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

By Carol Marians

But inside the merged materials of the glaze are diverse small molecules of materials, keeping their own identity. These will merge only through the cooling of the glaze. As the glaze cools these molecules--think of them as teenage girls--join hands, become cliques and gaggles, dancing and giggling along the street.  Little groups doing their own thing all together in one place. The glaze no longer harbors individual molecules but groups and clusters of them.

Or think of a train full of girls--train being glaze, girls being molecules--remaining individuals, seated quietly. Then something happens and they form groups, gaggles, clusters, which, through another event, become fixed in position.

But why would the glaze not look like its mother rock after going through what seems the same process? The picture of a mature glaze that has been properly melted and quickly cooled to room temperature is not of a uniform homogeneous mix. Under magnification the glaze would reveal fascinating landscapes, lakes, rivers, jagged pillars, dots, stars, a whole new world.
But if the rock had been sectioned, examined with the same microscope as the glaze made from that same rock, melted, quenched, examined again, it would not look like the glaze.

Because no matter how fast a glaze is cooled it never is 'quench
cooled'- taken from very high to room temperature in fractions of a second - and feasible only in a laboratory. Quench cooling 'freezes in' the structure of the molten liquid, creating glass in its purest form. In a pottery studio glaze always cools slowly enough to achieve some ordering of its structure.

Furthermore: a rock is a lump. Glaze on a pot is mostly surface, rather like a chiabatta bread which is mostly crust. Were a rock to be spread thin on a pot, and fired and cooled the same way, it would look similar.

Because, unlike our train which moves through space and time, rock and glaze pass through temperature and time.

The magma becomes rock by` cooling through millennia, going through what geophysicists call 'reaction series.' The magma cools slowly enough so that crystallization occurs. (Talk about slow cooling!) Glaze cools within hours, and as it cools, fractionally solidifies at each stage. Just as in cooking and baking the results are achieved through controlled heating and cooling. A 500oF oven for 15 minutes does not bake the chicken as a 350 o F oven will over an hour! nor would instant freezing achieve the ice cream a slow controlled freezing will. Controlled heating and cooling achieves specifically intended glaze results.

In cooling glaze passes through different temperature 'regions' in which different components solidify. How much solidifies depends on the time spent in the 'region'. As the newly formed, still growing, crystals sit in the liquid they may change again, due to the presence of the liquid. This phenomenon can achieve crystals with rings like the rings on a tree. The ultimate shape of the crystals depends on the speed of the cooling and solidification process.

Magma cools slowly enough to form crystals. Depending on rate of cooling Crystal A may form, followed by B and C, but A disappears and the final rock is made up of crystals B and C. Or different cooling crystal D forms, B and C 'coming in' later.

Some glazes, famously zinc ones, form tiny crystals, and microcrystalline glazes form crystals too small to be individually visible. Nevertheless rocks are mostly crystal with bits of glass. Glazes are mostly glass with bits of crystal.

We can achieve desired results with different coolings and different holds.
The whats whens and how longs require experimentation, intuition, and a great deal of patience to determine. Lets return to our trainful of girls from Molecular High,. Over the long trip, and as time passes, the girls regroup:
swimmers join gymnasts, the French and Spanish clubs get together , etc. The longer the trip, the more often the girls regroup. Only when the train stops at the last station do groups 'freeze'. It is up to the trainmaster to experiment with timing the trip and controlling train speed to achieve a certain desired grouping of girls when they get to their destination.

It is a wonderful and exciting journey. A thrilling trip. Welcome aboard.

Click to enlarge image

A tuff, a rock consisting of consolidated volcanic ash. Probably could be
used as a glaze.

Click to enlarge image
Pumice...a volcanic rock--which could have been obsidian, had it been
ejected sans gas. Gas bubbled through it during the volcanic eruption, and
when the rock cooled it kept the sponge like "bubbly" appearance.
The pumice was fired to c.6 in an electric kiln. In the process the
bubbles disappeared, leaving a waxy matte glaze.

The first photo was taken at Succor Creek Recreation Area in Eastern Oregon.

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Reproductions of this article are prohibited without expressed written permission from
Carol Marians © Copyright 2008 Carol Marians. All rights reserved worldwide. Visit Carol Marian's website at


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