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
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
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
>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