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from clay to bronze paul oestreicher

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.

kenyan dancer
Sculptor Tuck Langland works on the clay for his sculpture "Kenyan Dancer".

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 self-hardening/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.

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.

oscar robertson
Sculptor Blair Buswell made this larger-than-life armature to support his monumental sculpture of basketball legend Oscar Robertson. Blair covered this steel armature with foam before adding the clay.
alfred levitt
Sculptor Paul Oestreicher makes finishing touches to the clay version of his sculpture of painter Alfred Levitt.

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.

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).

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 waste-mold 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 Tuck Langland applies the first coat of rubber by brush to make the mold of his sculpture "Dawn". dawn
Above: Sculptor Tuck Langland applies the third coat of rubber by spatula. dawn
Above: Sculptor Tuck Langland applies the top plaster "jacket", or "mother mold", to hold the form of the mold.
pouring wax
A worker pours melted wax into the "mother mold" through a hole in order to make a wax positive.

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 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 the "wax chasing" 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.

wax chasing
A wax chaser at Schaefer Foundry in Arlington, TX, works on the wax with small tools to remove any imperfections. Photograph by Elizabeth Loggins.
A diagram of the sprue system by Sculptor Tuck Langland.

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 at Schaefer Foundry in Arlington, TX, attaches a sprue system to a wax portion. Photograph by Elizabeth Loggins.
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.

pouring bronze
The workers at Schaefer Foundry in Arlington, TX, carefully lift up the crucible, containing the molten bronze, out of its heating furnace. Photograph by Elizabeth Loggins.

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 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.

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.

Right: A worker at Schaefer Foundry in Arlington, TX, pours the molten bronze into ceramic shells. Photograph by Elizabeth Loggins. pouring bronze
break out
Sculptor Tuck Langland breaks apart a cooled shell to separate the metal inside from the shell.

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".

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 high-speed burrs, some of which are quite similar to those used by a dentist, to perfect the final bronze.

metal chasing
A metal chaser at Schaefer Foundry in Arlington, TX, works on the metal with small tools to remove any imperfections in the final bronze. Photograph by Elizabeth Loggins.

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 at Schaefer Foundry in Arlington, TX, carefully welds a bronze bird to a branch. The bird's wings need to be added as well. Photograph by Elizabeth Loggins.
A worker at Schaefer Foundry in Arlington, TX, uses the sand-blasting cabinet. Photograph by Elizabeth Loggins.

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.

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.

J.D. uses a blow torch to heat a bronze in preparation to apply the patina. Photograph by Elizabeth Loggins.
patina head of dawn
The bare metal, before the patina is applied, of "Head of Dawn" by Sculptor Tuck Langland. patina head of dawn
"Head of Dawn" by Sculptor Tuck Langland after the final patina is applied (with ferric nitrate over liver of sulfur).

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.

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.

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: The finished sculpture "Kenyan Dancer" by Sculptor Tuck Langland. kenyan dancer
tuck langland
Left: "From Clay to Bronze: A Studio Guide to Figurative Sculpture" by Sculptor Tuck Langland.

17. Summary: The Lost-Wax Process

Volumes have been written on the subject of the "lost-wax" process as well as the procedures of mold-making, casting, and patinazation. We hope you have found this overview of the "lost-wax" process interesting and enlightening.

For further information about the art of the “lost-wax” process, Sculptureworks recommends the book From Clay to Bronze: A Studio Guide to Figurative Sculpture (pictured at left) by Sculptor Tuck Langland, one of our Sculptureworks artists.

Also, the below diagram from Tuck's book gives a visual summary of the complex "lost-wax" process.

Right: Diagram by Sculptor Tuck Langland showing a summary of the “lost-wax” 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.

diagram of the lost-wax process
The above diagram is found in Sculptor Tuck Langland's book "From Clay to Bronze: A Studio Guide to Figurative Sculpture".

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, lighter-weight and less-expensive alternatives to bronze. Pewter, an alloy of nickel and silver, is another less-costly 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.

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