Topic: Ceramic Kilns

Techno File: Crystal Cool

Explore 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.

Define the Terms:

Crash Cool: Deliberately speed up cooling process after the kiln reaches
top temperature.
Float Glaze: A glaze in which titanium forms crystals during cooling, causing a variegated visual texture.
Pyroxene: A type of inosilicate mineral that forms with an adequately slow cooling rate in magnesium teadust glazes.

Cooling Speed—An Important Variable

Years ago, I put a glossy, transparent, gray/off-white glaze into a wood kiln and it came out matte, opaque, and super white. After scratching my head for a while, I realized the large, well-packed, well-insulated wood kiln had cooled much more slowly than the gas kilns I’d been firing in, allowing the glaze to form microcrystals and transform the surface. Cooling speed, it turns out, is one of the most important variables determining how our glazes come out.

Crystallization

Certain conditions (see below) 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.

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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.

3 Detail of a teabowl glazed with Ian’s Ash and crash cooled. It is greener because the piece was fired in heavier reduction, the glaze batch used an iron-bearing ash, and the glaze was applied thicker.

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:

  1. The melt is fluid (runny).
  2. There is a sufficient amount of one or more crystallizing oxide, including calcium, magnesium, titanium, iron, and zinc.
  3. There is sufficient cooling time at molten temperatures for oxides to precipitate out from the melt and form organized crystal structures.
  4. There is sufficient silica or boron to form crystals with the crystallizing oxides.
  5. Melt stiffeners are not present in high enough percentages (alumina, zirconium, magnesium (see Oxides and Their Crystallizing Properties)).

These guidelines are incredibly useful in learning to encourage, or discourage, crystallization.

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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 (see Cooling Schedules).
  • 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 (see Crystalline vs. Underfired/Unfused Mattes below).
  • 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.

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6, 7 Detail of two bowls glazed with a cone 10 amber teadust glaze. The one on the right (7) was cooled slower, and pyroxene crystals have started to form.

 

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8, 9 Detail of two vases with exactly the same glazes, a high-titanium glaze underneath layers of three different ash glazes. The first vase (8) was fired by itself in a kiln and allowed to cool naturally; the second vase (9) was fired using the slow cooling schedule. In this case, crystallized titanium has matted the glaze and dulled the colors.

Crystalline vs. Underfired/Unfused Mattes

There are two general types of matte glazes. The first type has a lower ratio of silica in comparison to alumina (there is more alumina). The refractory alumina prevents a complete melt of the glaze, resulting in a matte surface (10). If you were to open up the kiln at top temperature, these glazes would not be melted. The other type of matte glaze results from a complete melt, whether fluid (runny) or not, where crystallization during cooling creates a matte surface. These glazes would appear glassy if you opened the kiln at top temperature. Much of the time, if a glossy finish is desired, these glazes can be crash cooled to prevent crystallization and preserve the glossy surface.

Oxides and Their Crystallizing Properties

  • Calcium is the most common flux, and is found in almost every glaze at mid-range and high-fire temperatures. It is prone to forming calcium-silicate crystal mattes, especially in low-alumina glazes (1,12,13,14). Sometimes the mattes it produces are smooth and desirable, but they can also be chalky and dry. Fast cooling will prevent crystallization and preserve glossy surfaces. Calcium is usually sourced from calcium carbonate (whiting), dolomite (also a source of magnesium), or wollastonite.
  • Titanium is traditionally sourced from rutile, which is also a source of iron. Titanium can produce a wide range of crystalline effects, from mottled or float textures, to small visible crystals, to dry microcrystalline mattes (4, 5, 8, 9). Slow cooling will encourage crystallization, while fast cooling will prevent it. Titanium is sourced from rutile and titanium dioxide.
  • Magnesium has a high melting point that can prevent crystallization by inhibiting the melt, especially at lower temperatures. At cone 10, it can enter the melt and precipitate out during cooling, causing fatty or buttery mattes; though I am uncertain the mechanism is actually crystallization. Tony Hansen claims it is phase separation rather than crystallization. I have, however, crash cooled magnesium mattes and gotten semi-matte results. Magnesium can also produce pyroxene crystals in teadust glazes (7). It is sourced from talc, dolomite, magnesium carbonate, and certain frits.
  • In iron-saturated glazes, red-iron-oxide crystals can form, causing beautiful iron-red glazes.
  • Zinc can produce the kind of macrocrystals seen in what is popularly known as crystalline pottery.

10 This is an example of a glaze that is matte because of its high alumina content, not crystallization during cooling. This pot was crash cooled.

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11, 12 These are the same ash glaze. The vase on the left (11) is crash cooled, the one on the right (12) was allowed to cool naturally. While still semi-glossy, the vase on the right is matte in places due to calcium crystallization.

Cooling Schedules

The following are examples of mid- and high-fire cooling schedules. Give these a try or experiment on your own for your desired results.

Cone 6: Fast Firing with a Slow Cool

  • Segment 1: 150°/hour to 220°F
  • Segment 2: 500°/hour to 1978°F
  • Segment 3: 150°/hour to 2225°F
  • Segment 4: 999°/hour to 1900°F
  • Segment 5: 100°/hour to 1400°F

Slow cooling for gas kilns: Most people firing to cone 10 are using gas or wood kilns, which usually do not have programmable controllers. To slow down the cooling, pack the kiln as tightly as you can while still allowing for the flames to pass through the kiln properly. After reaching temperature, close all dampers, chimney, peep holes, and burner ports. If you have a hard-brick kiln, the heat retention of the bricks will help immensely. If, like me, you have soft brick, you have the perfect kiln for glazes that like to be cooled quickly.

13 This is a good example of a high-calcium glaze being completely changed by crystallization during cooling.

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14, 15 These two cups are glazed with exactly the same iron-bearing ash glaze; can you guess which one is crash cooled and which was cooled slowly? The answer: the cup in image 14 was slow cooled.

Fast cooling for gas kilns: In electric kilns, use fast, medium, or slow pre-programmed firing schedules and pack the kiln lightly. The kiln will cool faster in an unheated room in cold weather. If your kiln is vented, leaving your vent fan on may help cool it faster. Most glaze crystallization happens above 1500°F, so removing peep-hole plugs or cracking the lid below that won’t have much impact on glaze results. Cooling too quickly can damage your kiln bricks, kiln furniture, and ware.

In gas kilns, quick cooling can be achieved by leaving dampers open, just be sure to consult your kiln manual, as cooling too quickly can cause damage to your kiln’s bricks and mortar, kiln furniture, and ware.

the author Ian Hall-Hough is the manager of Clayworks Supplies, Inc. in Alexandria, Virginia. He has been a potter and glaze enthusiast for 20 years. Find him on Instagram @ihallhou.

1 Tony Hansen’s website, digitalfire.com, has an informative article on the subject, “Crystallization,” which I reference for the guidelines for crystal formation (adapted with permission).
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