Michael Wendt on sat 6 sep 03
We struggled here for many years to find a way to touch up and refire good
pots with a small flaw. The method that works best for me is:
surround the pot with insulating firebricks to create a saggar. This slows
both heating and cooling. I suspect that the cracking comes from
cristobalite inversion because none of the first fire works crack in this
way.
One way to tell would be to fire a load only to 500 F and let it cool. If
the refires are cracked, that is probably the cause.
Regards,
Michael Wendt
wendtpot@lewiston.com
Earl Brunner on sat 6 sep 03
I have noticed that when we try to reglaze fire larger pieces in our
Skutt electrics to cone 6, we often have a problem with cracking. I am
using the following firing schedule adapted from Ron and John's book.
100 degrees per hour to 350 (slow warm up)
250 to 850
150 to 1200
500 to 2100
100 to 2250 hold 20 minutes
100 per hour to 1800 and shut down.
The cracks appear to happen in the second glaze firing BEFORE the glaze
melts, (as apposed to on the way down during cooling) I infer this by
the way the glaze looks around the edges of the cracks.
(one piece leaned over and stuck to the shelf).
These are normally larger open forms or platters. I often place shelf
supports and denser things around the edges to try and create a heat
sink of sorts.
We are using an Aardvark Clay called Texas White.
Any ideas?
Michael Wendt on sun 7 sep 03
Good catch RR,
I failed to notice he was firning only to cone 6 and that is considered
below th epoint where cristobalite forms. Even at cone 10, we are told it
takes an extended soak to generate crisobalite.
Regards,
Michael Wendt
wendtpot@lewiston.com
RR wrote in part:
>Hi Earl,
I suspect the liner glazes - if you will send me the recipes I can
calculate the expansion and give a better opinion. Do the cracked pots
happen with all liner glazes or just some?
Uneven heating is also a possibility - if that is the case the ware that is
centered on a shelf will be less prone.
Some clays are more likely to crack when reheated - cristobalite would be
one reason but I don't suspect it at cone 6 - unless the body relies on
talc for fluxing. What is the absorbency of the body when fired to glaze
temperature?
Ron Roy on sun 7 sep 03
Hi Earl,
I suspect the liner glazes - if you will send me the recipes I can
calculate the expansion and give a better opinion. Do the cracked pots
happen with all liner glazes or just some?
Uneven heating is also a possibility - if that is the case the ware that is
centered on a shelf will be less prone.
Some clays are more likely to crack when reheated - cristobalite would be
one reason but I don't suspect it at cone 6 - unless the body relies on
talc for fluxing. What is the absorbency of the body when fired to glaze
temperature?
I think it unlikely that the cracking is happening during the fast part of
your firing but you may be going into it a bit soon. Cone 6 glazes are
usually pyroplastic at 1200F but maybe not the clay - it will begin to
soften after 1500F.
I bet we can figure this out but only if you try one fix at a time.
I can't resist trying a number of fixes for problems and consequently never
do find out which one was the real solution.
This is not usual for cone 6 ware - I would like to run a dilatometer test
of the clay - if you are willing I will tell you how to make samples. I
will need the name and manufacturer of the clay - if it's yours then I will
need the recipe so I can have a chance at coming to some useful
conclusions.
RR
>I have noticed that when we try to reglaze fire larger pieces in our
>Skutt electrics to cone 6, we often have a problem with cracking. I am
>using the following firing schedule adapted from Ron and John's book.
>
>100 degrees per hour to 350 (slow warm up)
>250 to 850
>150 to 1200
>500 to 2100
>100 to 2250 hold 20 minutes
>100 per hour to 1800 and shut down.
>
>The cracks appear to happen in the second glaze firing BEFORE the glaze
>melts, (as apposed to on the way down during cooling) I infer this by
>the way the glaze looks around the edges of the cracks.
>(one piece leaned over and stuck to the shelf).
>
>These are normally larger open forms or platters. I often place shelf
>supports and denser things around the edges to try and create a heat
>sink of sorts.
>
>We are using an Aardvark Clay called Texas White.
>
>Any ideas?
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Earl Brunner on mon 8 sep 03
I've had it happen with both of the following glazes at cone 6. On
these two specific pots, they were platters, 17-18 inches in diameter,
centered in the kiln and on the shelf. Not sure if the other breakage
was with either of these glazes. Both seemed to make it through the
first glaze ok, but needed some glaze touch up. One cracked into three
pieces and the other cracked right across the middle. They were both
what I would consider "good" weight or thickness, consistent throughout.
We have had others break as well, but these seem to be representative of
the lot. One common denominator is that they have all been fairly large
pieces.
I would have to ask the manufacturer of the clay about the talc and I
will have to do an absorbency test.
Oatmeal
Custer feldspar 49.1
Silica 5.3
Whiting 17.9
Zinc oxide 8.2
EPK 3.3
Bentonite 1.9
Granular ilmenite 1.7
Rutile 1.9
Nickel carbonate 0.7
Heino-Render Matt - This matt does craze on the clay body
Custer feldspar 30.147
Nepheline syenite 20.098
Whiting 17.083
Zinc oxide 8.63
EPK 14.012
Silica 3.554
Bentonite 2.009
Frit 3134 4.467
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Ron Roy
Sent: Sunday, September 07, 2003 11:42 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Cracking on reglazing.
Hi Earl,
I suspect the liner glazes - if you will send me the recipes I can
calculate the expansion and give a better opinion. Do the cracked pots
happen with all liner glazes or just some?
Uneven heating is also a possibility - if that is the case the ware that
is
centered on a shelf will be less prone.
Some clays are more likely to crack when reheated - cristobalite would
be
one reason but I don't suspect it at cone 6 - unless the body relies on
talc for fluxing. What is the absorbency of the body when fired to glaze
temperature?
Ron Roy on tue 9 sep 03
Hi Michael,
As you probably know I have a dilatometer and have measured hundreds of
clay bodies fired at cone 04, 6 and 10 - so I do have a pretty clear
picture about when to expect to see cristobalite in a body. The charts also
give a clear picture about amount of quartz as well.
I have found the dilatometer invaluable in diagnosing clay and glaze problems.
I urge you to read the article by Peter Sohngen in the Vol 8 #1 of Studio
Potter. It is complete with charts I did for him and clearly shows how to
avoid cristobalite at cone 10.
Cristobalite is very common with the types of stoneware bodies in use at
the present time even in normal firings. Microfine silica and not enough
spar is what it boils down to. This is a major problem with iron rich
bodies - because there is usually note enough KNaO into the clay to melt
the cristobalite as it forms.
I know you are interested in body formulation - if - after you have read
Peters article - you are interested in exploring this further please
contact me and perhaps I can cast further light on the subject.
By the way - I have only once seen any cristobalite in a cone 6 body - a
white stoneware partially fluxed with Talc and then barely enough to see.
Never any cristobalite in porcelains fluxed with spar - at any temperature
- because there is plenty of KNaO to melt the cristobalite seeds as they
develop.
RR
>Good catch RR,
>I failed to notice he was firning only to cone 6 and that is considered
>below th epoint where cristobalite forms. Even at cone 10, we are told it
>takes an extended soak to generate crisobalite.
>Regards,
>Michael Wendt
>wendtpot@lewiston.com
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on thu 11 sep 03
Hi Earl,
The oatmeal totals 85.70 up to and including the Bentonite - if that total
is right then it's crazing and will not crack pots. It's way short of
silica by the way.
The Matt glaze (totals 100.13) expansion is high as well.
So I know it's not the glaze thats cracking the ware. Maybe the clay is so
vitrified it is glassy and can't take any thermal shock.
You will just have to fire em really slow I guess - I'm sure it's uneven heat.
RR
>I've had it happen with both of the following glazes at cone 6. On
>these two specific pots, they were platters, 17-18 inches in diameter,
>centered in the kiln and on the shelf. Not sure if the other breakage
>was with either of these glazes. Both seemed to make it through the
>first glaze ok, but needed some glaze touch up. One cracked into three
>pieces and the other cracked right across the middle. They were both
>what I would consider "good" weight or thickness, consistent throughout.
>We have had others break as well, but these seem to be representative of
>the lot. One common denominator is that they have all been fairly large
>pieces.
>
>I would have to ask the manufacturer of the clay about the talc and I
>will have to do an absorbency test.
>
>Oatmeal
>Custer feldspar 49.1
>Silica 5.3
>Whiting 17.9
>Zinc oxide 8.2
>EPK 3.3
>Bentonite 1.9
>Granular ilmenite 1.7
>Rutile 1.9
>Nickel carbonate 0.7
>
>Heino-Render Matt - This matt does craze on the clay body
>Custer feldspar 30.147
>Nepheline syenite 20.098
>Whiting 17.083
>Zinc oxide 8.63
>EPK 14.012
>Silica 3.554
>Bentonite 2.009
>Frit 3134 4.467
>
>-----Original Message-----
>From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Ron Roy
>Sent: Sunday, September 07, 2003 11:42 AM
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Re: Cracking on reglazing.
>
>Hi Earl,
>
>I suspect the liner glazes - if you will send me the recipes I can
>calculate the expansion and give a better opinion. Do the cracked pots
>happen with all liner glazes or just some?
>
>Uneven heating is also a possibility - if that is the case the ware that
>is
>centered on a shelf will be less prone.
>
>Some clays are more likely to crack when reheated - cristobalite would
>be
>one reason but I don't suspect it at cone 6 - unless the body relies on
>talc for fluxing. What is the absorbency of the body when fired to glaze
>temperature?
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
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>
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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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