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Casting bronze

Recently, the MacKenzie Art Gallery has been offering “Twilight Tours” conducted by local artists. We visited Vic Cicansky’s studio, Joe Fafard’s foundry in Pense, and even toured back lanes with Wilf Perrault.

Of all the tours, I found the one to Fafard’s foundry most fascinating, because it shed some light on one of the oldest technological processes in existence: the lost-wax method of casting bronze..

The lost-wax method appeared sometime before 2000 B.C., and gave birth to the Bronze Age: it was used to make sculptures, weapons, tools, jewelry and household utensils all over the world, from China to India to Africa to Greece. In all the thousands of years since, it has changed little. Today we even use variations of it to cast complex devices like superchargers and gas turbines out of special high-temperature alloys.

A bronze sculpture begins with a sculpture in some other medium. Fafard uses a special type of wax that softens under a heat lamp, but you could also use clay or wood, or even cast a natural object such as a bird’s nest.

The first step of the casting process is to create a rubber mold. The original sculpture is painted with a thin layer of rubber, one side at a time. When the rubber dries, a protective mold of reinforced plaster is built around it. You end up with two rubber molds, one for each half.

Next, the inside of the rubber molds is carefully painted with liquid wax, creating a wax shell “like a giant chocolate Easter bunny,” as Fafard describes it. The wax shells from the two half-molds are joined together to create a complete wax copy of the original work.

Square tubes and a large cup that look like a cowbell, also made of wax, are then added to the sculpture. Eventually, this plumbing will be used to remove the wax and pour in the bronze, so it has to be very carefully positioned.

Next the wax sculpture is “invested,” covered with a rock-hard shell. For most of history, investment has consisted of a thick layer of plaster, sand and water. In the last 15 years ago, however, a new technology called ceramic shell has taken over.

To create a ceramic shell around the wax sculpture, it’s first dipped into a special slurry, then into a bath of fine silica sand. Then it’s allowed to dry. After several repetitions, a hard sandy shell a centimetre or so thick forms.

Once dry, the shell is heated in a kiln. The wax quickly melts and flows out through the tubing. (It’s very important that it all escapes: any that is left could explode when the hot bronze is poured in.) Then the temperature is boosted even higher to turn the sandy shell into ceramic.

The next step is to pour in the bronze. Bronze ingots are melted in a large crucible in a furnace in the foundry’s floor. The crucible is lifted out by two workers (using very long handles, because the bronze is well over 1000 degrees C), and poured into the sculpture, which is held in a metal rack, through the attached cup. This has to be done quickly, while the bronze is still fluid enough to fill every nook and cranny.

It only takes about an hour for a newly cast piece to cool enough to handle. The investment, which usually cracks as the bronze cools and shrinks, is removed. The tubing and cup are cut off, and small pieces that were cast at the same time are welded into place to fill any holes. Other imperfections must also be corrected. Sometimes, with large pieces, the sculpture must be assembled in sections, each cast separately.

The final step is patination, chemically applying color. Three water-soluble compounds form the basis for most colors. Ferric nitrate produces reds and browns, cupric nitrate creates greens and blues and sulphurated potash produces black. Although patination looks like paint, it’s not; the colors aren’t just slapped on the surface of the bronze, but are really a form of corrosion, the result of the bronze reacting to the patinating chemicals.

It’s intriguing to think, as you look at a work by Fafard or another artist, of the age-old technology behind it. So elegant is it that millennia from now, when our own vaunted 20th-century technology has long been forgotten, some artist somewhere will probably still be using the lost-wax method to create works of art.

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