Mark Mondloch on thu 21 feb 02
Hi,
I'm trying to advise my niece on what I believe is a dunting/shivering
problem. She's a high school art teacher, fires ^6 in a Skutt with
controller. The clay body is a commercial box mix ART 117 and she uses an
assortment of commercial glazes rated ^5-^6.
I gave her a copy of RR and JH's book and she changed her firing schedule to
try to improve her glaze results, adding a soak and ramping down the
cool-down. She had pieces crack, glaze shiver off, and a piece shattered
after it was out of the kiln. Previously she had a couple problems with the
clay, but seemed to fix it by turning off the vent before the 1100F quartz
inversion on the cool-down. I'm thinking that this is a poorly formulated
clay and the soak allowed more crystobalite to form and sent it way over the
edge.
Next year she's getting different clay, but this is what she has to work
with now. I hate to have her give up on doing the soak and slower cool down.
So I was thinking she could:
1. let the kiln drop quickly from top temp to 2000F and then soak (the Hamer
book says crystobalite forms above 2012F) and then ramp down slow from there
2. try to identify if certain glazes were on the offending pieces
3. She has 35mesh kyanite and grog on hand and a Peter pugger so I'm also
thinking that she could add kyanite to the clay which should strengthen the
clay and help lower the expansion. I don't know if the addition of grog
would help- maybe act to stop crack propagation?
Dunting/shivering is -fortunately- not something I've had to deal with
myself so I'm not very confident on the advice I'm giving and would really
appreciate any input.
Thanks,
Sylvia Mondloch
---
Mark & Sylvia Mondloch
Silver Creek Pottery & Forge
W6725 Hwy 144
Random Lake ,Wi 53075
HotArt@silvercreekpottery.com
http://www.silvercreekpottery.com
John Hesselberth on thu 21 feb 02
on 2/21/02 12:27 PM, Mark Mondloch at HotArt@SILVERCREEKPOTTERY.COM wrote:
> I gave her a copy of RR and JH's book and she changed her firing schedule to
> try to improve her glaze results, adding a soak and ramping down the
> cool-down. She had pieces crack, glaze shiver off, and a piece shattered
> after it was out of the kiln. Previously she had a couple problems with the
> clay, but seemed to fix it by turning off the vent before the 1100F quartz
> inversion on the cool-down. I'm thinking that this is a poorly formulated
> clay and the soak allowed more crystobalite to form and sent it way over the
> edge.
Hi Sylvia,
This is a bad mismatch between clay and glaze and won't be fixed by small
changes in firing schedule. When you say she has gotten by before, those
pots are still a disaster waiting to happen. Pour boiling water in one or
subject it to the freeze/boiling water cycles we recommend in the book and
see what happens. Carefully please--you are almost sure to break pots.
Please suggest to your niece that she find clay and glazes that fit each
other immediately before she has a bad accident with one of her pots. Don't
forget that some commercial glazes are as badly formulated as the worst ones
you find circulating around among potters. Just because it is commercial
doesn't mean it will fit any clay or is durable or, or , or ... You still
have to match clay and glaze and test to assure satisfactory performance.
Regards,
John
web sites: http://www.masteringglazes.com and http://www.frogpondpottery.com
EMail: john@frogpondpottery.com
"Pots, like other forms of art, are human expressions: pleasure, pain or
indifference before them depends upon their natures, and their natures are
inevitably projections of the minds of their creators." Bernard Leach, A
Potter's Book.
Dave Gayman on fri 22 feb 02
In addition to glaze-on-pot testing, you might want to test the unglazed
body.
Many an Ann Arbor potter in the 70s had problems when our clay supplier
adjusted his stoneware formula, resulting in too much free silica. A
carload of fire clay from a supplier that he had used for decades had
turned out to be woefully short, and he had added too much bentonite (and
probably other stuff) to revive its plasticity.
Personally, I lost an entire kiln-load several days before a summer art
fair... several hundred mortgage pots. They were fine on removal, but
anywhere from 4 hours to a day later, they cracked, they shattered, glaze
turned into razor-sharp flakes that fell off, and some pots rang like bells
and then fell apart.... a nightmare.
It was not the glazes, it was the clay body. On (naked eye) inspection,
the clay body was unusually vitrified and/or showed a tendency toward black
coring -- this was reduction firing. Ironically, I had produced most of
the wares in the kiln-load much thinner than usual for me, as it was a
juried art fair and I had wanted to show off.
Dave
At 08:20 PM 2/21/2002 -0500, you wrote:
>on 2/21/02 12:27 PM, Mark Mondloch at HotArt@SILVERCREEKPOTTERY.COM wrote:
>
> > She had pieces crack, glaze shiver off, and a piece shattered
> > after it was out of the kiln.
>Hi Sylvia,
>
>This is a bad mismatch between clay and glaze and won't be fixed by small
>changes in firing schedule. [snip]
>Regards,
>
>John
>
Mark Mondloch on fri 22 feb 02
Hi Dave,
Actually, I think she'll be glad to hear this. She's been beating her self
up over any little thing that she might be doing to cause this and I think
it's just bad clay. What you describe is exactly what she's been dealing
with.
Thanks,
Sylvia
---
Mark & Sylvia Mondloch
Silver Creek Pottery & Forge
W6725 Hwy 144
Random Lake ,Wi 53075
HotArt@silvercreekpottery.com
http://www.silvercreekpottery.com
----- Original Message -----
From: "Dave Gayman"
> In addition to glaze-on-pot testing, you might want to test the unglazed
> body.
>
> Many an Ann Arbor potter in the 70s had problems when our clay supplier
> adjusted his stoneware formula, resulting in too much free silica. A
> carload of fire clay from a supplier that he had used for decades had
> turned out to be woefully short, and he had added too much bentonite (and
> probably other stuff) to revive its plasticity.
>
> Personally, I lost an entire kiln-load several days before a summer art
> fair... several hundred mortgage pots. They were fine on removal, but
> anywhere from 4 hours to a day later, they cracked, they shattered, glaze
> turned into razor-sharp flakes that fell off, and some pots rang like
bells
> and then fell apart.... a nightmare.
>
> It was not the glazes, it was the clay body. On (naked eye) inspection,
> the clay body was unusually vitrified and/or showed a tendency toward
black
> coring -- this was reduction firing. Ironically, I had produced most of
> the wares in the kiln-load much thinner than usual for me, as it was a
> juried art fair and I had wanted to show off.
>
> Dave
>
> At 08:20 PM 2/21/2002 -0500, you wrote:
> >on 2/21/02 12:27 PM, Mark Mondloch at HotArt@SILVERCREEKPOTTERY.COM
wrote:
> >
> > > She had pieces crack, glaze shiver off, and a piece shattered
> > > after it was out of the kiln.
>
>
>
> >Hi Sylvia,
> >
> >This is a bad mismatch between clay and glaze and won't be fixed by small
> >changes in firing schedule. [snip]
> >Regards,
> >
> >John
Mark Mondloch on fri 22 feb 02
Hi John,
She brought over a couple of the pots for me to look at yesterday and we
discussed this some more. I think her clay is so bad that she'll have
dunting even without any glaze on it. In the catalog, they have an expansion
figure of 8.19 listed for the clay. Wouldn't that be really high for a
claybody? When I look at a glaze recipe in Insight I usually expect
anything over 8 will craze for sure on my clay.
Her immediate problem is that her students have quite a few pieces made with
this clay in the greenware state. And of course it's the best work they've
done, spent allot of time on, she doesn't want to disappoint them, et. I
suggested bisqueing them and then using cold finishes. I don't think she's
had any trouble with the clay up to bisque stage. She should be able to
bisque a bit higher too shouldn't she- say up to ^04?
If one was to try to match a glaze to this clay, I'd expect you'd try a
known high expansion glaze?
It seems tough for these teachers to do the necessary experimenting and
testing when they are expected to order all their supplies for the next year
ahead of time. I don't know how I'd do it.
Sylvia
---
Mark & Sylvia Mondloch
Silver Creek Pottery & Forge
W6725 Hwy 144
Random Lake ,Wi 53075
HotArt@silvercreekpottery.com
http://www.silvercreekpottery.com
> on 2/21/02 12:27 PM, Mark Mondloch at HotArt@SILVERCREEKPOTTERY.COM wrote:
>
> > I gave her a copy of RR and JH's book and she changed her firing
schedule to
> > try to improve her glaze results, adding a soak and ramping down the
> > cool-down. She had pieces crack, glaze shiver off, and a piece shattered
> > after it was out of the kiln. Previously she had a couple problems with
the
> > clay, but seemed to fix it by turning off the vent before the 1100F
quartz
> > inversion on the cool-down. I'm thinking that this is a poorly
formulated
> > clay and the soak allowed more crystobalite to form and sent it way over
the
> > edge.
>
> Hi Sylvia,
>
> This is a bad mismatch between clay and glaze and won't be fixed by small
> changes in firing schedule. When you say she has gotten by before, those
> pots are still a disaster waiting to happen. Pour boiling water in one or
> subject it to the freeze/boiling water cycles we recommend in the book and
> see what happens. Carefully please--you are almost sure to break pots.
>
> Please suggest to your niece that she find clay and glazes that fit each
> other immediately before she has a bad accident with one of her pots.
Don't
> forget that some commercial glazes are as badly formulated as the worst
ones
> you find circulating around among potters. Just because it is commercial
> doesn't mean it will fit any clay or is durable or, or , or ... You still
> have to match clay and glaze and test to assure satisfactory performance.
>
> Regards,
>
> John
>
>
> web sites: http://www.masteringglazes.com and
http://www.frogpondpottery.com
> EMail: john@frogpondpottery.com
>
> "Pots, like other forms of art, are human expressions: pleasure, pain or
> indifference before them depends upon their natures, and their natures are
> inevitably projections of the minds of their creators." Bernard Leach, A
> Potter's Book.
| |
|
