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Process

 

THE LOST WAX PROCESS

The lost wax technique is the most precise and popular method of art reproduction in bronze. An ancient process, it was refined in Egypt, China and Greece long before the Common Era. In this century, we have added a few modern elements to an otherwise unchanged technique.

While sometimes the casting is a one-of-a-kind, more frequently the casting is one of a limited edition. The number of reproductions in that edition is pre-determined by the artist and inscribed in each casting.

 

THE MOLD

In order to reproduce an artwork, a mold must first be made of the piece. This may be a one or two step process.

Sometimes, an artist may create work in clay that is not meant to be kept but is only created for reproduction. In this case, a less-destructible solid positive is required to be made first. This is called a waste mold: a plaster mold is formed around the soft clay, the original is removed in chunks, a mold release coated on the inside of the plaster mold and the mold filled with wet plaster. The original plaster mold is then chipped away (and wasted in the process) to reveal the plaster positive of the work. This cast remains the property of the artist, can be further worked in plaster, and if stored properly is available for many future molds to be made from the work.

When an artist has an original in clay or stone or whatever material, that they wish to keep intact and perhaps sell as the original, a flexible mold will be made, in however many parts are necessary. The mold can be of polyurethane or silicone rubber, and is surrounded by a supportive mother mold, made of fiberglass or plaster. Depending on the type of material used and the care taken in storage, a flexible mold can be re-used and stored for many years, but will not last forever.

 

THE WAX POSITIVE

Bronze sculptures are hollow or partially hollow, to minimize both weight and expense. To accomplish this, hot wax is poured, painted or slushed into the mold. When the wax cools, the mold is opened and the wax positive is removed. Depending on the size/complexity of the piece, other parts of the sculpture may be attached at this time or they may be cast separately and welded together later. The wax seam lines are chased and any surface flaws are fixed. At this point, the artist may sign the wax piece.

The thickness of the wax is very important as this will be the final thickness in metal; too thin may result in voids or weakness in the final cast, too thick may be heavy and more costly. For this reason foundries may not accept artists own wax casts or may reserve the right to refuse unevenly cast waxes positives.

 

SPRUING

Spruing

This is the term used for the system of wax tubes (gates) that will feed the molten metal into the piece during casting. A wax coffee cup is attached to the sprue system to allow the metal to be poured into mold. Correct spruing facilitates the flow of metal to reach all the areas in the casting, and can also help to minimize shrinkage in a piece with different thicknesses.

 

CERAMIC SHELL INVESTMENT

The wax positive is then dipped in the large vat of slurry. This is colloidal silica binder (a microscopic silica held in suspension) to which a refractory powder is added. The piece is then lightly dusted with refractory stucco and allowed to dry. This is repeated as many times as necessary for the size of piece. As the water evaporates, the silica binds closer with the refractory materials and thin layers of shells are built up into a strong casing. For larger sculptures, the ceramic shell may be reinforced with wire to ensure a successful casting.

 

DEWAXING & SHELL CURING

The ceramic shell now encases the wax positive. The wax is melted out of mold in a flash kiln, and then cured in a kiln at 1650 F for two hours. This negative mold is now ready to receive the hot metal, or it is cooled, inspected, and stored for a later pouring date.

 

POURING & BREAK-OUT

Evedure bronze, the best choice for sculpture, is a copper alloy consisting of copper, manganese and silicon. It is heated in a crucible to 2100 degrees F in the propane-fired melting furnace. The ceramic shell molds are pre-heated to avoid undue stress from the much hotter metal. After the pieces have cooled completely, the shell is chipped off, and the gates and cup are cut away.

 

FINISHING

The piece is now sandblasted to remove the last bits of shell. It is welded, chased and cleaned. Surfaces that were attached to sprues must be ground down and re-textured in the bronze. This can take hours, days, or weeks, depending on the size and complexity of the piece. There is a final sanding and inspection before bead blasting or hand sanding to prepare the work for patination.

 

PATINATION

Patination is the application of chemicals to the bronze to achieve colour. It can be an art form in its own right, with many different techniques, such as hot, cold, or buried patinas. There are also a wide variety of chemicals and pigments with which to spray, brush or dip. Outside sculptures might demand a different patina than an inside piece. The work should also be sealed, whether with hot wax or a commercial sealant.


Patina Process

 
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