Snail Scott on tue 19 mar 02
I was reading my new Ron&John book the other night,
noticing their assertion that the CoE for micro-
crystalline glazes (like the magnesium kind) cannot
be theoretically calculated, because matte glazes
are not homogeneous materials. I'll accept that,
but CoE of such glazes is still of concern to me.
Crazing isn't visually obvious in matte glazes, but
I expect it may still affect frostproofness in the
long run, and much of my work goes outdoors.
So, can anyone tell me, if the CoE will likely
increase or decrease with greater magnesium crystal
formation? For instance, if the CoE of my glaze is
determined, empirically, to be too high (crazed),
will it likely be improved by a longer soak (more
crystal formation) or a shorter soak (fewer
crystals)?
-Snail
Craig Martell on tue 19 mar 02
Hi Snail:
Crystalline silica has a higher COE than fused silica(glass). How much? I
think it depends on the amount of crystallization in the glaze. This can
get very tricky when a glaze is partially vitrified and partially
devitrifed. I have a magnesia matt that will craze when a certain amount
of pyroxene crystals develop. In the glassier phase it fits very
well. The calculated expansion is 6.7X10-6 and this number fits my
body. Except in the crystalline phase!! :>)
later, Craig Martell in Oregon
John Hesselberth on tue 19 mar 02
on 3/19/02 10:52 AM, Snail Scott at snail@MINDSPRING.COM wrote:
> So, can anyone tell me, if the CoE will likely
> increase or decrease with greater magnesium crystal
> formation? For instance, if the CoE of my glaze is
> determined, empirically, to be too high (crazed),
> will it likely be improved by a longer soak (more
> crystal formation) or a shorter soak (fewer
> crystals)?
Hi Snail,
I certainly can't give you anywhere near a complete answer to your question.
My gut tells me more crystalinity would result in a perceived higher
coefficient of expansion/contraction because crystals are pretty tightly
packed and organized entities. As they form I would speculate that the
mixture will shrink more as it cools than a glassy mixture would. Similarly
when they melt I would think they would expand more. But all that is only
SWAG (that's a sophisticated WAG).
Another twist to this question is one observation I made: I do know that on
a matte glaze I once got out of a dunting problem by decreasing the
CALCULATED coefficient of expansion. This, of course, is exactly opposite
what you would do on a glossy glaze. Would this hold true for all matte
glazes? I haven't a clue.
It is a fascinatingly complex system.
Regards,
John
Web sites: http://www.masteringglazes.com and http://www.frogpondpottery.com
Email: john@frogpondpottery.com
"Art is not a handicraft, it is the transmission of feeling the artist has
experienced." Leo Tolstoy, 1898
Ron Roy on wed 20 mar 02
Hi Snail
Only one way to tell - if you want to do the experiments I will do the
dilatometry.
RR
>I was reading my new Ron&John book the other night,
>noticing their assertion that the CoE for micro-
>crystalline glazes (like the magnesium kind) cannot
>be theoretically calculated, because matte glazes
>are not homogeneous materials. I'll accept that,
>but CoE of such glazes is still of concern to me.
>Crazing isn't visually obvious in matte glazes, but
>I expect it may still affect frostproofness in the
>long run, and much of my work goes outdoors.
>
>So, can anyone tell me, if the CoE will likely
>increase or decrease with greater magnesium crystal
>formation? For instance, if the CoE of my glaze is
>determined, empirically, to be too high (crazed),
>will it likely be improved by a longer soak (more
>crystal formation) or a shorter soak (fewer
>crystals)?
>
> -Snail
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Fax 613-475-3513
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