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Crystalline glazes

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

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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 the preferred crystal formation. That being said, if melt mobility can be achieved with considerable Al₂O₃ present, crystal growth should also be possible. A saturation of ZnO is also required, this is the magic crystallizing oxide. Adequate SiO₂ is needed to form zinc-silicate crystals. The ZnO provides the conditions for crystal growth, it is not physically providing nucleation points, but could be considered a catalyst.

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 and hold the temperature there. Experience and experimentation reveal at what temperature they grow best and how long to hold. An accurate electronic kiln controller is a must to make results repeatable. 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).

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 have 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.

Crystalline glazes need to be used on low-lignite bodies (ideally containing no ball clay). Bloating can occur because the kiln is being soaked for long periods and the glaze melts early and seals the body surface. This can impede the escape of gases of LOI and result in bloating and blistering of the body.

Since crystal glazes have a high thermal expansion (because they contain high KNaO) they will craze badly on most clay bodies. Increasing the silica content in your porcelain (as high as 40%) can greatly reduce or even eliminate the crazing (of course this will require careful cooling of the kiln through the quartz inversion phase). While many feel it is decorative, crazing drastically reduces the strength of the fired ware.

Crystalline glazes are most often likely not food safe, and for several reasons. They are flux saturated and the Al₂O₃, the very thing most needed to make a stable, durable glaze is normally almost zero. That means they will leach and lack fired hardness. In addition, the metallic fluxes are not securely bound chemically to the glass matrix (this is why they crystallize out), so parts of the surface, especially the crystals, are leachable in acids or bases. Crystalline glazes have high thermal expansion and craze, crazed glazes are not considered functional and they severely weaken fired ware. Also, because crystalline glazes have high melt fluidity they will run downward on the inside functional surfaces of ware forming a pool at the base. The thermal expansion of the thick glass mass 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).

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 a dry slowly. Bentonite additions of around 1-3% can suspend, harden and slow down drying. VeeGum also has 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. 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 are available for crystalline glaze firing, having heavier relays, mercury relays or even solid-state ones. You will also 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.

Related Information

Crystalline glazed vase by Rod and Denise Simair

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 crystals are typically hexagonal.

Crystalline plate made by Holly McKeen.

Notice the glaze is not crazing. That is because this is a high-silica porcelain.

Crystalline glazed vase by Rod and Denise Simair

Original File: Rod & Denyse Simair-2.jpg

Growing incredible crystals in a ceramic glaze is about the chemistry

Closeup of a crystalline glaze. Crystals of this type can grow very large (centimeters) in size. These 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 dominated by K₂O and Na₂O (from frits) and the catalyst is zinc oxide (enough to contribute a lot of ZnO). Because Al₂O₃ stiffens glaze melts, preventing crystal growth, it can be almost zero in these glazes (clays and feldspars supply Al₂O₃, so these glazes have almost none). The firing cycles involve rapid descents, holds and slow cools (sometimes with rises between them). Each discontinuity in the cooling curve creates specific effects in the crystal growth. These kinds of glazes are within the reach of almost anyone today since electronic controller-equipped kilns are now commodity items and anyone can fiddle with the chemistry and manage the testing of glazes in their insight-live.com account.

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 higher 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).

The melt fluidity of a crystalline glaze

The upper half of this small vase has a high-zinc crystal glaze. The lower half is just a functional cone 6 transparent (with the same amount of cobalt to produce the blue color). The piece was not slow-cooled to grow the crystals. The high degree of melt fluidity is clearly visible.

Example of catch basin dish used for crystalline glazing

The running glaze collects in a catcher sized to match the base of the piece. A calcined alumina or kiln wash paste (made using CMC gum or other binder) is applied to the base of the piece so that it does not stick to the catcher. After firing the catcher is broken off and the remaining sharp edges around the base of the piece are ground off.

This crystalline glaze has zero clay yet brushes well. Here is why.

A 500ml jar of this was made using 340g powder, 440g water, 5g CMC gum and 6g of Veegum. This is the most VeeGum that would likely ever be used in this type of glaze. But it suspends and gels the slurry well. This glaze must be gelled because it contains no clay to suspend it. But there is a problem: The amount of gel needed means it is impossible to remove the agglomerates of CMC and VeeGum by blender mixing (the slurry simply won't move enough in the jar to expose all of it to the propeller). That means either ball milling for an extended period or creating an initial batch that only includes the CMC gum and mixing that in the blender very thoroughly first. Then, on high speed, slowly pour in the VeeGum until it gels enough to suspend everything (don't add too much or it will over-gel immediately or overnight). Regular brushes won't load the gelled glazes. But a fan brush like this works surprisingly well and creates and even laydown (very important with crystal glazes). If you have only used commercial brushing glazes you may take the CMC gum they contain for granted. Dipping glazes made by potters normally contain no gum. After dipping the bisque typically absorbs the water quickly and the glaze dries to the touch in seconds. Trying to brush such a glaze would be impossible. But adding gum slows the drying dramatically and enables it to flow beautifully for brushing.

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Serious cracking in a crystalline-glazed P700 Grolleg porcelain. Why?

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