Crystalline glazes
Crystals can form during cooling and solidification in many kinds of glazes and they can be microscopic or very large, widely scattered or completely covering. Matte glazes are often such because of a dense mesh of crystals growing on the surface. Unwanted crystallization is called devitrification. However the term crystalline glaze generally refers to the pursuit of large macro crystals. People are captivated by them because they often seem to float on the glaze and they wrap to match the contour of the object. They can be of incredible size and beauty and have been demonstrated in infinite colors, shapes and patterns. But they only grow if the right conditions are present:
The chemistry: Glazes must have almost zero Al2O3 to produce a melt so fluid that it literally runs off the ware. In such glazes it is easier for the component oxides to migrate to the site of formation and they have more freedom to arrange themselves in crystal formation. A saturation of ZnO is also required, this is the magic crystallizing oxide. Adequate SiO2 is needed to form zinc-silicate crystals.
The time and temperature: Glazes prone to crystallization have a distinct "zone of crystallization". For the best results slow the firing at the peak to make sure all materials are fully dissolved in the melt and then cool to the point where the crystal forming material precipitates out into crystals and hold. Experience reveals at what temperature they grow best and how long to hold. An accurate electronic kiln controller is must to make results repeatable.
Most crystals are a different color than the surrounding glaze area (it is reduced in crystal forming oxides and is thus a 'depletion zone'). Larger crystals grow at the expense of smaller ones in a 'survival of the largest' situation. Crystals demonstrate the phenomenon of phase separation, where a glass melt separates into two or more liquids. Coloring materials tend to preferentially and selectively gather at one of these, (one coloring oxide colors the crystals, another the glassy areas). Crystal formation is actually a mechanical imperfection in the glass since it is disrupting the homogeneity of the matrix and imposing discontinuities between glass and crystal phases.
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Example of a crystalline glaze
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