Kara Rennert on mon 11 feb 02
I'm fairly new to pottery and have just started experimenting adding glass bits
when glaze firing. Since I am currently working in stained glass, that's where my
glass is coming from. So far, i put glass in the bottom of 3 pots and 2 tiles. the
pots came out quite nice, but the tiles broke into many pieces. the tiles were
basically a hand built flat surface with the sides pinched up to hold the glass in.
they were not well made, they were only built to test the glass. im wondering if it
was more likely that the tiles were poorly built, or whether the
expansion/contraction of the glass would have broken it anyway. anyone have
experience with this? or know some books where i could learn more about what glass
will do in the kiln when mixed w/pottery? some of the glass bottoms had a crackled
look which is fine with me, i just dont want to have the pieces fall apart.
thanks,
kara
Don Goodrich on mon 11 feb 02
Hi Kara,
The glass will almost certainly have a much higher coefficient of=20
expansion than the clay. This means that as it cools, the glass shrinks more=
=20
than the clay. At the same time, it's stuck to the clay very well. Since it'=
s=20
stuck to the sidewalls of the tile, there's considerable force pulling in=20
from those sides toward the center. This is what probably broke the tiles. I=
f=20
there's no spilled glass in the kiln, this happened while the piece was=20
cooling and shrinking. The heat transfer on a relatively small square tile=20
would be somewhat quicker and more uneven than in a round pot, which would=20
contribute to stress on the tile.
You can most certainly use glass on flat surfaces as a decorative element=
.=20
I've made many pieces using it. Slooowww cooling (around 100=B0/hr, down to=20
1200=B0F) helps a lot. Easy if you have a programmable kiln controller, more=
=20
work if you have to turn heat back on manually. Round, strongly made=20
stoneware should work fine, although porcelain slabs are fair game too.=20
I like to use it for candle holders, as the crackle that develops sparkle=
s=20
nicely in the flame. You can see some examples at:
http://members.aol.com/goodrichdn/candle.htm
I don't know of any books specifically devoted to the subject, but=20
Bullseye glass has a lot of material that might help. Their website is:
http://www.bullseye-glass.com/
They'll most likely be at NCECA, too.
If your stained glass is all from one manufacturer and formulated to be=20
fused with other colors in the same product line, you'll probably have bette=
r=20
luck. Some colored glasses, notable blue wine bottles, tend to form bubbles=20
at stoneware temperatures. Very annoying when the bubbles break during=20
cooling, leaving razor sharp edges on an otherwise lovely piece. Another=20
reason for slow firing: gives the outgassing time to get done, and the=20
surface to heal.
Until that book appears, you'll learn plenty by experimenting and keeping=
=20
careful notes.
Good luck,
Don Goodrich
goodrichdn@aol.com
http://members.aol.com/goodrichdn/
Cindi Anderson on mon 11 feb 02
Hi Don
I went to your site, but I'm not sure what I'm looking at. Is that a plate,
and a candle holder in front. Or is the large circle a close up of the
interior of the candle holder??? Or are they attached, and the candle light
reflects off the back surface?
Maybe this is like trying to see objects from those pictures of dots... I
just haven't the right perspective to see what I'm looking at!
Thanks!
Cindi
----- Original Message -----
From: "Don Goodrich"
I like to use it for candle holders, as the crackle that develops
sparkles
nicely in the flame. You can see some examples at:
http://members.aol.com/goodrichdn/candle.htmcareful notes.
Good luck,
Don Goodrich
Don Goodrich on tue 12 feb 02
Hi Cindi,
Sorry if those pictures are confusing. It never
occurred to me to provide a variety of viewpoints so it
would be more obvious what the sconces look like.
The candle cups are all attached to the plate parts,
and the whole thing is one piece. So your latter guess
if correct. Perhaps this drawing will help with the
perspective:
http://members.aol.com/goodrichdn/candle_2.gif
Cheers,
Don
>>I went to your site, but I'm not sure what I'm
looking at. Is that a plate, and a candle holder in
front. Or is the large circle a close up of the
interior of the candle holder??? Or are they attached,
and the candle light reflects off the back surface?<<
Kara Rennert on tue 12 feb 02
is there anything that can be added in that would effect the expansion as=
it cools
- some chemical or mineral or something?
thanks for the info, it's very helpful,
kara
Don Goodrich wrote:
> Hi Kara,
> The glass will almost certainly have a much higher coefficient of
> expansion than the clay. This means that as it cools, the glass shrinks=
more
> than the clay. At the same time, it's stuck to the clay very well. Sinc=
e it's
> stuck to the sidewalls of the tile, there's considerable force pulling =
in
> from those sides toward the center. This is what probably broke the til=
es. If
> there's no spilled glass in the kiln, this happened while the piece was
> cooling and shrinking. The heat transfer on a relatively small square t=
ile
> would be somewhat quicker and more uneven than in a round pot, which wo=
uld
> contribute to stress on the tile.
>
> You can most certainly use glass on flat surfaces as a decorative el=
ement.
> I've made many pieces using it. Slooowww cooling (around 100=B0/hr, dow=
n to
> 1200=B0F) helps a lot. Easy if you have a programmable kiln controller,=
more
> work if you have to turn heat back on manually. Round, strongly made
> stoneware should work fine, although porcelain slabs are fair game too.
> I like to use it for candle holders, as the crackle that develops sp=
arkles
> nicely in the flame. You can see some examples at:
> http://members.aol.com/goodrichdn/candle.htm
>
> I don't know of any books specifically devoted to the subject, but
> Bullseye glass has a lot of material that might help. Their website is:
> http://www.bullseye-glass.com/
> They'll most likely be at NCECA, too.
> If your stained glass is all from one manufacturer and formulated t=
o be
> fused with other colors in the same product line, you'll probably have =
better
> luck. Some colored glasses, notable blue wine bottles, tend to form bub=
bles
> at stoneware temperatures. Very annoying when the bubbles break during
> cooling, leaving razor sharp edges on an otherwise lovely piece. Anothe=
r
> reason for slow firing: gives the outgassing time to get done, and the
> surface to heal.
>
> Until that book appears, you'll learn plenty by experimenting and ke=
eping
> careful notes.
>
> Good luck,
>
> Don Goodrich
>
> goodrichdn@aol.com
> http://members.aol.com/goodrichdn/
>
> _______________________________________________________________________=
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Don Goodrich on tue 12 feb 02
Kara asked:
>is there anything that can be added in that would effect the expansion as
it cools >- some chemical or mineral or something?
If Karl Platt (glass guru) is still paying attention to this list, he must
be amused at our ignorant fumblings in his field of expertise. No doubt there
are things one could add to the glass and the clay that would bring them
closer in expansion. Doing this, however, amounts to mixing up your own
glass. This is what we do in glaze formulation anyway, so one might as well
just look up some glass recipes in some book like Conrad's Compendium and
start tinkering using glaze software. Likewise with claybody adjustment. You
could get a high-expansion body, but then you'd be stuck with very few other
glazes that would fit it.
If you had a lab analysis for your glass, you could estimate what would
change the expansion, just as with any glaze.
I'm in favor of accepting the effects one gets with the glass as is, and
designing my work accordingly.
Don Goodrich in chilly Zion, Illinois
goodrichdn@aol.com
http://members.aol.com/goodrichdn/
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