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.
100°C/hr to 250C
180°F/hr to 482F
250°C/hr to 500C
450°F/hr to 932F
500°C/hr to 1221C
900°F/hr to 2229F
999°C/hr to 1100C
1798°F/hr to 2012F
100°C/hr to 1000C
180°F/hr to 1832F
100°C/hr to 1050C
180°F/hr to 1922F
100°C/hr to 1020C
180°F/hr to 1868F
0°C/hr to 100C
0°F/hr to 212F
Start temperature assumed: 25°C or 75°F
"Fahrenheit degrees" is not the same as "degrees Fahrenheit".
A 100° reading on a Fahrenheit thermometer is equal to a 37° reading on a Celcius thermometer.
But "100 Fahrenheit degrees of temperature change" is equivalent "55 Celsius degrees of change".
That is an important distinction to understand the above temperature conversions.