Published May 11, 2020
Lately I have been experimenting with different firing schedules to see how my glazes are affected. One exciting discovery was the depth a slow cool added to a commercial glaze I had been using. With the slow cool, it has a crystal-milky quality that I just love!
In today's post, an excerpt from the Ceramics Monthly archive, Ian Hall-Hough gives some tips on how to encourage (or discourage) glaze crystallization. You'll see how controlling cooling at different rates can open up new and exciting results, solve your glaze woes, and shed light on common kiln-opening conundrums. –Jennifer Poellot Harnetty, editor
Certain conditions encourage oxides to form crystal structures with silica or boron on the surface of the glaze. The most common result is a more matte surface, but additional outcomes include visual texture and variation in a glossy glaze, as in titanium-float glazes (in which the titanium is often sourced from rutile). Iron-silicate crystals produce iron-red glazes. Zinc can form large crystals in glazes that are visible to the naked eye. These large crystals become the decoration (glazes with these attributes are typically called crystalline glazes). Even magnesium, which is normally a glaze stiffener, can enter the melt and precipitate out as pyroxene crystals in teadust glazes.
1,2 Detail of two pots glazed with Ian’s Ash. The vase (1) on the left was fired in a large gas kiln and allowed to cool naturally, crystallizing the calcium in the glaze. The mug (2) on the right was crash cooled, preventing crystallization and preserving the glassy character of the melt.
Quick- or crash-cooled glazes don’t form crystals for the same reason that ice cream made with liquid nitrogen is creamier and less grainy than churned ice cream. The super-cold nitrogen instantly turns the cream from a liquid to a solid, not giving the emulsified water in the cream a chance to precipitate out from the emulsion and crystallize (causing a grainy texture). During cooling, oxides act like the water in the cream in the sense that they can precipitate out from the melt and crystallize if they have enough time during the transition from a liquid to a solid, and enough silica or boron to form crystals with. If crash cooled, they don’t have that chance and solidify as a glass.
Tony Hansen’s website, digitalfire.com, has an informative article on the subject, which I referenced to compile the following guidelines for crystal formation.1 The following conditions encourage crystallization:
- The melt is fluid (runny).
- There is a sufficient amount of one or more crystallizing oxide, including calcium, magnesium, titanium, iron, and zinc.
- There is sufficient cooling time at molten temperatures for oxides to precipitate out from the melt and form organized crystal structures.
- There is sufficient silica or boron to form crystals with the crystallizing oxides.
- Melt stiffeners are not present in high enough percentages (alumina, zirconium, magnesium).
These guidelines are incredibly useful in learning to encourage, or discourage, crystallization.
4, 5 Detail of two mugs glazed with Slate Blue. The mug on the right (5) was fired in a loosely packed kiln in my garage during winter and allowed to cool naturally. The one on the left (4) was fired with the cone 6 slow-cool schedule. Crystallization of the titanium from the rutile in the glaze shows an interesting variegation and made the surface semi-matte.
The following are specific situations where the guidelines above might come in handy for troubleshooting. Note: This is not an exhaustive list.
- Your glaze fires glossy but is supposed to be matte. When cooled too fast, many matte glazes can fire glossy (5). If this happens, it means the glaze’s crystallization-prone oxide(s), like calcium or titanium, aren’t getting the amount of cooling time needed to form enough crystals to matte your glaze. It happens for me when I fire my small electric kiln, especially when it is loosely packed, and especially in the colder winter temperatures. If your kiln is in an unheated garage like mine, colder outdoor temperatures will encourage faster cooling and inhibit crystal growth, and thus matteness. Programming a slow cooling schedule is the solution).
- Your glaze fires matte and opaque, but is supposed to be glossy and translucent. This happened to me for years when I fired ash glazes in large gas kilns. I couldn’t understand why my results were so matte and opaque, when I saw that the same glaze recipes fired to a glossy and semi-transparent surface on other people’s pots. I discovered that if I crash cooled my kiln, those same glazes fired glossy (1–3). Ash glazes are high in calcium, and when also low in alumina, they are especially prone to forming calcium-silicate crystals. While crash cooling solves the problem, I am currently experimenting with adjusting calcia/alumina levels to achieve glossy results in slower-cooling kilns. Glaze matteness can also be caused by a low silica-to-alumina ratio.
- Your variegated or float glaze fires flat, unvariegated, and boring. The variation in these kinds of glazes, called float or variegated, comes from titanium. Again, cooling quickly can completely eliminate this effect in some glazes (4, 5). If your kiln tends to cool quickly, or you are firing in quick-cooling conditions, you might need to program a slow-cool firing schedule to achieve the results you’re looking for.
- Your zinc crystalline glaze isn’t forming crystals. I am by no means a crystalline potter, but zinc crystals need a very fluid glaze along with adequate cooling time to form. If your glaze isn’t running, it might have too much alumina or other glaze stiffener.
- Your teadust glaze is forming too many magnesium (pyroxene) crystals, or too few. Magnesium crystals, when distributed in the right concentration, can be quite beautiful. When cooled too slowly, they can completely cover the surface, turning the glaze totally matte and obscuring the tenmoku color of the glaze (7). This kind of glaze can be quite sensitive to different cooling rates. If your kiln is packed too tightly, it can crystallize too much, and if packed too loosely, it won’t crystallize at all. Keep trying to manipulate the cooling speed, via either the how densely the kiln was packed with ware or the cooling schedule, until you find what works in your kiln.