Bronze and other cast metal sculptures are the result of a collaboration between the artist and foundry. Unlike paintings and hand-made artworks that are appreciated as one-of-a-kind originals, most sculptures are produced in limited editions of vaying sizes. The practice was born out of the need to spread the manufacturing costs over a number of castings. Whatever the edition size, all castings made from first-generation molds are considered originals. Reproductions, or copies, come out of molds that have been formed, not from the artist's original model, but from a previously cast piece.
There are two basic methods of casting a bronze. The simpler technique of sand casting uses molds made of compact, fine sand. The lost-wax method translated from the French "cire perdue" is much more complex and common with modern foundries. Although technology has contributed to raising the consistency level in production quality, the steps involved in the lost-wax process have not changed since antiquity.
You can find many articles on the Web describing the lost-wax method. With a few edits of my own, the following was copied from the BronzeWorks semi-annual newsletters published by Mark Hopkins Sculpture, Inc., between Autumn 1995 and 1998. The Legends foundry now Starlite Originals applies essentially the same process in creating their Mixed Media sculptures. The color photos below were scanned from the Legends 1996 Collection Catalog; the black-and-white images are the property of Mark Hopkins Sculpture.
Sculptors usually work in clay or a similar material like plastiline, which is a mixture of clay, oil, and wax. The completed original model is delivered to the foundry, where the casting process begins with mold making. This procedure is very demanding due to the necessity of protecting the fragile original artwork from damage. Careful thought precedes any action at this point, as the level of difficulty of many future stages of casting are dependent upon a well executed mold.
The artwork must often be cut into multiple fragments if the angles, size, or complexity make it impossible to mold in one piece. The mold maker must make critical decisions concerning the division of the mold, as it will be made in two halves.
The original sculpture or fragment is surrounded, one half at a time, by a layer of liquid rubber which has been formulated to flow into every detail of the artwork. Another outside layer, or mother jacket, consists of plaster and holds the inner rubber layer firmly in place to prevent shifting or bending.
When the two halves of the plaster mold are completed around the original model, they are painstakingly pried apart and the artwork removed. The original is used again and again in the creation of more first-generation molds until the edition size is met, or the model deteriorates. The two halves of each mold are precisely reassembled; at their core is a vacancy in the exact shape of the original.
Through an opening which has been left for this purpose, hot wax is poured into the mold. The wax cools and takes on the shape of the artwork. The wax replica can then be carefully removed from the mold, ready for the next stage of the casting process.
When a wax sculpture is removed from the mold, no matter how carefully it has been molded and poured, there are small flaws which must be repaired, or chased. Equipped with sculpting tools, torches, and specialty waxes, foundry artisans smooth away bubbles and seam lines. They reassemble any parts of the piece which had to be molded separately. When finished, their piece will be an exact replica of the original work as it was created by the artist.
At this stage, the wax model must be prepared for an additional molding step. Wax sprues are attached to the wax sculpture to channel the molten metal through the mold and allow hot gases to escape. A ceramic cup is added which will act as a funnel through which the molten bronze will be poured.
The wax replica is taken into a temperature and humidity controlled room where timed mixing paddles keep liquid ceramic, called slurry, in constant suspension in large vats. The wax sculptures, which are very fragile, are carefully dipped into the liquid slurry, then dusted with fine silica crystals. They are allowed to dry for about four hours in the controlled environment, then dipped once more into the liquid slurry and dusted again with sand.
This dipping and dusting procedure is repeated over and over, using progressively coarser sand. With hours of drying time between each coat, the entire slurry cycle usually takes three days or longer. Eventually, the layers become thick enough to withstand the rigors of the remaining casting processes. The wax replica is now fully encased in a ceramic shell one quarter to one half inch thick, awaiting the fires of the next step.
Before the ceramic shell can be used to pour bronze, it must be freed from the wax sculpture around which the mold is formed. To remove the wax without damage to the shell, the mold is heated to melt out the wax. If not done correctly, the shell will be cracked by the expansion of the wax as it is heated.
There are several methods of wax removal. Some foundries use an autoclave, in which steam quickly melts out the wax in a pressurized chamber. Others like Mark Hopkins Sculpture use a more basic method called "flashing." A furnace is fired up to an intense heat, and each wax-filled shell is inserted into the furnace using a mobile, long handled rack suspended from the ceiling. The wax inside the shell is superheated and shoots out openings left for its escape.
The technician responsible for this stage watches carefully for signs that the pressure on the inside surface of the shell has been sufficiently relieved. When he feels the shell is ready, it is removed from the furnace and placed in a large melt-out oven. The remaining wax can then be melted out more slowly at a lower temperature. All the wax is thus removed, or "lost," leaving the empty ceramic shell ready to receive the bronze.
Just prior to the pouring stage, the shell is heated to nearly 2000°F to fuse the silica and prevent the mold from shattering when it comes in contact with the molten bronze. The metal is melted in a crucible in a furnace, then poured into the shell where it is left to cool.
The ceramic shell is hammered off to reveal bronze in the exact image of the wax sculpture on which the shell was molded.
Sandblasting removes the remaining shell material from the bronze, leaving it clean but rough and unfinished. Although the bronze has been cast, it still must be "chased" and polished. First, the gates and sprues that were added to aid in the casting process are removed, usually with a plasma cutter. The sculpture is then examined for casting defects and repaired as necessary. Welding can correct most problems. It can also unite the separate parts of a sculpture that had to be cast in several pieces due to an intricate design.
During metal chasing, welded areas are retextured, the stubs of spruing filed down, and other areas cleaned and polished. Each metal worker must be able to reproduce any needed texture or surface treatment to make sure that the sculpture remains true in every detail to the original design.
When all the metal work has been completed, the sculpture receives a final polish before the coloring, or patina, is applied.
Following specific instructions from the artist regarding the colors to use, patina workers meticulously apply various chemicals to the metal surface. Temperature and humidity can quickly alter the reaction of the bronze to the chemicals. Artisans at the Legends foundry use a blowtorch to speed the oxidation process, which fuses the acid color patinas to the metal surface.
When the sculpture has attained the desired coloration, the patina is sealed with hard waxes and polished to a soft luster. If it passes all quality inspections, a lasting piece of art is born.
Commissioned public monuments notwithstanding, most sculptures cast in bronze or other metals are produced in limited editions, the size of which is aimed at meeting profit objectives while coming just short of anticipated market demand. The sculptor's participation in the manufacturing process is usually limited to giving input on matters such as fragmenting the master model into parts for mold making, and what color patinas to use. Foundry artisans perform the tasks of creating molds, casting metal, and finishing.
Edition sizes may be as small as 10 or fewer, and as large as a few thousand. Heat and other factors inevitably put wear on the mold during each casting. The mold is discarded when it deteriorates to the point where it no longer meets quality standards. Depending on how large and detailed the design is, each mold may produce any number of castings, from just a few to 25 or more. The edition size dictates how many molds are made from the original clay model, and every sculpture created from these first-generation molds is an original. Reproductions come from molds that were formed from a finished sculpture or another artist's interpretation of the original artwork.
An artist proof (AP) is the first casting to come out of a mold. Subsequent castings are often referred to as LE's, for limited editions. To ensure the mold is correct and other phases of the manufacturing process adhere to specification, artist proofs go through more rigorous quality control inspections and typically cost more than the LE's. Since every mold yields one AP, limited edition sculptures will almost always have multiple AP's, which do not count toward the edition size.
Many collectors favor sculptures that carry a relatively low LE number. The desirability of a piece numbered 1 or 5 is purely psychological and has nothing to do with its physical characteristics. Indeed, an LE 25 may well be the final casting out of a mold before the latter becomes unusable and is tossed away, whereas a high LE number like 1274 could be only the second casting from another mold, following the AP. As long as the foundry enforces the proper quality standards, no original should be "superior" to another from the same edition.
LE numbers are usually stamped onto sculptures in the finishing stages of production. Except for numbers like an LE 1, it's unlikely that foundries sequentially stamp the numbers in the exact same order the sculptures were cast, unless the edition size is very small. In other words, an LE 250 out of 250 should not be represented as the last sculpture made unless the foundry expressly certifies it as such.
It is worth noting that an edition size of 150 doesn't always mean all 150 LE's were produced. Whether it's weak sales or any other reason, the foundry may decide to stop production of a sculpture and retire the piece before the edition size has been reached. The sculpture is reported as sold out when the entire edition has been distributed to authorized dealers and retailers.
The above text authored by manitouj.com (revised 2014). Copyright © 2012
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