Dave Finkelnburg on thu 17 jun 04
Carole,
I produced this fault by drying the rim too fast...got it to bone dry
while the rest of a platter was still leather hard. I couldn't see the
cracks, but they showed up in the bisque firing. Since then I take care to
keep the drying more even and have not had the problem.
That's my experience. Your mileage may vary...
Dave Finkelnburg in Idaho
----- Original Message -----
From: "Carole Fox"
Sent: Thursday, June 17, 2004 12:00 PM
> What causes radial cracks in the rims of platters, foot of pedestals, etc?
Carole Fox on thu 17 jun 04
What causes radial cracks in the rims of platters, foot of pedestals, etc?
I had a platter that was underfired during the glaze firing (cone 4 instead
of cone 6). When I re-fired to cone 6, it developed 5 small cracks in the
rim, each along a radius of the platter, and about half an inch in length.
In the same load, the foot of a colander with integrated pedestal developed
similar but much less noticeable and shorter cracks, which mostly filled
with glaze during the firing. The colander was being glaze fired for the
first time. All other pieces in the load (another platter, a teapot, some
mugs, bowls and several smaller objects)fired without incident.
Antoinette Badenhorst on thu 17 jun 04
Dear Carole,
Your problem has me puzzled for a little. For one, plates and other wide
bottomed stuff do not refire well unless you can create a roller effect
on which they can expand and contract in the kiln. That would cause one
large split from side to side. Here is what I want to guess is going on:
I think your tools might be blunt and you are pushing too hard on the
surface when trimming. It might also be that your pieces were too dry
when you trimmed them. If you handle the piece a lot while still
flexible (almost cheese hard) that might also causes your problem. I
would like to hear what others have to say, but that is my guess.
Antoinette Badenhorst
105 Westwood Circle
Saltillo MS
38866
662 869 1651
www.clayandcanvas.com
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Carole Fox
Sent: Thursday, June 17, 2004 1:00 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Radial Cracks
What causes radial cracks in the rims of platters, foot of pedestals,
etc?
I had a platter that was underfired during the glaze firing (cone 4
instead
of cone 6). When I re-fired to cone 6, it developed 5 small cracks in
the
rim, each along a radius of the platter, and about half an inch in
length.
In the same load, the foot of a colander with integrated pedestal
developed
similar but much less noticeable and shorter cracks, which mostly filled
with glaze during the firing. The colander was being glaze fired for
the
first time. All other pieces in the load (another platter, a teapot,
some
mugs, bowls and several smaller objects)fired without incident.
________________________________________________________________________
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Cynthia Bracker on thu 17 jun 04
Do you mean radial cracks as in spiral or cracks along a radius as in
half the diameter? If you mean spiral, Harry Fraser's book, "Ceramic
Faults and Their Remedies" cites the cause of this as over-extending the
clay during the throwing operation. Basically the pottery tries to pull
the clay up or out at a rate that the clay can't tolerate. The strains
cause spiral cracking to occur along lines following the rotation of the
clay. However, this usually this shows up after the bisque firing.
I'm also wondering if this problem (regardless of "spiral/radial" or
"along the radius" would be considered dunting? I looked it up in
Hamer&Hamer, which references Bisque (actually Biscuit - I love those
British!) Dunts, Firing Dunts and Cooling Dunts. I think yours would be
firing dunts. On that subject Hamers say
"Cristobalite and quartz inversions may cause dunts during the
warming-up period of a glaze firing. Such dunts are rare. The
inversions give the body a sudden increase in size, which, if not
allowed to happen uniformly throughout a pot, would cause stress between
two parts of the pot. This happens if the firing is too rapid."
Then halfway through the second paragraph: "Refires suffer if made from
bodies which are fine and dense and contain cristobalite as an anticraze"
Fourth paragraph says "Firing dunts may be ring cracks concentric with
the pot or vertical cracks from the rim, possibly spiraling around the pot."
Finally, the fixes:
"1. A slower rate of firing over the dunting points allows a pot to
expand as a unit as the silica inverts
2. A slightly higher biscuit firing will give a stronger biscuit to
withstand the stress.
3. A reduction of the free silica in the body will reduce the sudden
expansion proportionately"
Does this help clear it up or make it worse?
Good luck!
Cindy
PS. If any of this sounds plausible, I'd highly recommend looking up
dunting in Hamer & Hamer. There's a lot of other information there that
might also help you!
Carole Fox wrote:
>What causes radial cracks in the rims of platters, foot of pedestals, etc?
>I had a platter that was underfired during the glaze firing (cone 4 instead
>of cone 6). When I re-fired to cone 6, it developed 5 small cracks in the
>rim, each along a radius of the platter, and about half an inch in length.
>In the same load, the foot of a colander with integrated pedestal developed
>similar but much less noticeable and shorter cracks, which mostly filled
>with glaze during the firing. The colander was being glaze fired for the
>first time. All other pieces in the load (another platter, a teapot, some
>mugs, bowls and several smaller objects)fired without incident.
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
>
>
>
>
Carole Fox on fri 18 jun 04
On Thu, 17 Jun 2004 14:32:37 -0500, Cynthia Bracker
wrote:
>Do you mean radial cracks as in spiral or cracks along a radius as in
>half the diameter? If you mean spiral, Harry Fraser's book, "Ceramic
>Faults and Their Remedies" cites the cause of this as over-extending the
>clay during the throwing operation. Basically the pottery tries to pull
>the clay up or out at a rate that the clay can't tolerate. The strains
>cause spiral cracking to occur along lines following the rotation of the
>clay. However, this usually this shows up after the bisque firing.
>I'm also wondering if this problem (regardless of "spiral/radial" or
>"along the radius" would be considered dunting? I looked it up in
>Hamer&Hamer, which references Bisque (actually Biscuit - I love those
>British!) Dunts, Firing Dunts and Cooling Dunts. I think yours would be
>firing dunts. On that subject Hamers say
>"Cristobalite and quartz inversions may cause dunts during the
>warming-up period of a glaze firing. Such dunts are rare. The
>inversions give the body a sudden increase in size, which, if not
>allowed to happen uniformly throughout a pot, would cause stress between
>two parts of the pot. This happens if the firing is too rapid."
>Then halfway through the second paragraph: "Refires suffer if made from
>bodies which are fine and dense and contain cristobalite as an anticraze"
>Fourth paragraph says "Firing dunts may be ring cracks concentric with
>the pot or vertical cracks from the rim, possibly spiraling around the
pot."
>Finally, the fixes:
>"1. A slower rate of firing over the dunting points allows a pot to
>expand as a unit as the silica inverts
>2. A slightly higher biscuit firing will give a stronger biscuit to
>withstand the stress.
>3. A reduction of the free silica in the body will reduce the sudden
>expansion proportionately"
>Does this help clear it up or make it worse?
>Good luck!
>Cindy
>PS. If any of this sounds plausible, I'd highly recommend looking up
>dunting in Hamer & Hamer. There's a lot of other information there that
>might also help you!
>
>Carole Fox wrote:
>
>>What causes radial cracks in the rims of platters, foot of pedestals, etc?
>>I had a platter that was underfired during the glaze firing (cone 4
instead
>>of cone 6). When I re-fired to cone 6, it developed 5 small cracks in the
>>rim, each along a radius of the platter, and about half an inch in length.
>>In the same load, the foot of a colander with integrated pedestal
developed
>>similar but much less noticeable and shorter cracks, which mostly filled
>>with glaze during the firing. The colander was being glaze fired for the
>>first time. All other pieces in the load (another platter, a teapot, some
>>mugs, bowls and several smaller objects)fired without incident.
>>
>>__________________________________________________________________________
____
>>Send postings to clayart@lsv.ceramics.org
>>
>>You may look at the archives for the list or change your subscription
>>settings from http://www.ceramics.org/clayart/
>>
>>Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
>>
>>
>>
>>
>
>___________________________________________________________________________
___
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
(My apologies if this is a duplicate reply.) I meant cracks along a
radius. The cracks did not appear until after the referenced glaze firing,
which was glaze firing number 2 for the platter, number 1 for the
colander. The reference to firing too fast seems most plausible to me,
because the kiln did fire almost an hour faster than I had expected. I
have not had a repeat of this problem, but will look at the Hamer & Hamer
reference as recommended by Cynthia. Thanks!
John Kudlacek on fri 18 jun 04
Dear Carole,
It has been over 50 years that I had geometry in school so am a bit
confused by the term "radial cracks". I might be getting them and not
even know it. I have been having problems with what I refer to as
"concentric cracks". They are concentric(parallel)to each other as well as
to the circular form and rim and foot of the pot. These are occuring in
the trimmed area near the foot. These I attributed to poor throwing
technique (lack of compression). Then there are cracks which in essence
are perpendicular to the line of the foot or rim. Other cracks which
people are refering to as spiral, are more or less "diagonal" to rim and
foot.
I am sure Ivor or Lee have causes and cures for all these three types.
Hope your problem has been solved by now.
John Kudlacek
Joyce Lee on fri 18 jun 04
ok
----- Original Message -----
From: "John Kudlacek"
To:
Sent: Friday, June 18, 2004 9:36 AM
Subject: Re: Radial Cracks
> Dear Carole,
> It has been over 50 years that I had geometry in school so am a bit
> confused by the term "radial cracks". I might be getting them and not
> even know it. I have been having problems with what I refer to as
> "concentric cracks". They are concentric(parallel)to each other as well as
> to the circular form and rim and foot of the pot. These are occuring in
> the trimmed area near the foot. These I attributed to poor throwing
> technique (lack of compression). Then there are cracks which in essence
> are perpendicular to the line of the foot or rim. Other cracks which
> people are refering to as spiral, are more or less "diagonal" to rim and
> foot.
> I am sure Ivor or Lee have causes and cures for all these three types.
> Hope your problem has been solved by now.
> John Kudlacek
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
>
Ivor and Olive Lewis on sat 19 jun 04
Dear John Kudlecek,
Without seeing a sample I am always loathe to pass judgement. Frazer,
Hamer and Hamer seem to cover the ground quite well.
The only time I felt let down was when neither books could explain the
"Lightning Strike" patterns of crazed disintegration on a set of mugs
which quite literally exploded.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Ron Roy on sun 20 jun 04
Hi John,
You must say when the cracks happen or are seen - are they open or closed,
if the ware is glazed are the edges of the glaze sharp or melted.
Hamer gives the best descriptions because there are drawings of cracks and
explanations of each kind - how and when they happen.
RR
> It has been over 50 years that I had geometry in school so am a bit
>confused by the term "radial cracks". I might be getting them and not
>even know it. I have been having problems with what I refer to as
>"concentric cracks". They are concentric(parallel)to each other as well as
>to the circular form and rim and foot of the pot. These are occuring in
>the trimmed area near the foot. These I attributed to poor throwing
>technique (lack of compression). Then there are cracks which in essence
>are perpendicular to the line of the foot or rim. Other cracks which
>people are refering to as spiral, are more or less "diagonal" to rim and
>foot.
>I am sure Ivor or Lee have causes and cures for all these three types.
> Hope your problem has been solved by now.
>John Kudlacek
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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