Both of these mugs were soaked 15 minutes at cone 6 (2200F), then cooled at 100F per hour to 2100F and soaked for 30 minutes and then cooled at 200F/hour to 1500F. This firing schedule was done to eliminate glaze defects like pinholes and blisters. Normally the GA6-A glaze crystallizes (devitrifies) heavily with this type of firing, but an addition of 1% tin oxide to the one on the left has prevented this behavior.
GA6-A - Alberta Slip Cone 6 Amber Base Glaze
An amber-colored glaze that produces a clean, micro bubble free transparent glass on brown and red burning stonewares.
Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation.