The Story of Sculpture: From Clay to Bronze
Introduction: The Art of the Lost-Wax Process
The earliest known “lost-wax” castings date from the early dynasties of Egypt, nearly 7,000
years ago, when metal was poured into “investments” of fired clay that had been shaped with
the help of wax that was melted, or “lost”. Then, sometime between 4,000 and 3,000 B.C.,
bronze was discovered, probably by accident, as being a metal that was harder than copper or
tin alone. Thus began the era known as the Bronze Age.
Ancient “lost-wax” bronze castings have withstood the centuries, visually telling the tale of past
cultures, their religions, and their social structures. For example, Chinese bronzes depicted
ceremonial images; Indian and Egyptian castings symbolized deities; Africans cast images of
nature; and the Greeks recreated the human form.
Many of these cultures have since grown obsolete, their religions have evolved and societies have
changed. Elements of the “lost-wax” process have been refined. Yet today, bronze casting is essentially
the same as it was in 2,000 B.C. during the Akkadian period.
Bronze is an alloy of 95% copper, 4% silicon and 1% manganese with traces of other elements such as
iron. Silicon bronze has been the bronze of choice for fine art castings since its development in the 1920s.
It is corrosion-resistant, strong, resilient, formable and weldable. Also known as "hot-cast" bronze, a fine
art "lost-wax" casting of silicon bronze is created through many labor-intensive steps.
1. Making the Original Clay Sculpture
To begin the process of making a bronze sculpture, most sculptors
choose to make an original out of clay. There are multitudes of different
clays available for the sculptor. The three main categories of clay used
for sculpture are water-based clay, oil-based clay, and selfhardening/low-fire clay. Choosing the proper clay for a particular project
may simply be a matter of preference. Experience, however, is perhaps
the best teacher when deciding what type of clay to use.
A sculptor works on the clay for his
sculpture.
Most of the artists with Sculptureworks prefer to work with oil-based clay
(also known as plasticene, plastilene, or plastilina). The main reason for
working with oil-based clay is because it never dries or hardens. It can
always be softened and reworked if a change needs to be made. The
main concern with using oil-based clay is how to have the clay soft
enough to build the sculpture rapidly and yet at the same time have the
clay hard enough to produce good detail. The usual solution is to choose
a clay that is hard enough for detail work and then just use heat to
soften the clay for buildup.
Very small sculptures are often modeled directly in wax which, though
more difficult to model, allows for maximum detail.
2. The Use of Armatures
An armature in a sculpture is a material used for supporting the weight of
the clay. An armature is usually necessary when portraying a figure in
motion or in a standing posture. The armature serves as the underlying
support to the sculpture. In some cases, a skeletal formation can be made
out of wire, or other materials such as pipe or aluminum, that will help
hold the clay in place, so that the clay can be worked and detailed.
When working with large sculptures, weight is an important issue. Thus,
after the initial armature is constructed, sculptors often use “fillers” such
as styrofoam or aluminum foil to “fill out” the form and reduce the amount
of clay being used. If most of the mass of the armature can be made with
filler, then the clay on top will be just a thin skin instead of a huge thick
mass that risks falling off.
Once the armature is built, the clay sculpture can then be formed around
and on the armature. The armature holds the parts of the sculpture in
place, allowing the sculptor the freedom to move the clay along the
armature to get the form that he or she wants.
For bas reliefs, an armature can be constructed which consists of a grid of
screws connected with wires to prevent the clay from sliding.
The sculptor made this larger-than-life
armature to support his monumental
sculpture. He then covered this steel
armature with foam before adding the
clay.
3. Finishing the Clay in Preparation for Molding
Once the final touches have been made to the original clay sculpture,
the final procedure before molding is touching up delicate details and
smoothing the surface of the clay to perfection. This is quite important
because any error or imperfection in the clay would be copied in the
molding process and would appear in every subsequent stage. Thus,
the smoother the clay, the less work will be needed on the wax replica
and the finished bronze casting itself.
Hook tools, or rake tools, are often used to cut the clay down to a new
contour and to leave small raised ridges on the surface. For smoother
surfaces, rubber or metal kidney tools can be used. For really fine
surfaces, a brush and a solvent are used to whisk away all tool marks.
Sculptor makes finishing touches to
the clay version of his sculpture.
Once the desired smoothness has been achieved, the piece should be
allowed to dry thoroughly before continuing to the molding process.
4. Making a Mold of the Clay Original
When the clay sculpture is completely smooth and finalized, a mold must be
made of the original clay. Excellent surface replication of the original can be
achieved with a polyurethane mold compound or a high-quality silicon
rubber.
The most common material used to produce a mold today is a material
known as Smooth On. It is a latex rubber that is very carefully formulated to
be able to pick up the minutest details in a work. The rubber mixture is
applied directly to the surface of the clay, using a brush to actually “paint” it
onto the surface. Special attention needs to be given to avoidance of air
bubbles that may be trapped within the rubber mixture.
Once the first coat of rubber has been applied to the surface, it must be
allowed to thoroughly dry (usually for 24 hours) before the next coat is
applied. All of the details from the original clay are now picked up within the
rubber material that has been painted on. A mold most generally consists of
three to five coats of rubber (applied over the course of several days).
Above: Sculptor applies the first coat
of rubber by brush to make the mold
of his sculpture.
Once the final coat of rubber has dried, a firm outer "jacket" is made to help
retain the shape of the more flexible rubber mold for pouring the wax
replica. This “jacket” is usually made out of plaster, Hydrocal, resin, or
epoxy, and is called the “mother mold”.
When all of this is complete and it is all dry, the outer "jacket" is removed,
and the rubber is then cut away from the clay, on what is called a parting
line (simply the division line for the mold). The sculpture has now gone from
a positive form to a negative form.
If the sculpture is large or complex, it will usually have to be divided into
smaller pieces, with each piece needing its own individual mold. The pieces
will be rejoined later, after the sculpture is cast in bronze, to make the
complete sculpture whole again.
The learning process for making an excellent multiple piece rubber mold can
take years. A novice should hire a professional, or make a plaster wastemold of the original and make a plaster cast (a positive). From this solid
positive a rubber mold can be made without risk of "losing" the original. The
highest quality mold will save much time and money in production.
Above: Sculptor applies the third coat
of rubber by spatula.
Above: Sculptor applies the top plaster
"jacket", or "mother mold", to hold the
form of the mold.
5. Pouring a Wax Replica From the Mold
With the “mother mold” complete, positive forms can now be generated
by pouring a wax replica. Pouring a wax is usually done in four stages or
coats. The first coat is applied by heating the wax to a temperature of
approximately 220 degrees Fahrenheit. While the wax is quite hot, the
mold is rotated about, in an attempt to coat the entire internal surface of
the mold with the melted liquid wax, which is poured into the “mother
mold” through a hole. The second coat of wax is applied slightly cooler,
at around 200 degrees. The final two coats go on at approximately 180
to 185 degrees.
A worker pours melted wax into the
"mother mold" through a hole in order
to make a wax positive.
A completed wax should be approximately 1/4” thick, or just slightly
thinner. When the wax cools and the mold is removed, a wax positive of
the sculpture emerges.
If the sculpture is a bas relief, or if it has a large mold that opens wide, it
is usually much easier to brush on the wax. Following the same
temperature guidelines per coat as above, the wax must be brushed on
beginning in the deepest hollows of the mold to avoid any drips.
6. Wax Chasing
"Wax chasing" is the procedure of repairing all of the imperfections that were
created during the pouring process of the wax to maintain the original details
within the mold. There will most always be a few air bubbles, seams, and
mold lines which will need to be removed from the surface of the wax
positive.
"Wax chasing" is usually performed at the foundry by a professional "wax
chaser". However, if the sculptor prefers, they can do it themselves.
The "wax chasing" procedure is done with small, delicate tools for scraping,
and with a hot tool for filling in the imperfections within the wax. Once the
wax is back to a perfectly finished state, the sculptor inspects the wax. If the
wax receives the sculptor's approval, it is ready to go to the foundry for
spruing.
A wax chaser works on the wax with
small tools to remove any
imperfections.
7. Spruing a Wax
The finished wax is a positive replica of the
original clay. A network of wax rods, called
sprues and gates, are next attached to the
positive wax model. These sprues will serve as a
type of channel system, which will feed the
molten metal to all of the areas of the sculpture,
as well as allow gases and air to escape. Also, a
wax funnel (called a pouring cup) is attached to
the gates for use during pouring.
A worker attaches a sprue system to a
wax portion.
A diagram of the sprue system by
Sculptor.
During the spruing process, special care is given to areas such as the tips of a
rabbit ear, or the fingers of a frog, to ensure that these fine areas will not “short
pour”. Once the spruing is completed, the wax is ready to go on to the shelling
process.
8. Ceramic Shell (Investment Casting)
The ceramic shell process requires a series of dipping the wax positive into a mixture called slurry to
create a hard shell. This ceramic shell, once dry, becomes a hard, durable shell around the entire
sculpture that is going to receive, hold, and shape the molten metal to produce the bronze figure.
The wax is first dipped into a solvent, which cleans any loose particles or debris from the surface of the
wax. The shell process is about to begin. The clean wax is dipped into a solution called prewet,
followed by two coats of a very fine grained slurry. This is known as the primary coating. This is where
all of the fine detailing in the piece is picked up. It is almost like grained silicon flour.
The shell then progresses through the slurry process, into different slurry mixtures, which are various
grades, gradually becoming courser with each coating. Each time the wax is coated with the ceramic
slurry mixture, it is allowed to dry thoroughly. Each coat adds support to the overall strength of the
shell. The ceramic shell process consists of seven to nine coats of slurry mixture. Once all of the coats
have dried, the ceramic shell is then sent to be “dewaxed”.
9. Melting/Burning Out the Wax
When the ceramic shell is complete, it is placed in a high pressure sealed oven, known as an autoclave.
High temperatures (1500 to 1800 degrees Fahrenheit) and pressure force the wax from the shell and the
wax melts out, thus becoming "lost". This is where the name “Lost-Wax Process” has been derived.
The wax has now left the shell, leaving the detailed impression within the shell, which will soon become
filled with the molten bronze. This step also hardens the ceramic shell in order to prepare the shell for
the extreme temperature of the molten bronze which it is about to receive.
10. Casting/Pouring the Bronze
When the ceramic shell is empty of wax, it is then re-fired and
made ready to have the molten bronze poured into it. All of
the hot ceramic shells are taken to the pouring floor, where
they are either placed in sand to stand by themselves, or they
are wired to a support frame to be held in place.
The workers carefully lift up the crucible, containing
the molten bronze, out of its heating furnace.
The solid blocks of bronze, meanwhile, are heated to a
temperature of approximately 2250 degrees Fahrenheit so that
liquid bronze is created. The liquid bronze is stirred and
prepared for the pour.
When the molten bronze is ready, the foundry workers very
carefully lift the crucible, containing the liquid bronze, out of its
heating furnace. The workers must wear protective face shields,
clothing, gloves, and boots.
Right: A
worker
pours the
molten
bronze into
ceramic
shells.
Moving quickly and very precisely, the foundry workers pour the
liquid bronze into each awaiting ceramic shell. When the ceramic
shells are all full with the poured bronze, they are then left to cool
for several hours. The negative space within the ceramic shells
have now become positive bronze castings.
11. Break Out
Once the bronze and ceramic shell has cooled, the ceramic shell is
broken off to separate the bronze metal from the shell. This is done with
hammers, tools, power tools, and sand blasters.
This process needs to be done very cautiously, though, in order not to
cause unwarranted damage to the metal surface. Once the entire
ceramic shell has been removed, the sprues (which have now become
metal) must also be cut away or sawed off.
At this stage, the uncovered bronze is considered a "raw metal".
Sculptor breaks apart a cooled shell to
separate the metal inside from the
shell.
12. Metal Chasing
"Metal chasing" is the process of finishing the metal back to the appearance
of the original. This process usually involves a fair amount of welding with a
high frequency welder. Grinders are often used to remove sprues. Angle
grinders with a variety of different grades of Roloc pads are used to finish the
surface of the bronze. Always start out with a courser pad and then work your
way down to a very fine pad when chasing the metal.
It may also be necessary to retexture some areas of the bronze. This can be
done with a tool known as a pencil grinder. This tool uses a variety of highspeed burrs, some of which are quite similar to those used by a dentist, to
perfect the final bronze.
A metal chaser works on the metal
with small tools to remove any
imperfections in the final bronze.
13. Metal Welding/Assembly
If the sculpture being created was rather large or complicated, it might have
been cut into pieces during the mold stage. This is necessary in order to be
able to make molds that would be small enough to pour bronze into evenly.
That is, instead of one big mold for a large piece, the piece may have been
broken up into five or ten different smaller pieces, each piece with its own
mold. Each smaller mold would thus be a different part of the whole
sculpture, so that when all of the pieces have been poured and chased, they
can then be fit back together to create the whole piece.
If this is the case, all of the separate bronze pieces must be welded together
to create the whole sculpture again. A metal welder will use a high frequency
welding torch to carefully weld the sculpture together. After the sculpture is
welded back to one piece, all of the weld lines must also be chased away so
that they disappear and it looks as though the sculpture was all one piece to
begin with.
A welder carefully welds a bronze bird
to a branch. The bird's wings need to
be added as well.
14. Sand-Blasting the Bronze
When the final piece is all welded together and chased to perfection, the
bronze is next sand-blasted (or bead-blasted) to make it very smooth and
shiny. Using a sand-blast cabinet is the most convenient way to sand-blast a
sculpture. The sculpture is placed inside of the cabinet and the lid is closed
securely. A worker can insert their arms into the cabinet through holes on
the side of the cabinet which have gloves attached to protect the worker's
arms. Wearing these protective gloves, the worker can then operate the
sand-blasting compressor while monitoring the progress through a window in
the cabinet.
A worker uses the sand-blasting
cabinet.
After the sculpture is sand-blasted, the sculptor once again inspects the
piece for accuracy. Now that the bronze has been sand-blasted, its true
gold-like color, almost like glowing platinum, is visible. The sand-blasting
also makes the surface of the bronze more porous, allowing the top surface
to be more open to penetration of the patina in the next stage.
15. Patinazation
The "patina" is the color of the bronze. "Patinazation" is
the process whereby a calculated chemical reaction
between bronze, acidic chemicals, and high temperatures
oxidize the surface of the metals. Certain chemicals will
produce certain predictable colors when they are sprayed
on the bronze and then heated up.
The patina process begins by heating (usually with a torch)
the surface of the bronze to a temperature of
approximately 450 to 475 degrees Fahrenheit. The surface
will have a nice golden appearance when it is ready. A
wide variety of techniques can then be used for chemical
application, the most common being done with natural
bristle brushes. Chemicals can also be applied with spatter
guns, spray bottles, and airbrushes.
Artist uses a blow torch to heat a bronze in preparation to
apply the patina.
The chemicals applied to the bronze will interact with the metal, creating a
colored or textured appearance to the surface of the bronze. The patina
has become quite an art in itself, and many beautiful combinations can be
achieved.
Browns, golds, and reds are typically produced through the use of Ferric
Nitrate. Greens and blues can be produced with Cupric Nitrate. Whites and
beiges can be produced with Bismuth Nitrate and Zinc Nitrate. Silver
Nitrate is very expensive, but if applied very hot and with persistence and
expertise, it can create a beautiful silvery-gray patina.
The ancient Asians would bury their bronzes to naturally oxidize them,
sometimes for years, in order to create patinas. Today, the oxidation and
coloring of bronze sculpture can take place within a matter of hours. But it
is still a delicate process that should be performed with caution and with
the proper equipment, preferably by a trained patineur.
The bare metal, before the patina is
applied
.
After the patina is applied, the patineur and the sculptor decide between
two sealants to protect the patina. The traditional method of protection is
to apply several thin coats of clear paste wax to the surface, which is then
lightly buffed with a soft, clean, cotton cloth. This patina should last
indefinitely indoors. However, if people repeatedly touch certain areas, the
bronze may be expected to eventually shine through.
Because today's outdoor atmosphere is more corrosive and carries a
higher content of manmade pollutants, a more durable lacquer, metal
protectant is recommended for any sculptures placed outdoors. Incralac™
is one such sealant designed specifically for copper and its alloys. Since
Incralac™ produces a plastic-looking, high-gloss finish, the sculpture must
be waxed to create the soft look of a traditional patina.
After the final patina is applied (with
ferric nitrate over liver of sulfur).
16. Completion: The Finished Sculpture
After the completion of all of the above steps, a unique bronze sculpture will have been produced and
can now be sold. Of course, if this piece sells and the sculptor wants to make another one, that sculptor
will then have to return to the “mother mold” and go through all of the steps again in order to make
another bronze sculpture.
One of the main things to keep in mind when viewing sculpture is that the "lost wax" process creates
truly unique sculptures. Although the sculptors examine each of the waxes, and double check each
finished metal, due to the handmade nature of this process, each cast and each patina will be similar but
still distinctively unique.
Right: Diagram showing a summary of the “lostwax” process (from clay to bronze) with solid, or
traditional, investment.
Top row, left to right: the original clay sculpture,
the rubber mold with shell (or "mother mold"),
pouring wax into the mold, the hollow wax cast,
and adding the sprue system.
Bottom row, left to right: the wax with
investment, burning out the wax, pouring the
bronze, and breaking the cast.
The "lost-wax" process explained above is primarily used for casting sculptures in bronze. Other
options, however, for casting sculpture include bonded bronze, bonded marble, and pewter. Bonded
bronze (AKA "cold-cast" bronze) is primarily resin. The surface is a skin of resin into which bronze
powder has been blended. Bonded marble is resin with marble powder. With a low-luster white surface,
it is subtle and quite beautiful. Bonded bronze and bonded marble are hand-cast, hand-finished, lighterweight and less-expensive alternatives to bronze. Pewter, an alloy of nickel and silver, is another lesscostly alternative.
Also, if you are interested in creating a sculpture using the "lost-wax" process, it is important to note
that shrinkage will occur in any casting process. The mold shrinks. The wax shrinks. The metal shrinks.
A sculpture which in clay was 26" long may be 25" long in metal.
18. Care for Bronze Sculpture
All bronze sculpture is an investment which must be properly cared for.
For indoor bronzes, the bronze should be wiped with a clean, soft rag once a year. Use a soft brush
and apply a coat of Johnson's Paste Wax or Tree Wax to the sculpture. Allow it to sit for an hour or so
and then buff it with a soft brush or rag. This will protect your bronze from the oil of human hands,
dust, and grease.
Outdoor bronzes should be treated twice a year by cleaning and waxing the metal. Generally, this
cleaning should be performed right before and right after summer. Waxing the bronze right before
summer is especially important because this will protect it during the hot summer months.