MacIntire, Matt on mon 13 aug 07
I had always imagined that the texture that arises in glazes, especially
layered glazes, was due to mixing caused by outgassing during the melt.
(such as with an oil spot glaze, or some raku glazes) This past weekend
I pulled draw tiles over the entire firing range and found that a highly
textured glaze combination that I am using does not ever go through a
"boiling" phase. So now I am confused...
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What causes the aggregation and separation we observe in highly textured
glazes that do not "boil"? =20
Does anyone know?
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Thanks in advance
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Ivor and Olive Lewis on tue 14 aug 07
Dear Matt MacIntire,
I cannot recall the details of the glazes you are using or your firing =
program.
I feel you may be making assumptions about the behaviour of your raw =
materials as kiln temperature is rising, ignoring the potential for =
crystallisation as your kiln is cooling, disregarding variations in =
thickness of clay beneath your mature glazed surface and forgetting =
about the interplay of light on transparent, translucent and opaque =
areas of your pots. Other undefined factors may influence your work.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
George Koller on tue 14 aug 07
Hallo,
I might have something to contribute on this one as I spent some 2 months developing a glaze effect that I originally thought was a crystaline effect. This phenomena started showing up when I was putting a final clear glaze coat over a base glaze.
Turned out in our case that the effect happened during cooling. Critical was that one layer had a different "freezing" tempurature. We learned that 100% for certain (or as close as I get to 100%) with repeated experiments. I was working with a true glaze expert and the way he explained it has stuck with me - "think of the a lake freezing over". This is how I visualized it :: The lower level has a higher concentration of colorant. If it should cool first there can be "chunks" of solidifying glaze pushed and shoved about at different angles with the clear over glaze dropping down here and there as needed to satisfy the needs of surface tension.
This is how I believe the two layer depth effect that I stumbled upon was caused, I have no particular reason to believe it is the only way that "depth" can occur...
Best,
George
MacIntire, Matt on tue 14 aug 07
George, thanks for a plausible explanation. Your idea gets me partway
there.=20
I think you are correct, at least in part. The draw tiles clearly
showed that the darker glaze layer I applied first was fluid much
earlier than the outer whitish layer. Presumably this would hold true
during cooling.
What I still don't understand is why the glaze layers separate (or
clump) into this textured arrangement even on horizontal surfaces where
there is no flow.
Could this be some effect of surface tension in the stiffer glaze, like
crawling sort of, but happening over the fluid layer of glaze
underneath? That is my current suspicion, after understanding Georges
remarks.
I posted some close-ups of the glaze combination I am working with on
the Flickr Clayart pages. Basically it is a fluid darker glaze
underneath a stiffer whitish glaze.
http://www.flickr.com/photos/clayart/
I realize that in these shots, there is some crystallization during
cooling, which shows up as flecks. I like that too, but what I want to
understand better is the texture, since it forms even when cooling is
too rapid for much crystallization.
Thanks in advance to anyone who cares to comment further.
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