Closeup of a crystalline glaze by Fara Shimbo. Crystals of this type can grow very large (centimeters) in size. They grow because the chemistry of the glaze and the firing have been tuned to encourage them. This involves melts that are highly fluid (lots of fluxes) with added metal oxides and a catalyst. The fluxes are normally B2O3, K2O and Na2O (from frits), the catalyst is zinc oxide (alot of it). Because Al2O3 stiffens glaze melts preventing crystal growth, it is very low in these glazes (clays and feldspars supply Al2O3, so these glazes have almost none). The firing has a highly controlled cooling cycle involving rapid descents and holds (sometimes multiple cycles of these). Between the cycles there are sometimes slight rises. Each discontinuity in the cooling curve creates specific effects in the crystal growth. Thousands of potters worldwide have investigated the complexities of the chemistry, the firing and the infinite range of metal oxides additions.
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