The glaze on a fired pot is generally an amorphous supercooled liquid. As the glaze is melted and cooled in the kiln, glass molecules bond together in random strings. Crystals occur if the glaze is fluid enough to allow molecules to move more and hot enough long enough to allow the glaze molecules to arrange themselves in structured strings, or crystals. For this to happen, there must be an extended time at higher temperatures to allow time for crystal growth, and the glaze must have the right type of chemical composition. These are the first two of three factors that potters deal with when working with crystalline glazes. Generally speaking, crystals begin to form as needle-like shapes at about 2084 F/1140 C. If the temperature is held at about 2012 F/1100 C, a double-axehead shape will usually form. Holding the temperature between 1994-1850 F/1090-1010 C will encourage the shape to round out. Fully rounded crystals give a distinctly flower-like effect. Crystalline glazes are lower than normal in their alumina content. In addition, the amount of free silica in both the glaze and the clay body must be kept to a minimum. Otherwise, cristobalite may form, making the pot much more brittle and susceptible to thermal shock. Because of these requirements, crystalline glazes tend to be quite runny. Pots should be fired on a bisque pedestal-saucer to catch all drips. The pot’s bottom may need to be ground and polished after removal from the kiln. Cobalt is the strongest; it will override the attraction of the other colorants and move into the crystal structure alone. For example, if cobalt and manganese are both present, the cobalt will migrate into the crystals making them blue, and the manganese will remain in the glaze matrix, making it yellow. If cobalt is not present, nickel takes the next precedence in migrating into the crystal, then manganese, then copper. Copper, if by itself, will color glaze and crystal fairly evenly.
