Ron Roy on mon 28 jul 03
Sounds to me like the inside glaze had too low an expansion for the clay -
if the outside glaze had too high an expansion (crazed) then this would
have contributed to the failure.
RR
>About twenty five years ago I built my second kiln, my first in Oz. In the
>very first firing there was a small bowl. The glazes were superb, taken
>from E. Cooper's Recipe book and applied to our common S. Aust stoneware.
>Outside a soft cobalt blue, inside a strong copper green. But a total
>disaster. The bowl was in two equal pieces, split asunder with one of
>those vertical fractures.
>
>This was a classical dunt caused by the impingement of a jet of cold or
>cool air at a critical moment during cooling through the silica phase
>change. Perhaps you have the same problem. Consult F. Hamer's Dictionary.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Earl Brunner on tue 29 jul 03
I don't know Gayle, I used to get them too. And I don't know when it
was that I stopped getting them; don't know any thing that I do
different. I hardly if ever get them now, and haven't for years. But I
still use , plastic, masonite or particle board and I don't remove them
onto anything else. Maybe it's just becoming a better thrower and not
over working or over soaking the bottoms.....
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of claybair
Sent: Tuesday, July 29, 2003 10:23 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: making sense of B-mix
I've been thinking about this for several weeks now.....
yeah, I know .... get a life!!!
Anyway, the original posting was concerning s-cracks.
I used to have s-cracks and couldn't figure out why I
haven't had them in several years. Of course it could be that my skills
have improved but I believe something else was at work.
iandol on tue 29 jul 03
Dear Ron Roy,
You say <for the clay - if the outside glaze had too high an expansion (crazed) =
then this would have contributed to the failure.>>
Plausible I admit. But I have had combinations of glazes where that =
happened as well and the effect was far more spectacular, especially on =
thinly thrown ware. This bowl was thickly thrown as well as thinly =
glazed and would have had the strength to withstand the pressures. =
Furthermore crazing would have been in evidence on the outer glaze and =
there was none.
I did have a spectacular example of the effect caused by the technology =
you describe. The mugs, and I still have one, were shattered to give =
fractures that looked like strokes of forked lightning. Beautiful jagged =
edges to every fracture and abrupt changes in direction. Moreover, in =
some examples the curvature of the cylinder opened out to a larger =
diameter, proof positive of the tension which can be created and the =
pressure which can be exerted by layers of glaze with opposing =
properties. In the case of some large globular carafes the event =
happened some time after the pots were take, cold, from the kiln. These =
exploded with the report of a 3030 rifle round. Not a pleasant =
experience when you are throwing pots, even when there is no flying =
shrapnel.
Best regards,
Ivor Lewis. Redhill, South Australia=20
claybair on tue 29 jul 03
I've been thinking about this for several weeks now.....
yeah, I know .... get a life!!!
Anyway, the original posting was concerning s-cracks.
I used to have s-cracks and couldn't figure out why I
haven't had them in several years. Of course it could be that my skills
have improved but I believe something else was at work.
I think I have found one possible culprit..... that being
what kind of bat is being used. A plastic or masonite bat
will not let the bottom dry at the same rate as the rest of the pot
which is exposed.
I used to leave the pots on my masonite or plastic bats
after running a wire under them. for the past few years
I have been using drywall and recently switched to
hardibacker. Both of them are absorbent though
I think the hardibacker is better.
I made several sets of bowls in the past few weeks.
The weather has been very hot and dry. I threw them and placed
them on my boards and just left them uncovered.
None of them have developed s-cracks.
Now I didn't try it on a plastic bat but will
today to see what happens.
So...there is my theory... and like Ann Elk and her dinosaur theory.... it
is mine!!!
Gayle Bair
Bainbridge Island, WA
http://claybair.com
Donn Buchfinck on tue 29 jul 03
B mix is interesting stuff. I use it and I like it.
I think one of the problems is wheel wedging,
that is when youy are centing and you work the clay up and down several times
to get that "just right feeling" of being in center.
I think if you wedge realy well off the wheel, try putting the wedged ball of
clay on its side and see what happens. this might counteract all the kenitic
energy you are building up in the clay.
Donn Buchfinck
San Francisco
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