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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 (e.g. high CaO) are often such because of a dense mesh of micro-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 preferred 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 hold the temperature there. 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 (which 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 coloring 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.

The advent of hobby electronic kiln controllers, online communities and glaze chemistry and record keeping software has brought these glazes within the reach of thousands of potters worldwide. International shows, lots of available books, research and liberal information sharing by many are a catalyst to the explosion in popularity. The study of these crystals and the conditions that affect their growth has become quite technical (e.g. people measure and record the rate of crystal growth per period of hold time).

Scattered crystals on a highly melt fluid zinc free glaze

Crystals do not just grow on zinc glazes. These were fired by Bill Campbell. The glaze is lithium fluxed and colored with iron. There is a metallic halo around the crystal, the crystal is usually a hexagon.

What could make glazes grow these incredible crystals?

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.

Raw and calcined zinc oxides in a crystalline glaze

Zinc oxide calcined (left) and raw (right) in typical crystalline glaze base (G2902B has 25% zinc) on typical cone 6 white stoneware body. This has been normally cooled to prevent crystal development. The melting pattern is identical. Note how badly these are crazed, this is common since crystalline glazes are normally high in sodium.

Why is this crystalline glaze not crazed? Even in the pool at the bottom?

Because this is Plainsman Crystal Ice, it contains 40% silica (quartz). It also does not vitrify, so as much of the quartz remains undissolved as possible. This produces a body with a much high thermal expansion so it can put more of a squeeze on the high-expansion glazes used in the crystal glazing process (it is very common for such glazes to be crazed, it is accepted as part of the process).

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By Tony Hansen

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