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coeff of expansion/ivor

updated tue 26 mar 02


Craig Martell on mon 25 mar 02

At 01:59 PM 3/21/2002 +1030, you wrote:
>The tables available to me give the following values for forms of Silica.
>Quartz // to the axis 8*10 -6. Perp to the axis 14*10 -6, Fused (glassy
>state) 0.4*10 -6. Which of these would be chosen for inclusion in a glaze
>calcs program?

Hello Ivor:

Sorry to have taken so long to respond but your posts and questions always
require a bit of careful thought and as usual, I'm out of practice.

I don't know which of these axes would absolutely compatible with what's
used in a glaze calc program. I do know, as I'm sure you know too, that
the COEs in calc programs are based on fused, non crystalline glass.

>The formula I gave is for fully vitreous substances. Once devitrification
>starts it becomes a can of worms. You say you have pyroxenes. Have you
>identified them?

Since the glazes contain Mg and Ca as auxilliary fluxes to the feldspar,
I've always assumed that the pyroxenes involved are enstatite and
diopside. I actually tried to get my old Mineralogy prof from college to
take a gander at my glazes but he's retired now and said "maybe, when I'm
not golfing."

>These are problems which will not be solved by inventing an algorithm
>based on assumed values for substances which do not exist in the material
>we are dealing with. The answer will come through testing, making
>specimens and heating and cooling them while measuring the degree of
>expansion or following the empirical trial and error pathway.

If I had the time and energy I would think seriously about getting hold of
a dilatometer to attempt to make some quantitative headway with glaze
expansion in differing phases. But I'm usually making pots and wondering
if any of this will ever be assigned a solid number. I think the main
thing we, as potters, can do is be aware of the chaos that can arise when
our glazes devitrify. As of this moment, I have yet to experience any
disasters that would send me to the laboratory for and answer.

>If it was my intention to make a microcrystalline glaze, then I would
>anticipate a change in the Coeff.of Exp. and make pre-emptive adjustments

Most bright, non crystalline glazes with an expansion of 6.7X10-6 will fit
my porcelain pretty well. The magnesia matts that develop the pyroxenes
have been adjusted to 6.2X10-6 and they fit a lot of the time if I don't
have a prolonged cool from a lot of thermal mass. As I said before, no
disasters or serious problems yet so I try my best and am usually happy
with the results even if there is a bit of crazing.

Thanks for your post, Craig Martell in Oregon