The crystals on pots are thought to be zinc silicate (willemite), one of the chemicals used when mixing up a glaze. Crystals are formed by a fine balance of chemical compesition and firing style.
To make the crystals grow we use titanium to activate the zinc which happens when the temperature in the kiln is held at a over 1100 degrees for an extra 3 hours than normal. Once again the results are anything but predictable.
The consequence for the potter is that the glazes are very mobile, tending to slip off the pot all over the kiln. They must be fired in oxidation to hold the zinc and cooled very slowly.