These two mugs have the Alberta Slip base cone 6 GA6-A glaze on the inside. The left one is cooled normally (kiln off at cone 6 after soak). For the mug on the right the kiln has been soaked for half an hour at 1800F on the way down. This was done to develop the rutile blue glaze on the outside, but during this period crystallization occurred on the inside. If you need to cool slow (for the Alberta Slip rutile blue) but would like the liner glaze to be transparent, add 0.5-1% tin oxide to the GA6-A to impede crystal growth.
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.
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.