karen gringhuis on sat 4 jun 05
Kathy -
Don't bother rebisquing.
First of all, WARPING happens only in the forming &
drying/leather hard process or as a result of being
over-fired to a temperature at which the clay body
slumps. Some shapes and those with thin walls are
more likely to warp than others. Real warping has
nothing to do with bisque firing.
Look at the cracks in your glazed pots. If there is
glaze IN the cracks, then they occurred on the way up
in either your bisque or glaze firings. If no glaze,
then on the way down in the glaze firing.
To determine if your unglazed bisque ware has dunting
hair line cracks, brush it with a weak solution of
water and india ink or food coloring (which will burn
out in a glaze firing). This should show up any
cracks. If none show up, then the problem probably
wasn’t your bisque firing. Do your subsequent glaze
firing more carefully.
If you find cracks, then they were there but invisible
from the bisque. High probablility that they resulted
from going too fast on the way up. Perhaps you weren’t
firing up as slowly as you thought. Note below that
both temps are below red heat. Crystobalite inversion
is at hot home oven temp. Also what’s the material of
your gas kiln? It may cool faster than your electric
particularly if it’s a fiber kiln.
Dunting has three main causes:
Firing too fast thru quartz inversion temp. (1063 F)
on the way up.
Cooling too fast thru quartz inversion on the way
down.
Opening kiln and removing work too soon when its temp.
is above crystobalite inversion (439 F). Cryst. forms
at high temps so it is a danger only on the way down
The higher & slower one glaze fires, the more cryst.
forms. (There is an extensive discussion of this in
Hamer’s Potters Dictionary --- IMHO a
don’t-leave-home-without-it resource.)
If you didn’t remove the glazed work when it was too
hot, then the probability is going thru quartz
inversion too fast on the way up in the bisque firing
or thru it too fast on the way down in the glaze
firing.
Karen Gringhuis
KG Pottery
Box 607 Alfred NY 14802
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Ron Roy on sun 5 jun 05
Hi Karen,
A couple of points -
I don't belive dunting happens on the way up - either in bisque
or in a glaze firing. I have fired fast accidentaly and did not have the
problem - and the raku potters certainly do it with bisque fired work.
So I do think it is on the way down when it happens.
I have been measuring a number of cone clay bodies recently that have a lot
of cristobalite in them - somethig I have never seen before at cone 6. I
have come to the conclusion that it is coming from a raw material.
Some low fire ware actually has some cristobalite added to help with
crazing - so I know it can be introduced.
Best regards - RR
>First of all, WARPING happens only in the forming &
>drying/leather hard process or as a result of being
>over-fired to a temperature at which the clay body
>slumps. Some shapes and those with thin walls are
>more likely to warp than others. Real warping has
>nothing to do with bisque firing.
>
>Look at the cracks in your glazed pots. If there is
>glaze IN the cracks, then they occurred on the way up
>in either your bisque or glaze firings. If no glaze,
>then on the way down in the glaze firing.
>
>To determine if your unglazed bisque ware has dunting
>hair line cracks, brush it with a weak solution of
>water and india ink or food coloring (which will burn
>out in a glaze firing). This should show up any
>cracks. If none show up, then the problem probably
>wasn=92t your bisque firing. Do your subsequent glaze
>firing more carefully.
>
>If you find cracks, then they were there but invisible
>from the bisque. High probablility that they resulted
>from going too fast on the way up. Perhaps you weren=92t
>firing up as slowly as you thought. Note below that
>both temps are below red heat. Crystobalite inversion
>is at hot home oven temp. Also what=92s the material of
>your gas kiln? It may cool faster than your electric
>particularly if it=92s a fiber kiln.
>
>Dunting has three main causes:
>
>Firing too fast thru quartz inversion temp. (1063 F)
>on the way up.
>
>Cooling too fast thru quartz inversion on the way
>down.
>
>Opening kiln and removing work too soon when its temp.
>is above crystobalite inversion (439 F). Cryst. forms
>at high temps so it is a danger only on the way down
>The higher & slower one glaze fires, the more cryst.
>forms. (There is an extensive discussion of this in
>Hamer=92s Potters Dictionary --- IMHO a
>don=92t-leave-home-without-it resource.)
>
>If you didn=92t remove the glazed work when it was too
>hot, then the probability is going thru quartz
>inversion too fast on the way up in the bisque firing
>or thru it too fast on the way down in the glaze
>firing.
>Karen Gringhuis
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
=46ax: 613-475-3513=20
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