Priscilla Hollingsworth on fri 30 jul 99
To fire clear glass in the bottom of a ceramic form such as a bowl,
you should expect heavy crazing unless you go to the glass experts for
help. I have talked to artists who cast glass, and I have done a little
myself. The problem with a thick layer of glass, whether it is glaze or
glass, is that it must be fired down to avoid cracking. Glass artists
"anneal" their work, which is holding the glass at a certain temperature
range (the kiln is dark, but hot) until the glass releases its internal
stresses. The annealing time increases geometrically as you increase
the thickness of the glass.
If you want to solve this problem by the seat of your pants, you
might run a test piece by adding a day to your electric kiln's cooling
cycle. After the kiln has hit the high temperature and is cooling, wait
until the air color disappears and turn the kiln back on low for several
hours. It's important not to open the kiln too soon. A glass artist
told me to wait until the internal kiln temperature was down to about
100 degrees F - this will help keep the glass from cracking.
Also, every kind of glass has its own melting temperature (and
annealing range, too). I have had good luck with window glass at about
cone 01. Bottle glass may be similar. You can also get into lead
crystal - it's very beautiful, and it fires lower.
The way to solve your problem technically would be to get a computer
controller for your kiln that would allow you to enter a special program
with a long cooling ramp. If you're not a gearhead (like me), you can
get somewhere with this problem by understanding some of the technical
principles and making some educated guesses. Or you could do like my
students do. They don't take "ifs" and "buts" for answers - they melt
broken glass and marbles in their pieces in the raku kiln. After ten
minutes of reduction, they pull out their work and spray water on it.
The glass cracks like crazy, but it does stick to the clay underneath,
and it's very appealing visually.
-Priscilla Hollingsworth
Date: Wed, 28 Jul 1999 09:29:19 EDT
From: Peter Atwood
Subject: puddled glass
----------------------------Original message----------------------------
Hi All,
Here's a question for you. I've checked the archives this time before
posting (aren't you proud of me?:)) and really couldn't find a specific
answer. So here goes...
I'm looking for a noncrazing clear glass or glaze to melt into a puddle
in
the bottom of a bowl. What I've done is to create a textured sort of
environment and I want to encase it in at least a quarter inch of clear
glass. How can I best accomplish such a thickness? Would just throwing
in
some busted up Corona bottles be the best way? Or should I mix a clear
glaze
and dump a pile of the dry powder in the bottom of the bowl?
Of course I'll probably try both of these methods on test bowls anyway
but I
thought I'd put the question out to the list and see what others have
done
before me.
Thanks for all the responses to my Hamada Green question. I'm doing a
glaze
firing this weekend and will test them out.
--Peter Atwood
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-----------------------
Patsy Catsos on sun 1 aug 99
In a message dated 7/30/99 4:08:30 PM, you wrote:
<broken glass and marbles in their pieces in the raku kiln. After ten
minutes of reduction, they pull out their work and spray water on it.
The glass cracks like crazy, but it does stick to the clay underneath,
and it's very appealing visually.>>
I agree that it's very appealing visually, but the crazing can be so severe
that you can cut your finger if you run it across the surface, and little
splinters of glass can pop up. Certainly not food-safe, either, due to the
crazing and the unknown components of the glass.
Patsy Catsos,
Cape Elizabeth, ME
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