Vermiculite has a bad reputation because it is often mined in the same places as asbestos, which can contaminate the vermiculite. Bullseye Vermiculite Board comes from a mine that is certified asbestos free. It is stronger, more durable, and less expensive than most fiberboard and can be cut and tooled like wood or particle board. Work in a well-ventilated area and wear a NIOSH-approved respirator whenever generating dusts. If you want your finished piece to be level and square, it is important to cut the vermiculite boards accurately.
Also, pre-drill and countersink screw holes so the board does not bloat or blow out when you screw it together. Use stainless steel screws to put the mold together as they will hold up to repeated firings without flaking. Do not use galvanized steel screws because upon firing, the galvanization will release toxic fumes and the screws will flake and cause contamination in your kiln.
Cut two long side boards, two short side boards, and one base board. Lay the boards out as an open box and pre-drill holes in the flat surface of the long side boards to connect them to the ends of the short side boards using a bit that has a diameter slightly smaller than the diameter of the stainless steel screws. Be sure to drill your holes on center to avoid blowing out the side of the board. Then screw the sidewalls together.
Next, set the base board on top of the assembled side boards and pre-drill holes to join it to the sides, and then screw it together. Then take the entire box apart and fire the vermiculite board at a rate of 500°F (278°C) per hour to a temperature of 1580°F (860°C) or about 55°F (30°C) higher than the temperature at which you will cast the glass. Hold at that temperature for half an hour, and then crash cool the kiln. Once the boards are cool, take them out and reassemble the sides using the stainless steel screws. Cut a piece of 3 mm fiber paper and set it on the base board, then set the assembled sides on top of the fiber paper, and screw the box together. Line the side walls with 3 mm fiber paper, making sure that it fits tightly, without bowing or leaving gaps in the corners.
Stephen Richard wrote:To avoid excess separator on my sand, I use an old sock (with no holes!) to dust the separator over the sand. This gives an adequate separation, without over doing the thickness of the separator.
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