Paul B on mon 30 jan 06
The people at columbus clay told me they use calcined silica as grog for
their porcelain bodies. I was thinking about trying it for my stoneware
body because most of the other types of grog i have used had some problems.
But i wonder if using calcined silica would cause problems with
crystobalite? I believe they sell it in 100 mesh. Anyone have experiece
with this or know anything about it?
thanks,
Paul
Ron Roy on wed 1 feb 06
Hi Paul,
I have never heard of calcined silica - what exactly is it?
First of all - adding silica sand to a clay body can present certain
problems - mainly bisque dunting - those chunks of silica have to go
through the beta to alpha inversion at 573C - and any extra silica added to
a body certainly increases the amount of the volume change.
If there is micro fine silica added - it can change to cristobalite easily
unless there is enough KNaO to melt it as it forms - the more silica you
add the more KNaO you need to control the cristobalite.
You can make successful clay bodies with silica sand but particle size is
important and you may need to make adjustments to prevent dunting.
Most porcelain bodies have lots of silica in them already - and lots of
KNaO - adding some silica sand can be an aid to throwability - especially
if the sand is sharp - as opposed to rounded (as in beach sand rounded by
wave action) - perhaps that is what they mean - hot quartz dropped into
water which would break up into sand?
100 mesh is very fine however and I would not expect it to help with
throwing - it would be better at 40 or 50 mesh - best to check how even the
sizing is for best results.
RR
>The people at columbus clay told me they use calcined silica as grog for
>their porcelain bodies. I was thinking about trying it for my stoneware
>body because most of the other types of grog i have used had some problems.
>But i wonder if using calcined silica would cause problems with
>crystobalite? I believe they sell it in 100 mesh. Anyone have experiece
>with this or know anything about it?
>thanks,
>Paul
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
John Britt on wed 1 feb 06
Paul,
Are you sure they don't mean calcined kyanite , a.k.a. mullite?
John Britt
www.johnbrittpottery.com
Jon Pacini on thu 2 feb 06
Greetings All --Hi Paul
I'd have to echo Ron's comments -- I've never heard of Calcined Silica
either. And as for adding it to your body, I'd be very hesitant to do so
for all the reasons mentioned. I'd check back with them and see if that's
what they really meant to say and not Calcined Kaolin or some other sized
and Calcined clay mineral.
Laguna uses a lot of sand in the western clays but it's a feldspathic sand
not a straight silica sand and that makes a big difference in the
performance of the material.
Best regards,
Jon Pacini
Clay Manager
Laguna Clay Co
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