Crystalline glazes

A type of ceramic glaze made by potters. Giant multicolored crystals are grown on a super gloss low alumina high zinc glaze by controlling multiple holds and soaks during cooling in the kiln.

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Crystals can form during cooling and solidification in many kinds of glazes. They may be microscopic or very large, sparsely distributed or sufficiently numerous to cover the entire surface. Many matte glazes, especially calcium and zinc mattes, owe their surfaces partly or largely to networks of microscopic crystals. Unwanted crystallization in a glaze intended to remain glassy is commonly called devitrification.

Crystalline glazed vase by Rod and Denyse Simair
This award-winning couple are-all in on crystal glazes. They have learned that success is about data. A lot of data. Thousands of pictures, hundreds of firing schedules, hundreds of recipes, endless notes all come together in the growth of crystals like these! Notice the clear background, no micro-crystals fogging it up. Notice that two fundamentally different types of crystals are being grown. Not to be ignored is the throwing skill it takes to make a porcelain piece like this, these are not small pieces.

However, the term crystalline glaze normally refers to glazes intentionally fired to grow visible macrocrystals. These crystals can appear to float within or on the glaze, and their radial forms wrap around the contours of the ware. They have been produced in an extraordinary range of colors, sizes, shapes and patterns. The most familiar macrocrystalline glazes are zinc-silicate types in which the principal crystals are willemite, Zn₂SiO₄.

The Chemistry: Macrocrystalline glazes normally contain very little Al₂O₃. Alumina stiffens a glaze melt and helps produce a stable glass network, whereas large crystals require sufficient melt mobility for crystal-forming ions to diffuse through the liquid and arrange themselves at a growing crystal surface. That being said, if melt mobility can be achieved with some Al₂O₃ present, crystal growth is also possible. These glazes normally contain a high concentration of ZnO together with sufficient SiO₂ to form willemite. ZnO is not simply a catalyst or a material that creates nucleation points: zinc and silica are incorporated into the crystal itself. High ZnO also influences melting behavior and melt viscosity. Other oxides and mineral additions can affect nucleation, crystal morphology, color and the temperature range in which growth occurs. Titanium-bearing additions, for example, can promote nucleation under suitable conditions.

Melt Fluidity: The glaze must be fluid enough for diffusion and crystal growth, but not so fluid that all of it runs from the ware. This balance is difficult because compositions capable of growing spectacular crystals are commonly very mobile at peak temperature.

The Firing: It normally has three broad stages: melting, nucleation and crystal growth and rapid cooling. Melting is thorough because the glazes are fired high enough to make them very fluid, producing a relatively homogeneous liquid. The high melt fluidity enables firing rapidly to the peak temperature. Glazes prone to crystallization have a distinct "zone of crystallization" where willemite can nucleate and grow (e.g. 1900-2000F, although some sources quote the range as wider). Individual craftsmen distinguish their work by studying how specific glaze formulations, colorants and nucleating agents form crystals according to dwell times within this range. The best practitioners in this field do hundreds, even thousands of firings, carefully recording the schedules, recipes, procedures and pictures (an account at insight-live.com is excellent for this). After the desired growth period, the kiln is cooled more rapidly to reduce further crystallization and preserve the developed pattern.

Crystals and the Surrounding Glass: Most crystals are a different color than the surrounding glaze area (which is reduced in crystal-forming oxides and is thus a glass 'depletion zone'). Larger crystals grow at the expense of smaller ones in a 'survival of the largest' situation. The phenomenon can be considered a glass-ceramic composed of willemite crystals in a residual glass phase. Coloring materials tend to preferentially and selectively gather at one of these (one coloring oxide coloring the crystals, another the glassy areas).

The advent of hobby electronic kiln controllers and online communities has brought these glazes within the reach of thousands of potters worldwide. They are accurate enough to make results quite repeatable. That being said, electric kiln elements must be kept in good condition to ensure the kiln can follow the programmed schedule.

International exhibitions, books, online communities and generous information sharing have greatly increased the popularity and technical sophistication of this field.

Crystalline glazes need to be used on low-LOI bodies (ideally containing little or no ball clay), avoiding gas bubble production or body bloating.

Crystalline plate made by Holly McKeen.
Notice the glaze is not crazing. That is because this is a high-silica porcelain.

Since crystal glazes have a high thermal expansion (because they contain high KNaO) they will craze badly on most clay bodies. While many feel it is decorative, crazing drastically reduces the strength of the fired ware. Consider an extreme: Because crystalline glazes have high melt fluidity, they will run downward on the inside surfaces of ware, forming a pool at the base. The thermal expansion of the thick mass formed will almost certainly be higher than the body, thus having the power to impose its cracks onward into the body matrix. This power is often evident when the base of a mug, for example, simply falls off (leaving razor-sharp edges). There is a way to address the thermal expansion mismatch between body and glaze: Increase the silica content in the porcelain (as high as 40%). This can greatly reduce or even eliminate the crazing (of course, this will require careful cooling of the kiln through the quartz inversion phase to avoid dunting).

The leachability of crystalline glazes used on food surfaces should be tested. They are flux-saturated, and the Al₂O₃, the very thing most needed to make a stable, durable glaze, is normally very low. Thus, by their very nature, they should leach and lack fired hardness. The tendency to craze noted above also affects food safety.

Because these glazes have very low (or zero) percentages of clay, the slurries do not suspend well in the bucket or harden well during drying. It is common to use CMC gum to improve hardening, but this produces sticky slurries that drip a lot during application and dry slowly. Bentonite additions of around 1-3% can suspend, harden and slow down drying. VeeGum has also been found to work well, but typically less than 1%. VeeGum has an added benefit, according to Holly McKeen. She observes: "Not only does it suspend better but than with CMC, the crystals got smaller and blander over time. A few times I applied from an old vs new batch - same formula, side by side, and the new was always best. Now, with VeeGum, no difference".

The firing schedules needed for this type of glaze put extra demands on the relays of electric kilns (especially hobby kilns). This is because schedules that hold or slowly decrease temperature need to switch relays much more often than with typical fast rises or drops. Special kilns, having heavier-duty relays, are available for crystalline glaze firing.

Even with catch plates, you will have accidents, where glazes run more than expected and get onto kiln shelves, so a good kiln wash is important. For smaller kilns, it may be practical to make your own shelves (we use calcined alumina, zircon or refractory kaolin/grog mixes for this). This can be a benefit since you can make them much thinner so there is less of a heat-dampening effect on the programmed firing schedule.