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Each shell must be prepared before it can be dewaxed and poured. Most pieces simply have the bottom of the cup removed. Larger pieces, however, may require additional venting or clips. Clips are placed along the edges of pieces to help hold them together as this is often an area of greater pressure. Additional venting consists of drilling the piece in the end opposite of the cup. This allows the wax to escape and relieve pressure in places where it might otherwise rupture from the expanding wax.
Dewaxing and Curing
Once the shell is prepared, the kiln is preheated to approximately 1100 F. The pieces are placed in the kiln and the wax is rapidly melted out of the piece. The high temperature allows the wax to be melted out quickly, without causing stress on the shell from expansion. The wax removed from the shell is then captured and recycled.
Most of the wax will drain out of the piece in the first fifteen minutes, but the kiln is run a full hour to make sure that the shell has time to cure and harden. It is also necessary for the shells to be around 1650 F for the pour itself. Too low a heat will cause the shell to not cure completely as well as being subject to thermal shock when the bronze is poured into it. Too high a heat will cause the shell to become brittle and crack.
Debris Removal and Patching
Once the piece is removed from the kiln, air is blown into the cup of the piece to help remove excess shell, dust, and other debris from the cup, sprues and vents. The shell is quickly examined for defects, and all cracks and drilled holes are patched.
Burying the Piece
Once the piece has been patched, the last step before a piece is poured is to partially bury it in sand. The sand holds the piece in place during a pour, helps to contain the metal in the event of a leak or blowout, and can even be used to help insulate the piece causing it to retain heat if necessary. After it is buried it is capped with a piece of fiberfrax to keep random debris from falling into the cup of the piece and to help the shell retain heat until it has been poured.
Heartland Art Bronze Inc.
11628 254th Street
Lawrence, Kansas 66044-7331
Phone # 785-842-3626
Fax # 785-842-7011
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