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Kiln-drying plaster/silica molds

This is part two of a multi-part series on air-drying plaster/silica molds.  Click here to go to part one.

When a plaster/silica mold is heated, the mold goes through three phases.  Regardless of the size or shape of the mold, it's important to recognize each of these phases and to soak the mold at each phase so that the mold will dry without causing cracking or other distortion. 

The first phase is at around 225 degrees F (107 C).  At this temperature physically absorbed water within the mold can be driven off, so that the process of drying the mold can take place.  For small molds (under 8 inches/20 cm) it's sufficient to soak a wet mold for around two hours to drive off physically absorbed water.

The second phase, which involves the removal of chemically bound water in the plaster, takes place at around 350 F (177 C).   A two hour soak will generally work well to remove chemically bound water from small molds.

The final critical phase occurs in the temperature range from 500 to 850 F (260 to 450 C).  This is where the plaster starts to shrink as it completes the drying process.  It's important to fire slowly (take about three hours for small molds) through this range to avoid cracking the plaster or fracturing the mold. 

For larger pieces the firing times and soak times will need to increase.

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Copyright 2005 Brad Walker.  All rights reserved.

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Copyright 2005-2006 by M. Bradley Walker.  All rights reserved.

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