The crystals in a crystalline glaze grow during the cooling cycle. As in nature, these crystals are the result of a slow cooling of a molten mass containing many chemicals, giving like molecules, moving through the melt, time to find each other and align in pattern, one by one. So the trick to growing crystals is to control cooling so that the optimum conditions exist for the crystals we want to grow. In our case, 'optimum' conditions are conditions of glaze viscosity. The glaze must be cool enough to allow zinc silicate molecules to lock into a position in the lattice, but warm enough to give them the mobility to find that position in the first place.
The temperature range at which zinc silicate crystals will grow in a glaze is said by some sources to be from 1180° to 1010° C (or about 2160° to 1850° F), and by others to be 1050° to 750° C (1922° to 1382° F). My own experience has been that my glazes will form crystals starting at about 160° C from peak temperature and will continue to do so within a range of about 70° C below that. Every new glaze should be tested carefully to see where its best crystal-growing range happens to be under your own firing conditions.
By choosing carefully the point at which you hold (or don't hold) your temperature, you can change the shape of the crystals you grow. At the high end, crystals tend to be acicular (needle-like). As the temperature is lowered through the growing range (something often referred to as a 'slant soak' the crystals 'fan out' and produce more variety of shape (as in Illustration 7.4). You can, if you like, hold your temperature at one point and get a crystal whose internal structure is very uniform, or allow the temperature to change and produce a more flower-like effect. Note also that the crystal color lightens as the temperature is lowered.
|2||250°C/hr to 500C||450°F/hr to 932F||0||2:00|
|1||100°C/hr to 250C||180°F/hr to 482F||0||4:30|
|3||500°C/hr to 1221C||900°F/hr to 2229F||0||6:27|
|4||999°C/hr to 1100C||1798°F/hr to 2012F||0||6:34|
|5||100°C/hr to 1000C||180°F/hr to 1832F||0||7:34|
|6||100°C/hr to 1050C||180°F/hr to 1922F||30min||8:34|
|7||100°C/hr to 1020C||180°F/hr to 1868F||30min||9:22|
|8||Freefall°C/hr to 100C||0°F/hr to 212F||0||9:22|
Shimbo Crystal Holding Pattern 2
Electric firing: Crystal Glazes 2 by Fara Shimbo (page 138)