Hanne Bjorklund on wed 7 feb 01
My wonderful cone 5-7 base glaze seems to craze when I add 7% copper as
the only colorant. I have put the clear base glaze through the freeze /
boil / India ink test, and it passed with flying colours. Not a single
craze line!
So my question is this: Why does it craze with the added copper, and is
there a solution?
I fire quite slowly and soak for half an hour at 200deg and 600deg, and for
1.5 hours at 1200deg.
I sometimes unload perhaps a bit too soon, like maybe at 175deg. Could this
have something to do with the copper crazing?
HANNE
bjorklund@clear.net.nz
Martin Howard on wed 7 feb 01
A copper glaze crazed with me. It was only 3%.
But I also had the habit of opening at 200 degrees and listened to the
tinkle, tinkle sound of crazing taking place.
Martin Howard
Webb's Cottage Pottery
Woolpits Road, Great Saling
BRAINTREE, Essex CM7 5DZ
England
martin@webbscottage.co.uk
Dennis Mummert on thu 8 feb 01
Martin - I usually open the kiln at around 500. It is a top loader,
otherwise I'd wait till about 300. I don't get crazing unless the glaze
would craze anyway.
Dennis Mummert
Earthen Fire Studios
Fairborn, Ohio
USA
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of Martin Howard
Sent: Wednesday, February 07, 2001 1:49 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: copper craze
A copper glaze crazed with me. It was only 3%.
But I also had the habit of opening at 200 degrees and listened to the
tinkle, tinkle sound of crazing taking place.
Martin Howard
Webb's Cottage Pottery
Woolpits Road, Great Saling
BRAINTREE, Essex CM7 5DZ
England
martin@webbscottage.co.uk
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Cindy Strnad on thu 8 feb 01
If the glaze crazes when you open the kiln at 200 degrees, whether
Fahrenheit or Centigrade, it will eventually craze anyway. The copper must
increase the expansion of your glaze. At any rate, very few glazes with this
much copper (Was it 7%? I don't remember) will be suitable for food use, so
a little crazing doesn't matter unless you don't like the look of it.
If you don't like the appearance with the crazing, you can lower the
expansion using various means . . . increase the silica, decrease the
spar---it depends on your glaze, and you'll probably want to use a glaze
calculation program to do this. Sometimes you can lower expansion without
changing the appearance of the glaze, but not always.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
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