robtcope@mfire.com on thu 6 apr 06
How does the mesh size of silica effect a cone 6 stoneware
body? Does it matter?
Thanks
Bob Cope
Madisonville, TX
Daniel Semler on fri 7 apr 06
Hi Bob,
I think the person you want on this is Ron Roy, but I'll take a shot, as I've
been reading a bunch on silica specifically relating to body expansion
over the
last six to nine months.
The big thing about very fine mesh silica in a body is its significantly
increased tendency to convert to cristobalite. This occurs in both cone 10 and
cone 6 stonewares. Some components in a body have a catalytic effect on this
conversion too. The big antedote to it is sufficient feldspar in the mix, if
you can't avoid it. There is a great article on this exact topic in Studio
Potter Dec 1999, by Peter Sohngen. RR mentioned it the other day on another
thread.
The upshot of greater cristobalite formation is increased expansion and a
greater susceptibility to thermal shock failure at lower temps, the
cristobalite inversion commonly occuring around 220 C (though it varies) -
pretty much where hot oven cooking is done.
I understand that significant proportions of very fine mesh in silicas may be
more common now than they used to be.
> How does the mesh size of silica effect a cone 6 stoneware
> body? Does it matter?
Thanx
D
Ron Roy on sat 8 apr 06
Hi Bob,
My experience - extensive - says no. I don't think there is enough time for
it to happen. I have seen a very small amount in a talc fluxed body at cone
6 but nothing I would be concerned about.
I have - on the other hand - measured a number of cone 6 bodies by other
manufactures and have found - in some cases - a lot.
I have tried to find out why but the manufactures are not interested in
helping me track it down. I assume it is coming in with a raw material.
There is some grog available that is made from crushed used soft fire brick
for instance. Depending on how high a soft brick is fired - and how often -
there could be a lot of cristobalite present.
I don't think it would be impossible to find cristobalite in some of the
materials we use - depending on processing or even as as part of a mined
material.
RR
>How does the mesh size of silica effect a cone 6 stoneware
>body? Does it matter?
>Thanks
>Bob Cope
>Madisonville, TX
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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