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.
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