Zinc crystalline glazes- how do they form?

Zinc crystals naturally form around 2000 degrees Fahrenheit.  The glaze is applied quite thick and fired to around 2300 degrees.  This causes the glaze to run and flow around (and off) the pot, distributing the zinc oxide evenly.  The kiln is programmed to cool to anywhere between 1950 and 2050 degrees and held at this temperature for up to 4 hours.  The crystals form as straight rods that stack and radiate as fractals off of each other.  Temperature and duration of the hold phase influence the shape, size, and nature of the crystal.  Oxides such as copper carbonate, iron oxide, manganese dioxide, nickel oxide, and cobalt carbonate add color to the crystal, to the background, or both.  Because of the tendency for crystalline glazes to run, small saucers called "catchers" are used under each piece, along with a biscuit made of kaolin, alumina hydrate, and sawdust.  The pedestal or biscuit is chiseled off the piece and the bottom of the piece is ground smooth with a silicon carbide disk.  Failure is possible at every step of the way, which makes large pieces with extraordinary crystals a real treasure.  Crystalline glazes are not sturdy enough to be used with food or drink, and should not be washed in the dishwasher.  Any crystalline piece that I make that might have a functional application will have a food-safe glaze used on all contact surfaces.  There are many wonderful books and websites that can help describe further this magnificent glaze effect.