|Monthly Tech-Tip |
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
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 and hold the temperature there. Experience and experimentation reveals at what temperature they grow best and how long to hold. An accurate electronic kiln controller is 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 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.
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 purposely 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, it will thus have 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 that with CMC, the crystals got smaller and more bland over time. A few times I applied from an old vs new batch - same formula, side by side, and new was always best by far. Now, with VeeGum, no difference".
The firing schedules needed for this type of glaze put extra demands on the relays of electric kilns. Special kilns are available for crystalline glaze firing, having heavier relays (or mercury relays). 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 for this). This can be a benefit since you can make them much thinner and there is less of a heat-dampening effect on the programmed firing schedule.
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).
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.
Original File: Rod & Denyse Simair-2.jpg
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.
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.
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.
The cracks appear to have happened on heat-up (because they have widened). Bisque firing was done around cone 04. Issue 1: The cone 10 electric firing was up-ramped at 400F/hr to 2330F (so it whizzed pass quartz inversion on the way!). Issue 2: Wall thickness variations in the pieces, they produce temperature gradients that widen as firing proceeds. Issue 3: Abrupt contour changes and sharp corners, especially when coincident with thickness variations, provide failure points that rapid temperature changes exploit. Issue 4: This new body is more plastic than the previous Grolleg porcelain used, that was likely an enabler to making these thin wall sections even thinner. But remember, practically any piece (unless it has huge in-stresses from uneven drying) can exit a kiln crack-free if firing is done evenly and slowly enough. Results of past firings are the main guide to know what to do in future ones, this is now a "past firing". So the first obvious fix here is slower heat-up, especially around quartz inversion (1000-1100F). Second: more even wall thickness.
Glass vs. Crystalline
In ceramics, understanding the difference between what a glass and crystal are provides the basis for understanding the physical presence of glazes and clay bodies.
Designing a good kiln firing schedule for your ware is a very important, and often overlooked factor for obtained successful firings.
Phase separation is a phenomenon that occurs in transparent ceramic glazes. Discontinuities in the internal glass matrix affect clarity and color.
Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation.
Non-functional ceramic glazes having very high percentages of metallic oxides/carbonates (manganese, copper, cobalt, chrome).
Crystal glazes technical information links from Phil Hamlin
Crystalline glaze forum (very active)
William Schran cone 6 crystalline glaze overview and recipes
Comprehensive crystal glaze links page: Phil Hamlin
The Awesome Crystalline Glaze Gallery of Tilton Pottery
|Oxides||ZnO - Zinc Oxide|
|Oxides||Al2O3 - Aluminum Oxide, Alumina|
B1 - Bory 1 Fara Shimbo Crystalline Glaze
High Boron, Opaque Cone 6-10 glaze
Cone 6 Crystal Glaze Plainsman
Five-Step firing with no holds