Crystallization (also called devritrification). You can see the tiny crystals on the surface of this copper stained cone 6 glaze (G3806C). The preferred orientation of oxides in crystalline, especially when metal oxides are present. When kilns cool quickly there is simply no time for oxides in an average glaze to organize themselves and crystals do not grow. But if the glaze has a fluid melt and it cools slowly through the temperature at which the crystals like to form, they will.
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