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offcntrThermal expansion. Clay and glazes both expand when heated, contract when cooled. While the glaze is molten in the kiln, any difference in expansion between the two doesn't matter. On cooling, though, the glaze solidifies into glass, and any difference in shrinkage between glaze and clay body can be a major issue.
Crazing. If a glaze shrinks more than the clay body it's applied to, a network of fine cracks can appear. This is called crazing. In stonewares, it's not a huge problem, but porous clay like earthenware will leak liquids if the glaze is crazed, and I've seen it happen on porcelain occasionally. Sometimes used decoratively, when it's called a crackle glaze.
Shivering. When the clay shrinks more than the glaze coat, bits push away from the pot, especially where there's extra pressure--rims and edges, mostly.
Crawling. When melted glaze beads up, leaving patches of bare clay. Has a variety of causes--dusty pots, high surface tension, too much raw clay.
Calcine. To expose to high heat, generally bisque temperatures. Calcining zinc oxide drives off moisture. Calcining is also used to turn gypsum into plaster and limestone into cement. Calcining clay drives off moisture and organic impurities and changes the structure so the clay will no longer expand or contract on exposure to water.
Kaolin. China clay. A white, primary clay used in porcelain.
Ball clay. A secondary clay--one that's moved by water from the feldspar deposit where it originated. This means it has finer particles, more plasticity, also a little more organic impurities. Usually still fires white. Shrinks more in drying.
Crazing. If a glaze shrinks more than the clay body it's applied to, a network of fine cracks can appear. This is called crazing. In stonewares, it's not a huge problem, but porous clay like earthenware will leak liquids if the glaze is crazed, and I've seen it happen on porcelain occasionally. Sometimes used decoratively, when it's called a crackle glaze.
Shivering. When the clay shrinks more than the glaze coat, bits push away from the pot, especially where there's extra pressure--rims and edges, mostly.
Crawling. When melted glaze beads up, leaving patches of bare clay. Has a variety of causes--dusty pots, high surface tension, too much raw clay.
Calcine. To expose to high heat, generally bisque temperatures. Calcining zinc oxide drives off moisture. Calcining is also used to turn gypsum into plaster and limestone into cement. Calcining clay drives off moisture and organic impurities and changes the structure so the clay will no longer expand or contract on exposure to water.
Kaolin. China clay. A white, primary clay used in porcelain.
Ball clay. A secondary clay--one that's moved by water from the feldspar deposit where it originated. This means it has finer particles, more plasticity, also a little more organic impurities. Usually still fires white. Shrinks more in drying.