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Glaze is Not a Rock
An Introduction to Cooling and Holding Cycles in an Electric Kiln

By Carol Marians

Digital kilns with excellent pyrometers allow studio potters to achieve remarkable glaze effects through planned and controlled cooling and 'holding' cycles.

But how are these points and times decided upon? And what, exactly takes place? In a research-level glass laboratory we could do many experiments, construct the relevant phase diagrams, extrapolate from them to determine our holds.

Unlike metals, ceramics do not come to equilibrium in a time span corresponding to human activity. While physical chemists have techniques for dealing with this, potters do not! All we can do is experiment within the framework provided by a few facts, such as that nothing happens below the temperature at which the glaze is a solid. We test between the temperature of the desired cone, and the point where the glaze is a solid.

A look at those few facts.

Potters make their glazes from ground-up rocks.

Most of us get these rocks pre-sorted, cleaned, segregated, and ground into powder. It wasn't always so, as books about the making of glazes from primitive materials show. Even today glazes can be, and are, made by pulverizing rock, mixing it with some material so it can be spread on the pot, and firing it! A viable glaze results! Of course, not ALL rocks lend themselves to this treatment, but plenty do.

Does the final glaze look like the rock it came from? No. If you pick up a rock and examine it under strong light, possibly with a magnifying glass, you will see speckles, streaks, bands, maybe variegated bits of different colors. Ground up, melted, re-solidified rock is not at all like its 'parent'.  The difference between the rock and the glaze made from it is time: the interval during which the original rock cooled, and the interval during which the glaze cooled from cone whatever to room temperature!

Geophysicists know that enormous cooling time, largely under equilibrium conditions, leads to a rocky outcropping containing quartz. (This is not the only way Nature achieves natural quartz) What matters to potters is that conditions and rate of cooling---slowly over millennia --can turn the same magma into a verity of rocks.

(The well-known exception is lava: magma spurted out from volcanoes, splashing to earth, cooling very quickly. There is no chance at crystallization, and the solidified lava forms a host of glassy rocks ranging from obsidian to pumice.)

At high temperatures molecules move fast and easily, tend not to stick to each other, and can form the kind of bonds that, cooled very slowly, achieve ordered form--namely crystals. With rapid cooling the molecules get 'rattled', form the wrong kind of bonds--ending up as glass.

Cooled slowly enough most rocks will end as a conglomerate of many crystalline minerals.

Molten glazes never had the uniformity of a geologic magma, and cool infinitely faster, forming little crystal and much glass. Cool slowly--crystals. Cool fast--glass.

Looking now just at glazes.

What happens as a glaze melts? What happens as it cools? The paths are different but the process quite similar.

Never a uniform melt, glaze is more like a slurry of mud. But no water, as that would have evaporated early in the firing! This slurry forms

a glassy matrix in which float many liquids and solids. There are globules and bubbles of a variety of glasses, like a concoction of house paint, petroleum jelly, honey, vinegar, crankcase grease, mixing together. The heat does the mixing.

As glaze melts lots of different viscous liquids form. They don't move far nor fast, but slowly merge. When the various solids have been incorporated into liquids, and the mixture has spread itself evenly over the pot, and cooled, we speak of a matured glaze.

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