Techno File: Crystal Cool Ian Hall-Hough
Appears in the May 2020 issue of Ceramics Monthly.
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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.
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:
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.
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.
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
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
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.
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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