taube wilson on fri 4 feb 00
Hi all,
One of my students had some very strange cracks appear
in almost all of her pieces. These were bowls(about 4"tall,
5" diameter cereal-bowl shapes) and vases(cylindrical, about
6 or 7" tall, 4-5" diameter). They came through the
bisque firing just fine but developed spiral cracks on
the vertical surface, just above the foot, on the outside
only(the cracks didn't go all the way through to the inside).
She had about 8 pots with this problem- all fired together.
The pots were fired to ^6 ox, in an unvented kiln with a
computer controller. Nothing else in the kiln was damaged
in any way. This happened with two different glazes. What's
more, other pots with the same glazes were fine. If I remember
correctly, the cracks were somewhat filled in with glaze, but
I'll have to go back to the studio and check again to be sure.
She's throwing Highwater Loafer's Glory, but so are lots of
other people who are not having this problem.
Several of us looked at it and one of the other teachers
checked Harry Frazer's Glaze Fault book, but none of us
could figure it out. Could it be something about the way
she throws or trims? The walls were a little uneven, but
certainly not any worse than lots of other student pots.
One person speculated that she is somehow putting a twist
in the pot by handling it roughly when she puts it onto the
wheel to trim (she slides it onto the damp wheelhead and gives
it a little tap on the bottom). I'm more inclined to think
it's in the throwing, but why doesn't it show up until the
glaze firing?
Has anybody ever seen anything like this before? Any help
here is much appreciated.
TIA,
Taube Wilson
tpottery@hotmail.com
______________________________________________________
Get Your Private, Free Email at http://www.hotmail.com
Anthony Allison on sat 5 feb 00
Hiya Taube,
Two words, uneven drying. Could have been aggravated by firing a bit too
fast through the 1000 F - 1200 F range.
Tony
-----Original Message-----
From: taube wilson
To: CLAYART@LSV.UKY.EDU
Date: Friday, February 04, 2000 2:45 PM
Subject: cracks in glaze firing-weird
----------------------------Original message----------------------------
Hi all,
One of my students had some very strange cracks appear
in almost all of her pieces. These were bowls(about 4"tall,
5" diameter cereal-bowl shapes) and vases(cylindrical, about
6 or 7" tall, 4-5" diameter). They came through the
bisque firing just fine but developed spiral cracks on
the vertical surface, just above the foot, on the outside
only(the cracks didn't go all the way through to the inside).
She had about 8 pots with this problem- all fired together.
The pots were fired to ^6 ox, in an unvented kiln with a
computer controller. Nothing else in the kiln was damaged
in any way. This happened with two different glazes. What's
more, other pots with the same glazes were fine. If I remember
correctly, the cracks were somewhat filled in with glaze, but
I'll have to go back to the studio and check again to be sure.
She's throwing Highwater Loafer's Glory, but so are lots of
other people who are not having this problem.
Several of us looked at it and one of the other teachers
checked Harry Frazer's Glaze Fault book, but none of us
could figure it out. Could it be something about the way
she throws or trims? The walls were a little uneven, but
certainly not any worse than lots of other student pots.
One person speculated that she is somehow putting a twist
in the pot by handling it roughly when she puts it onto the
wheel to trim (she slides it onto the damp wheelhead and gives
it a little tap on the bottom). I'm more inclined to think
it's in the throwing, but why doesn't it show up until the
glaze firing?
Has anybody ever seen anything like this before? Any help
here is much appreciated.
TIA,
Taube Wilson
tpottery@hotmail.com
______________________________________________________
Get Your Private, Free Email at http://www.hotmail.com
I.Lewis on sun 6 feb 00
------------------
Subject: cracks in glaze firing-weird
Looked in Frank Hamer=92s book, first edition, Cracks, pages 82-3, =
illustrations
Lb and M.
Hamer says it is due to stress between the glaze and the body which cannot =
be
absorbed. Calls it a displacement rupture. He also says it may be due to =
biscuit
dunt which was not noticed at the glazing stage. That the cracks are infused
with glaze suggests biscuit dunting.
Wouldn=92t think it was due to throwing unless her way of opening is =
excessively
vigorous and rate of climb were also fast. Slowing down the speed of the =
initial
pull up may reduce the chance of steep spiral stresses. Pull up twice =
instead of
once.
Just had a pot crack on me as well, after two years service. Two cracks =
across
the diameter vertical down from the rim of an up and down cylinder used to =
cure
olives. Interesting=21
Ivor Lewis.
Paul Taylor on sun 6 feb 00
Dear Taube.
Some Times cracks such as these appear in the biscuit firing . If the
ware is under biscuit fired or received a stress at sinterring temperature
like a cold draft of air or bad placing and/or If they are badly structured
ie thin bits or highly exaggerated throwing rings. They can look just fine
until glazed fired.
Paul T
----------
>From: taube wilson
>To: CLAYART@LSV.UKY.EDU
>Subject: cracks in glaze firing-weird
>Date: Fri, Feb 4, 2000, 10:43 pm
>
>----------------------------Original message----------------------------
>Hi all,
> One of my students had some very strange cracks appear
>in almost all of her pieces. These were bowls(about 4"tall,
>5" diameter cereal-bowl shapes) and vases(cylindrical, about
>6 or 7" tall, 4-5" diameter). They came through the
>bisque firing just fine but developed spiral cracks on
>the vertical surface, just above the foot, on the outside
>only(the cracks didn't go all the way through to the inside).
>She had about 8 pots with this problem- all fired together.
>
> The pots were fired to ^6 ox, in an unvented kiln with a
>computer controller. Nothing else in the kiln was damaged
>in any way. This happened with two different glazes. What's
>more, other pots with the same glazes were fine. If I remember
>correctly, the cracks were somewhat filled in with glaze, but
>I'll have to go back to the studio and check again to be sure.
>She's throwing Highwater Loafer's Glory, but so are lots of
>other people who are not having this problem.
>
> Several of us looked at it and one of the other teachers
>checked Harry Frazer's Glaze Fault book, but none of us
>could figure it out. Could it be something about the way
>she throws or trims? The walls were a little uneven, but
>certainly not any worse than lots of other student pots.
>One person speculated that she is somehow putting a twist
>in the pot by handling it roughly when she puts it onto the
>wheel to trim (she slides it onto the damp wheelhead and gives
>it a little tap on the bottom). I'm more inclined to think
>it's in the throwing, but why doesn't it show up until the
>glaze firing?
>
> Has anybody ever seen anything like this before? Any help
>here is much appreciated.
> TIA,
> Taube Wilson
> tpottery@hotmail.com
>______________________________________________________
>Get Your Private, Free Email at http://www.hotmail.com
Becky Age on mon 7 feb 00
The problem is most probably with the clay. I have heard that Highwater had
some sort of problem with the Loafer's Glory recently. If you contact them
about it, I'm sure they could fill you in. (I've also heard that for at
least one concern they replaced the order of Loafer's Glory with another type
of clay entirely.)
Becky
Ron Roy on thu 10 feb 00
------------------
A crack in a bisque pot will come out of a glaze fire open and with
some glaze melt in the crack edges - not sharp.
A crack due to a low expansion glaze (low contraction on cooling)
will have sharp edges because it will only crack after the glaze has
set (become rigid) and the crack will be open because of the force of
the glaze. The opposite of a high expansion (high contraction on
cooling) which results in crazing.
The Hamer book is very useful for helping to identify and understand
various forms of cracking.
RR
=3ELooked in Frank Hamer=92s book, first edition, Cracks, pages 82-3, =
illustrations
=3ELb and M.
=3E
=3EHamer says it is due to stress between the glaze and the body which =
cannot be
=3Eabsorbed. Calls it a displacement rupture. He also says it may be
=3Edue to biscuit
=3Edunt which was not noticed at the glazing stage. That the cracks are =
infused
=3Ewith glaze suggests biscuit dunting.
Ron Roy =3Cronroy=40total.net=3E
93 Pegasus Trail,
Scarborough, Ontario
Canada. M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
| |
|
