Martin Rice on sun 29 sep 02
After sending off the letter about crazing a little while ago, I took a
close look at the pieces that I hadn't put in the dishwasher -- turns out
that they are now crazed, too. So it seems it was a matter of time rather
than the diswasher until crazing appeared.
Something else. There was another piece that I wasn't paying much attention
to that I glazed with something I made up myself: 1 Duncan clear glaze, .5
water, .5 slip and 1 teaspoon cobalt oxide. This piece is not yet crazed.
The finish is not as glossy as the other pieces, I guess it's a semi-gloss.
Reasons for this not crazing yet?
And two other pieces of information: all the crazing is in the overglaze.
The painting I did with the underglazes has not crazed yet at all. And the
clear glaze I use I had diluted, 2/3 glaze, 1/3 water. Could that have
contributed to the crazing?
Thanks so much,
Martin
Lagunas de Barú, Costa Rica
www.rice-family.org
Marjorie on sun 29 sep 02
Martin : a possible clue to the particular piece that did not craze, is
your addition of slip. You have added some Al203 and Si02 to the formula ,
the alumina would account for part of the matting, the silica also, as you
have slightly raised the melting point of the mix. I don't think that the
thinness of the first batch caused the crazing. The base glaze could have a
higher expansion than is appropriate for your clay body. Did you peek too
soon and unload too hot? Another cause of crazing.
MarjB
----- Original Message -----
From: "Martin Rice"
To:
Sent: Sunday, September 29, 2002 10:21 AM
Subject: About crazing - additional information
> After sending off the letter about crazing a little while ago, I took a
> close look at the pieces that I hadn't put in the dishwasher -- turns out
> that they are now crazed, too. So it seems it was a matter of time rather
> than the diswasher until crazing appeared.
>
> Something else. There was another piece that I wasn't paying much
attention
> to that I glazed with something I made up myself: 1 Duncan clear glaze, .5
> water, .5 slip and 1 teaspoon cobalt oxide. This piece is not yet crazed.
> The finish is not as glossy as the other pieces, I guess it's a
semi-gloss.
> Reasons for this not crazing yet?
>
> And two other pieces of information: all the crazing is in the overglaze.
> The painting I did with the underglazes has not crazed yet at all. And the
> clear glaze I use I had diluted, 2/3 glaze, 1/3 water. Could that have
> contributed to the crazing?
>
> Thanks so much,
> Martin
> Lagunas de Barú, Costa Rica
> www.rice-family.org
>
>
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John Baymore on mon 30 sep 02
Did you peek too soon and unload too hot? Another cause of crazing.
You are headed in a good direction with this comment and I hate to nit-pi=
ck
here .............but this might be more accurately stated that "peeking
and unloading hot" can serve to ACCELERATE the apperance of crazing that
would have happened eventually anyway.
Delayed crazing is a pretty common thing.
As you said correctly........ the cause of crazing is the difference in t=
he
reversible coefficient of thermal expansion between the glaze and the
underlying clay body. They don't both expand or shrink on heating and
cooling at the same rate. Eventually ....something gives....... usually
the glass..... that is somewhat weak in tension.
Rapid or repeated heating and cooling can serve to make it show up more
quickly.
Best,
..............................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752 (s)
800-900-1110 (s)
JohnBaymore.com
JBaymore@compuserve.com
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