So, exactly how does a clay sculpture become a bronze sculpture? Each time you visit a museum, a studio, or a gallery, knowing the answer to this question will enrich your art experience. That is precisely why I have included this explanation. I have attempted to distill the ancient and complicated process into five steps which explain each part of the Lost Wax Casting process. No wonder bronze statues are costly. It is not only the talents and time of the artist, but this casting process itself is very time consuming and requires skilled artisans. Enjoy!
THE ANCIENT LOST WAX PROCESS
The origins of the lost-wax process of metal casting, called cire perdue, began in the Bronze Age, somewhere between 4,000 - 3,000 BC. Although materials have improved over time, bronzes are still cast today using the very same process as in ancient times. As you will learn, the high cost of a bronze sculpture is due to the many hours of hands-on-labor involved. The clay sculpture is the first step to a long process of completion. The entire process in the foundry takes from six weeks to four months to produce a single bronze.
STEPS IN MAKING A BRONZE
In order to simplify a very complicated process, think of the process as five alternating positive and negative steps. For example, your foot is a "positive" form. When you walk along a beach and leave a footprint, that is the "negative" form. The negative form is the exact same as your foot, but in reverse.
Step #1: The Artist Completes a Sculpture (1st Positive)
The original sculpture can be in clay, wood, stone or metal. This is the first positive.
Step #2: Making the Mold (1st Negative)
A mold is made of the original sculpture by coating it with several layers of rubber. Another mold is made around the rubber, out of plaster, perhaps with burlap mixed in, to support the supple rubber. This is called a "mother mold". Most sculptures cannot be "molded" in one piece. For example, a sculpture with raised arms would require the arms to be molded separately. Depending on the original, several molds might be needed. The original clay is now removed from the mold. The mold is now empty. The empty mold is the first negative of the process.
Step #3: A Hollow Wax Replica (2nd Positive)
Molten wax is poured into and out of the mold. About three coats or so will be necessary to make a hollow replica of the original piece. This process must be done smoothly and evenly so a workable mold is produced. Cool wax or uneven pouring will "pancake". The wax replica should be a consistent thickness of about 1/8". The hollow wax replica is removed from the mold after it is completely cooled. Any surface imperfections in the wax are corrected. This is called "working the wax". Each casting in a sculpture edition required a separate wax. The Mold can be used for the entire edition, but a new wax is poured each time.
Step #4: Preparation/Engineering for the Bronze Pour (2nd Negative)
For the wax replica to be used to cast, it must be "sprued" and "gated". This means that a wax pouring cup, wax rods, called "sprues" that will later serve to channel molten metal into the piece, and air vents to release trapped air when the metal is poured, are strategically attached to the wax replica. All of this preparation allows for a bronze pour. At this point, an investment, or "ceramic shell", is applied to each wax by dipping it into a heat resistant liquid, or "slurry", and then coating it with a heat resistant sand or "stucco". Six to twelve coats are applied to the wax replica. Each coating must be completely dry before the next coat is applied. This step normally takes between one and two weeks. This ceramic shell over the wax replica has created the second negative.
Step #5: "LOST" Wax is "lost" (3rd and final Positive)
The step creates a negative in the shell. When the ceramic shell covering the wax is completely dry, the piece is inverted and placed in a kiln at 1800 degrees which causes the shell to become strong. The term "lost wax" comes from the fact that at this temperature, the wax inside the ceramic shell melts and pours out. This empty ceramic shell, the second negative, is placed with the pouring cup (open end) uppermost. 2200 degree molten bronze is poured into the cup and down through the sprues into the cavities of the shell. As the bronze cools, the last positive is created.
Step #6: Finishing the Bronze Casting
When the bronze has cooled, the ceramic shell is broken away to reveal the bronze casting. The (bronze) sprues are cut off, and the sculpture is sand blasted. This is the point of the process when any pieces that were cast separately are welded back together. The piece is then worked so that the seams are unnoticeable. The piece will resemble the surface of the original sculpture. This reworking of the surface is called "metal chasing". It takes many hours of labor intensive work.
Step #7: Patination - Coloring the Bronze
The final step is the "patina" - the coloring of the bronze. The ancient Asians would bury their bronzes to naturally oxidize them, sometimes for years. Today the oxidation and coloring can take place within hours.
What exactly is patination? It is an art in itself! Patination is the process of applying various chemicals to the bronze to give it different colors and effects on its surface. There are three main chemicals that are used to produce the colors:
1. Potassium sulphide is used for golds, browns and black
2. Ferric nitrate is used for golds, reds and browns
3. Copper nitrate is used for greens and blues
Many chemicals and mixes of chemicals can be used for this process. There are also many ways to apply/complete this step. The most widely used is the chemical application. The "patina" is most widely applied by brushing or spraying various chemicals onto the metal with or without heat. Different chemicals are used to create a variety of colors. Often acrylic is used to simulate chemical colors.
Step #8: Applying Wax Polish
With the patination now completed, it is ready for the application of wax polish. The wax polish enhances the bronze, revealing the subtle markings. The skill and understanding of a good patination only comes with experience, as it is hard to fully comprehend what the outcome of the color will be until the wax is applied. The wax is applied before the bronze has fully cooled. The bronze is now left to cool completely before it is buffed with a soft polishing cloth. It then has another application of wax, applied cold, and buffed up again to produce a wonderful shine on its beautiful surface.
Each bronze is truly unique because of the extensive hands-on process that it goes through. It is extremely difficult to get even two pieces in any edition to look exactly the same. It takes both the Artist and the Foundry to create a bronze sculpture. Sculpture is truly an art form that lasts a lifetime . . . and beyond.
Once a year the bronze should be wiped with a clean, soft rag. 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 gease.
The book, The Care of Bronze Sculpture, by Patrick Kipper.
Outdoor bronzes should be treated twice a year by cleaning and waxing the metal.