The Art of Lost Wax Bronze
The following is designed to give an overview of the steps and processes that Paul Lindquist and other sculptors go through in order to deliver the finished fine art bronze sculpture. Each step requires a team of artisans, specific skills, many tools and considerable time. This is done either by the artist and or the foundry to create the finished fine art bronze sculpture. A piece of fine art that can last for centuries.
In the third millennium B.C., somewhere between the Black Sea and Persian Gulf, an artist crafted a vision in beeswax, covered it in liquid clay and cooked it in a fire. In the flames the wax melted and was replaced by empty space. Tin and cooper – alloys of bronze – were gathered and heated. Once melted, the metal was poured in to the empty cavity of the fire hardened clay. The metal cooled and the sculptor knocked the clay from the metal. The first bronze was cast. Ancient “Lost Wax” bronze castings have withstood time and remained for centuries, being one of the only remaining records of past cultures. Elements of the “Lost Wax” process have been refined, yet today bronze casting is essentially the same as it was in 2000 BC during the Akkadian period.
Before any art form can exist as a completed project, it must first exist as an idea in someone’s mind. The first step is the idea, the concept, the inspiration of the sculpture. This sometimes is the hardest step. Being able to conceive an idea that will work with balance, physics, and of course gravity, all play into having sculpture becoming a reality. This requires much research, many drawings, and planning.
Create Clay Original Sculpture:
Creating the image, sculpting the subject in clay and making the original sculpture is the next step in this process; this is called the model or positive. Determining how the finish sculptures will look and getting all proportions accrete is an integral part in the sculpting process. The way the clay sculpture develops and looks is exactly the way the finished bronze will look. Artists use many tools in order to create the clay sculpture, they include knives, scrapers, armatures, wire, and molds or reference materials.
Create Flexible (Rubber) Mold:
The mold is the negative shape that is made from the positive, into which the casting material is poured. In a way, a mold is a casting in itself, since it is usually formed of soft material or liquid that is placed around the positive and allowed to harden, taking a perfect negative impression of the positive. A common used term for flexible molds is rubber molds, though the material used is rarely actual rubber molds, most are made from silicone. The reason for a flexible mold is if you have a richly textured surface with a lot of detailed parts sticking out here and there, the flexible mold takes care of that. Also a flexible mold gives a very exacting detail, are easy to make and use. The use of molds allows an artist to reproduce his sculpture over again giving him the ability to make multiple copies.
Create Mother Mold:
The mother mold is a rigid shell that fits over the rubber to prevent it from flopping when it is removed from the sculpture and filled. This supporting mold (mother mold) is constructed over the rubber with plaster to firm the rubber mold and hold the shape of the sculpture. Once both molds (rubber & mother) are created, the next step is ready for the wax.
From the “negative” rubber mold, a wax “positive” is created. Wax is melted to about 200 degrees F, poured or painted into the rubber mold and evenly coated. This is repeated three times using cooler wax each time to avoid melting the previous coat, until the wax is approximately 3/16” thick. When the mold is opened and the rubber mold is peeled away, an almost perfect wax reproduction of the clay sculpture is removed.
“Wax Chasing” is the delicate process of removing the seams and repairing imperfections to the wax reproduction. This is done with heated customized soldering irons and fine tools. The goal is to make this wax reproduction as perfect as possible, because the way the wax looks is exactly the way the bronze sculpture will look.
Gating and Sprucing:
The “gates”, “spruces”, and “vents” are made of wax sticks and form the channels through which the wax will exit the “investment” (final mold) and the molten bronze will travel to create the sculpture. These wax sticks are affixed to the wax reproduction with heated tools. Later in the casting process, the space occupied by spruces, gates and vents become the runways through which the molten metal flows and trapped gases escape. Distribution of the bronze, low turbulence, ventilation and shrinkage are all important considerations in the science of gating and sprucing.
Investment (Ceramic Shell):
"Investment" is the process of building a rock-hard shell around the wax sculpture. Later in the process, when the wax has been melted out, the investment will serve as a mold for the molten bronze. For most of history, an investment consisting of plaster, sand and water was used to accomplish this task. In the last 15 years, a new technology called ceramic shell has become the industry standard. The ceramic shell technique begins by dipping the gated wax into vats of slurry followed immediately by a bath of sand. This process builds a very thin wall of silica around the wax. When repeated approximately 9 times, allowing for drying time in between dips, a hard ceramic shell, about ½" thick, forms around the wax. Prior to the invention of ceramic shell, solid plaster investment was used. To invest by the solid plaster method: tarpaper was loosely wrapped around the wax reproduction in the shape of a cylinder. The enclosed space surrounding the wax was then filled with a wet plaster and sand mixture. When the plaster hardened, the tarpaper was removed and a solid plaster investment is ready for "de-wax." Whether ceramic shell or plaster is used to make the shell, the wax is a "positive" which must disappear in order to create a cavity or "negative" for the bronze to fill. Thus the phrase "lost wax casting" comes from the process of the wax being melted or "lost" from the shell. Plaster built shells are then "de-waxed" in a high-pressure steam chamber known as an autoclave; ceramic invested shells are de-waxed in a kiln, melting out the wax.
The wax sculpture must disappear in order to create a cavity or “negative” for the bronze to fill. Thus the phase “lost wax casting” comes from the process of the wax being melted or “lost” from the shell. The ceramic shell molds are “de-waxed” in a large kiln and fired at 1600 degrees f.
While the bronze ingots are melted in the melt furnace between 2000 degrees F and 2100 degrees F, the ceramic shell molds are being heated in a kiln to about 1300 degrees F. When the “Dance of the Pour” begins, the crucible is lifted by a crane out of the furnace. At the same time, the glowing ceramic shells are brought out of the kiln to the pour area. Two Artisans operate the crane which holds the crucible in a “jacket”. The Artisan with the controls is the “lead pour”, the other Artisan maintaining the crucible balance is known as the “dead man”. A third member of the pour team pushes away dross and slag on the surface of the molten bronze. The alloy cast is known as Silicon Bronze. The metal is made up of the following elements: copper 95%, Silicon 4%, Manganese 1%.
“De-vesting is the process during which the investment (ceramic shell) is removed from the metal. Approximately one hour after the pour, the piece is cool enough to handle. Skill and strength are combined with hammers and power chisels to knock the investment off the freshly solidified metal. The gates and spruces must also be removed with a high intensity electric arc that can cut through the bronze like butter. The final step is to sandblast the fine investment from the bronze. When clean, the sculpture advances to the metal shop.
Like the wax chasing, bronze must also be chased or cleaned to address the slight imperfections that may result from the casting or shell process. On larger sculptures, where assembly of cast sections is required, chasing is essential to take down weld lines formed by the joining of two planes. Metal chasing usually starts with large electric or pneumatic grinders to remove the bulk of the unwanted metal. Then, more refined and smaller tools such as die and pencil grinders are used to re-create the artist’s subtle surface texture. Much as a house needs wood frame to stand, many monumental bronzes require a stainless steel internal structure to support the bronze “skin”. Larger than life-size bronzes may be analyzed by a structural engineer who recommends a support structure that can withstand earthquakes and high winds.
Patination is enhancement of the bronze by the chemical application of color and use of heat. The sculpture must be clean in order to accept the patina. Some patinas go on cold bronze, but most are applied to hot bronze, which means the sculpture must first be heated. A flame from a torch is then used to drive off the moisture in the bronze. It also warms the surface of the bronze and helps the surface accept the application of the chemical (Patina). These chemicals create a wide range of colors, both transparent and opaque, for example, Ferric Nitrate produces reds and browns, Cupric Nitrates creates the greens and blues , Sulphurated Potash produces black and Liver of Sulfur which can be the base to added other patinas on to it. Each foundry develops its own proprietary patinas that result from a carefully orchestrated blend of different chemicals, pigments and application techniques. The final step is putting a thin coat of clear wax over the bronze to enhance and preserve the patina.
The Mounting and the Base:
Not all sculptures need bases. Some sculptures stand perfectly well on their own with no base, or are modeled with an integral base. The base can operate like a frame of a painting, setting the sculpture apart from the world, lifting it and defining it as fine art. Bases take many sizes and shapes from round, to oval, to square. Bases are made from many different materials, wood, marble, metal and stone. The sculpture needs to be mounted into the base to secure and stabilize the sculpture; this requires drilling and tapping for screws or bolts and or welding the sculpture to the base. Once this is done it is ready to be packaged and shipped to its new owner or the next art exhibit.