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Moldmaking techniques match artist's sculpting style.

Moldmaking Techniques Match Artist's Sculpting Style

Surrounded by rich farmland overlooking Gardiners Bay at the eastern tip of Long Island's North Fork, sculptor Robert Berks has converted an old schoolhouse into a superbly equipped art studio. Here he has developed a distinctive, knobby artistic style as well as several unique applications of new moldmaking techniques and materials to facilitate the faithful bronze reproductions of his difficult-to-cast work.

An internationally recognized sculptor, Berks has created hundreds of portraits sculptures of important personalities. Two of his noted pieces include a large bronze monument of Albert Einstein, and his most complex piece, a large kneeling figure of Carolus Linnaeus, originator of the science of botany. The bronze statue of the kneeling Linnaeus surrounded by shrubs, flowers and wildlife was commissioned for the Chicago Botanic Garden in Glencoe, IL.

Berk's unique modeling technique utilizes great random facets of clay to create rough, dramatic accents and high-lights. By its very nature, however, this technique creates technical problems in casting his plasteline clay models into bronze.

In planning the Linnaeus work, Berks was faced with the challenge of making a rubber mold from this, his largest sculpture, because of the sculpture's numerous deep cavities. From the rubber, a wax duplicate of the original was made. The wax was coated with a ceramic shell into which bronze was cast after the wax was melted out of the shell. Through the skillful use and combination of materials, he was able to overcome the difficult technical problems.

His problem was not so much one of removing a rubber mold from the clay (because the clay would normally be destroyed when the mold was removed) but, rather, how could a rubber mold stiff enough to reproduce the original be fabricated without sagging and distorting. It had to be applied easily to the largely inaccessible interiors of the numerous fist-sized cavities then be withdrawn from the wax cavity without breaking the fragile wax or tearing the rubber and leaving it and other debris in the cavity.

A polyurethane mold compound allowed Berks to accurately reproduce every detail of his rough-surfaced clay model. He found that deep, negative draft cavities could be transferred accurately from clay to bronze with a process he developed using collapsible bladders in the rubber mold. Then, he looked for a better way to apply the uncured rubber with a mayonnaise-like consistency to his clay model.

A few other artists had used the rubber, premixed in containers with an added thickener, to cover their plasteline models, applying the mixture by brush or trowel. Berks went one step further. He combined the precise mix of components all in one application using a spray machine that precisely metered the chemicals. He blended the isocyanate component with a flexibilizer component to make Part A of the mixture, and the polyol component with the thickening agent to make Part B. The machine-metered blend yielded the exact mix ratio by volume required, making the mold much more efficiently and effectively.

The mixture, with the thickener already added to the polyol portion prior to spraying, remains liquid for several seconds after the Part A and Part B components are mixed as they pass through the spray gun. The liquid immediately fills the deepest cavities, flowing into the finest detail, and gels to a thixotropic, viscous paste that does not flow off the clay surface.

The rubber that Berks used remains in a pasty state for about 30 minutes, and can be trowelled and moved about during this period if desired. Using this technique, Berks applied the first coat of rubber about 1/8 in. to 1/4 in. thick onto every surface and cavity of the clay Linnaeus model.

While the first coat of rubber was curing, the wire bases of mold-to-shell fasteners were pressed gently into the first coat of soft rubber. The base of each anchor consisted of a 3 in. x 3 in. square of 1/4 in. hardware (wire) cloth. From the center of each square extended a rod of clay encasing a double piece of strong cord anchored through the hardware cloth to a nail. Subsequent coats of rubber firmly buried the anchor in rubber while the erect clay rod extended up through the succeeding layers of rubber and ultimately through the shell. After the shell was in place, the clay rod was removed exposing the loop of stout cord which had been encased in the clay. By inserting a stick through the cord and twisting, the rubber was held tightly against the rigid shell to maintain its shape once the mold was removed from the model.

While the first coat of rubber was in the pasty state, Berks pressed sheets of polyethylene film (spanning the deepest section of each cavity) into the rubber to create a pocket beneath the plastic film. When the next coat of rubber was sprayed onto the sides of the cavity, it created a new bottom wall, literally a rubber-surrounded, air-filled bladder or pocket beneath it. This process was repeated several times, if required, until all undercuts were filled with multiple bladders, one above the other. The rubber-covered model presented a smooth surface ready for the shell to be applied. A small vent hole was poked through all the bladders so they would compress easily when required. The rubber was stiff enough to maintain the desired shape yet compressed during demolding and able to spring back to its original shape.

A "mother mold" or shell is necessary as a backup for a rubber mold and rigid and dimensionally stable enough to hold the thin-walled rubber mold in its original position. The rubber mold reproduces detail and undercuts, but the rigid shell maintains the overall shape.

Shell molds are often made of plaster, but for such a large mold, a conventional "splashed up" plaster shell would have been too thick and so heavy the wet plaster could have caused distortion of the model. Berks used a special spray chopper gun, often used in boatbuilding, to apply a fiberglass shell. The gun sprays liquid polyester resin and chopped strands of fiberglass over the rubbermold that had been coated with a release agent. The resin/fiberglass mix hardened to a strong lightweight shell.

Although the fiberglass shell performed well, it was expensive, flammable and had a disagreeable odor. Berks found that he could spray on a thin, lightweight, fiberglass-reinforced plaster shell that was no more than 1/4 in. thick. This shell proved to be very strong, less costly, easily handled, odorless and durable. The shell for the mold of Linnaeus was made of fiberglass reinforced plaster.

As with the rubber, several coats are involved in constructing the shell. After the first coat of plaster is sprayed on, burlap strips or ties are imbedded in the wet plaster. When the first coat is hard, pipes bent to fit closely, but several inches above the shell, are tied into place with the burlap strips. At the ends of the pipes crossing each section of the mold are steel plates welded with holes punched in each plate. The pipes from one section of the shell are joined to the pipes from another section by bolting together the plates. The next several coats of plaster and chopped glass fiber are applied. When cured and the sections of the shell are taken apart, they can be reassembled rigidly and in perfect alignment by bolting the matching plates at the pipe ends.

Berks' studio is better equipped than most, with MIG and TIG arc and gas welding equipment, drill presses, lathes, a giant bandsaw and tools of every description. Outside his studio, he has built a 60 ft x 60 ft concrete platform with a steel turntable in the center covered by a self-furling canopy. This allows him to work on his large pieces under natural light conditions. He has a crane to reach high parts of his creations, and uses a precision 9:1 pantograph, constructed from accurately machined aluminum beams, to scale up his small originals. He even has heavy-duty mixing and extruding machines to make his own plasteline.

New rubber materials and innovative uses of fibers, plastics and tools are all functional but lifeless materials. In the hands of Robert Berks and other talented artists, they are stimulating new techniques and aids that allow the creation of broader forms of cast metal art for society's enjoyment.

Linnaeus would understand and applaud that.

PHOTO : Bronze monument by sculptor Robert Berks commissioned for the Chicago Botanic Garden, Glencoe, IL.

PHOTO : Hedgehog peering out from his deep burrow is an example of the large cavities Berks managed in making the Linnaeus molds.

PHOTO : Berks created collapsible polyethylene film bladders to fill deeply undercut cavities to allow rubber molds to be withdrawn without damage to the wax or mold. The urethane covered plastic sheets act like springs to maintain exact shape of model's wall.

PHOTO : Rubber-covered portion of sculpture ready for application of a rigid shell. Rods to hold the mold to the shell project form rubber; aluminum strips divide mold into sections.

PHOTO : Berks (l) assists in removing urethane rubber mold from the model's massive face.
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Title Annotation:special section: Artcasting '90; sculptor Robert Berks
Author:Bex, Tom
Publication:Modern Casting
Date:Jul 1, 1990
Words:1524
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