David Finkelnburg on tue 20 sep 11
As many on the list have noted in response to Paul Gerhold's question,
slow cooling permits time for matte glaze surfaces to develop. Why is that=
?
Here's a long-winded explanation. :-)
Why does it take time to grow sugar crystals in the bottom of the syrup
jug? Because it takes time for the sugar molecules to move through the
thick syrup to the crystals. The crystals are pure sugar. The syrup is suga=
r
plus water. The process of crystallization requires changing where in the
container the sugar molecules are. That is called, in scientific terms, a
transport phenomenon. Sugar molecules have to move.
In precisely the same way, for crystals to grow in a glaze, atoms of
certain elements have to move through the molten glass. The longer the glas=
s
is molten, but at a temperature below the crystallization point for a given
crystal, the greater the opportunity for atoms that will make up the crysta=
l
to move through the melt and join the crystal. Crystals can form very
rapidly in some cases. Glass scientists literally drop molten glass samples
through the air to cool the glass to a solid and stop it from crystallizing=
.
The cooling time to solidification may be less than a second, the cooling
rate thousands of degrees C per second, yet glass scientists still fight
crystals in their samples that are supposed to be amorphous glass, not
crystals!
Glazes, however, are intentionally not runny, are usually rich in
alumina and consequently viscous, so growing crystals in glazes may take
quite a bit of time. That's where the slow cooling helps. I use as lot of
ash glazes. For some of them fast cooling produces a very glossy result, bu=
t
slow cooling the same glaze produces a nice matte finish. It's all a matte=
r
of time!
Please note that none of the above applies to glossy glazes--true
glossy glazes won't turn matte in even the slowest cooling cycle.
Dave Finkelnburg
http://www.mattanddavesclays.com
Dannon Rhudy on tue 20 sep 11
Dave, I knew in general how the slow cooling worked for
crystals, but your analogy with sugar crystals is a gem.
Makes a picture one cannot forget. And it will help when
I'm cooking down the maple sap next spring.....
Thanks.
regards
Dannon Rhudy
....... As many on the list have noted in response to Paul Gerhold's
question,
slow cooling permits time for matte glaze surfaces to develop. Why is that=
?
Here's a long-winded explanation. :-)
Why does it take time to grow sugar crystals in the bottom of the syrup
jug? Because it takes time for the sugar molecules to move..........
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