then
transfer the shells from the kiln to a rack near the furnace and crucible. The shells are hung,
open cup-side-up, on a rack. They then hoist the crucible from the furnace and maneuver it
to the rack, finally pouring the bubbling, lava-like molten bronze into the cups.

The Breakout
After a few hours, the newly poured pieces have cooled enough and foundry workers use
hammers and other tools to break the silica shell from the bronze. Once the large pieces of
the shell have been removed, the piece is taken to an enclosed sandblaster where the rest of
the shell is blown away.

The Chase & Finishing
Skilled finishers then take the raw piece and use a variety of air-powered tools to clean it
thoroughly - and bring it ever-closer to the artist’s vision. Very large sculptures are usually
cast in multiple pieces and finishers assemble the parts as they would a jigsaw puzzle,
welding the pieces together and then “chasing” the welds with tools. Finally, the piece is
cleaned, but still has a dull finish and needs one final step.

The Patina
Good patineurs are highly regarded as artists in their own right. Using heat, water and
specific mixtures of chemicals they apply tone, color and hue to the piece. Patina is not paint.
The mix that is applied to the sculpture reacts with the bronze naturally,
Some bronze artists sculpt in an
oil based clay, others in wax or
in wet clay - such as that used
for ceramics.
Camarino Sandoval pours wax
into a mold at Frostad Atelier
Foundry in Sacramento, CA.
Marcy Randolph cleans and
smooths the wax pieces and
attaches sprues to the piece to
help hot bronze flow evenly
through the piece.
The finished wax is coated in may
layers of silica left to dry to a very
hard shell.
After the wax melts out, the empty
shells are pulled from the furnace.
Wearing protective gear, foundry
workers pour molten bronze into the
shells and let it cool for many hours.
Expert finishers use air-powered
tool, welders, grinders and cutters
to clean the raw bronze and make it
ready for patina.
John Mitchel puts a high polish
on a Bennett sculpture.
Photos taken at Frostad Atelier in
Sacramento, Ca. The foundry is
one of the few woman-owned
foundries in the U.S.
creating an endless variety of finishes for sculpture. Finally, special wax is applied to the
still-warm bronze to bring out its inner luster. The bronze piece has arrived at the end of its
long and complex journey, ready to be enjoyed and appreciated.



© Darby Patterson, 2008
At left, Frostad patineur Jamie Fantozzi
sprays a chemical mix on a piece
while heating it with a torch. The
various chemicals react with the
metal, water and heat to create various
natural finishes on a bronze piece.