Plaster in your clay
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Plaster in your clay
Be careful not to get plaster in your clay. In the studio the wedging table, various molds and bats are made of plaster. Plaster (calcium sulfate) in the clay is transformed by firing to become calcium oxide or quicklime. Quicklime is unstable--it absorbs moisture from the atmosphere. When it has absorbed enough water to become stable calcium hydroxide, the lime will have expanded to 30 times it's original volume. If this happens in the wall of a pot the pressure of the expanding lime will blow out the surface of the pot leaving a crater with fluffy white powder at the bottom. This will usually happen after the bisque firing. Plaster molds are easily chipped or scratched which can dislodge bits that get into clay. Be very careful when working with plaster, and never use metal tools on a plaster surface.
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| Joe Wallace |
