Craig Martell on sun 4 jan 98
Hi:
I read Veronica's post and Ron's response and Veronica stated that she got a
calculated expansion of 8.8 for that glaze, which is high and I had stated
that it was low, so what's the deal?? I ran the glaze through Insight again
and got the same expansion as Veronica. My only explanation for this, and
the shivering problems is that the calculated expansion and the actual
expansion are not the same. As I recall, Lithium was the major oxide
present in the molecular formula of this glaze, which should affect the
expansion, and lower it.
All I can say is that according to Insight, this is a high expansion glaze,
but in reality, it acts like a low expansion glaze due to the shivering
problems that people are experiencing. I am at a loss to explain this!
Craig-stumped and I'm 45 miles south of "Stumptown".
Tom Buck on tue 6 jan 98
CM: I have this confusion too so I make the mental effort to link the body
and the glaze on its surface. A body often has an expansion number of 5 to
6 x10-6. And a good glaze often has a number of 6.8 to 7.3 x10-6.
Ok, as the pot cools in the kiln the glaze will contract more than
the body (that is what the numbers mean). So the glaze will squeeze the
body as it nears room temp. And if the compression is too much the glaze
will at some point bulge up and crack off (shiver).
So when is too high a glaze number too much? Theoretically,
anything beyond 7.6 is pushing it, especially on a porcelain body. But in
practice things never go nicely with theory. As Ron Roy has noted often,
if cristabolite forms in a porcelain body at high temperature its
expansion/contraction level changes, usually its coefficient of (linear)
expansion, or COE, number will rise substantially perhaps matching that of
a high COE glaze...hence, all will be well with the pot. (But not on a
Porcelain that does not produce cristabolite; then, another waster).
Also, another phenomenon may occur (this has not been much
reported in the literature I have seen): namely, the body either Porc. or
StoneW. may interact with the melting glaze at the boundary layer in such
a fashion as to intermix alumina/silica contents. If a high COE glaze
receives a generous dose of Al2O3/SiO2 from the body, its COE number will
decrease perhaps into the "safe" range (7.5x 10-6, more or less).
Hence, although glaze calculation and analysis can get a body -
glaze combination into the "ballpark" it cannot, being based on certain
premises, guarantee a happy combination leading to a commercial-grade pot.
That is where the eye of the potter comes into play. Each potter has a
unique system that only she/he can fine-tune using the numbers as a good
starting point and guide. Good tests. Good pots. Til later. Tom.
Tom Buck ) tel: 905-389-2339 & snailmail: 373 East
43rd St. Hamilton ON L8T 3E1 Canada (westend Lake Ontario, province of
Ontario, Canada).
Kris Baum on wed 7 jan 98
I wanted to jump in here with another weird observation (?) - I got a
copy of Glaze Simulator (nice program) and was playing around with
it, putting in the formulations for the glazes that I had tested over
the last few years. I expected to get a clustering of expansions for
glazes that fit my clay body (ies) without crazing or shivering, and
to have the crazing/shivering glazes on my claybodies lie somewhat
farther away on the expansion scale. To the contrary, they were all
over the place! And perhaps only one or two had small amounts of
lithium. Not sure what to conclude about this ...
Totally confused in the D.C. suburbs ...
> ----------------------------Original message----------------------------
> Hi:
>
> I read Veronica's post and Ron's response and Veronica stated that she got a
> calculated expansion of 8.8 for that glaze, which is high and I had stated
> that it was low, so what's the deal?? I ran the glaze through Insight again
> and got the same expansion as Veronica. My only explanation for this, and
> the shivering problems is that the calculated expansion and the actual
> expansion are not the same. As I recall, Lithium was the major oxide
> present in the molecular formula of this glaze, which should affect the
> expansion, and lower it.
>
> All I can say is that according to Insight, this is a high expansion glaze,
> but in reality, it acts like a low expansion glaze due to the shivering
> problems that people are experiencing. I am at a loss to explain this!
>
> Craig-stumped and I'm 45 miles south of "Stumptown".
>
>
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Kris Baum, Shubunkin Pottery
mailto:shubunki@erols.com
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