Kathy McDonald on tue 3 apr 07
I opened the kiln today to a %^$#@#$ mess!!
Not sure what happened is indeed called spalling but it sure
is a *&$$ off.
I've had this happen to the odd piece in the e-kiln
and chalked it up to careless glazing, poor bisque, dirt,
etc.
But this occurred on pretty near a whole load of pots.
Some were fine, but about 50% of them were ruined..the
glazes literally
flew off the pots and landed on shelves and other pots.
It is a Geil 20 cu ft downdraft with a DDE controller
so I controlled the rate of temp rise to about 100 deg/hr.
This ware was made of the same clay, bisqued slow to 05...
washed before glazing as I normally do
, and was dried for about a week prior to firing.
These glazes are glazes I have used successfully in this
kiln
in the past in exactly the same manner.
What is really strange is that the glazes literally flew off
some pots onto
the shelf. I seemed to affect all glazes and not just some.
The other weird thing is that pots with the same clay, same
glaze, same shape
thrown, bisqued and glazed together were affected
differently .
There could be two identically produced pieces on the same
shelf..one would spall
the other would be just fine.
Wondering if anyone else has had a whole load ruined in this
way
and what do about it.
Kathy
...who spent the morning with a hammer!! Mosaic stepping
stones
might become my specialty.
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Paul Herman on tue 3 apr 07
Kathy,
It's called shivering, and shows a glaze fit problem. The glaze is
under so much compression that it fails and peels off.
I knew a potter in Reno who had a whole bunch of stuff crack and
shiver, and the clay company offered to replace the clay. That is one
of the things that convinced me to mix my own.
If you haven't changed your glazes, then it may be a clay formulation
problem.
Did you recently get a new batch of clay?
best,
Paul Herman
Great Basin Pottery
Doyle, California US
http://greatbasinpottery.com
On Apr 3, 2007, at 12:39 PM, Kathy McDonald wrote:
> Wondering if anyone else has had a whole load ruined in this
> way
> and what do about it.
>
>
> Kathy
>
> ...who spent the morning with a hammer!! Mosaic stepping
> stones
> might become my specialty.
>
Marcia Selsor on tue 3 apr 07
Doesn't sound like spalling to me. Spalling (as I know it) usually
happens when pieces get moisture
behind the glaze where the clay is porrous (not in the kiln) and
the glaze pops off.
Marcia
>
Marcia Selsor
http://marciaselsor.com
William & Susan Schran User on tue 3 apr 07
On 4/3/07 3:39 PM, "Kathy McDonald" wrote:
> What is really strange is that the glazes literally flew off
> some pots onto
> the shelf. I seemed to affect all glazes and not just some.
This spalling issue - is the glaze coming off and melting on the shelf or is
it peeling from the pots and chips of melted glaze that has fallen off the
pot during cooling?
These are two different problems, with different solutions.
The first could be glaze applied too thick or glaze not adhering to the
bisqueware.
The other issue is called shivering, happens during the cooling of the kiln,
after the glaze has melted and set on the pot. This is an incompatibility
between glaze and clay. Specifically, the glaze expansion is too low for the
clay body. Adjustments can be made but we also need to know:
If you are using same clay & glazes as in previous firings, but only now
having this problem, then something has changed. Is this a new batch of
clay? Did you mix new batches of glazes or have they been sitting around for
a long period of time? Think hard, sure nothing changed?
More information is needed.
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Kathy McDonald on tue 3 apr 07
Paul ,
These are clays and glazes that I have used for
a while now for c6, and it happened on two different clays
with several different glazes. I haven't added anything
to the commercial bodies and got 10 boxes of each
last shipping. Cone 6 porcelain (Dakota clay) and
Brownstone (Dakota Clay)
There have been other problems with glaze fit with this clay
but never had the glaze completely "fly" off pieces before..
I had shivering and delayed cracking (ie: pieces shattering
after 24-
48 hours) with this company's Whitestone clay so I switched
to the porcelain to avoid that problem.
The problem has not been evident with electric kiln firing
only in the gas, perhaps that's my answer, the clays might
not be designed for the gas firing.
What was really puzzling is the fact that 3 mugs on a shelf
were fine, and 2 had the glaze "jump" right off the pot.
Same with several bowls on a lower shelf.
There was no cracking or even crawling of glazes on the
pieces.
The glaze just migrated completely off the pots that were
affected.
I Kathy
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG]On Behalf Of
Paul Herman
Sent: Tuesday, April 03, 2007 4:44 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Spalling glazes: what causes it?
Kathy,
It's called shivering, and shows a glaze fit problem. The
glaze is
under so much compression that it fails and peels off.
I knew a potter in Reno who had a whole bunch of stuff crack
and
shiver, and the clay company offered to replace the clay.
That is one
of the things that convinced me to mix my own.
If you haven't changed your glazes, then it may be a clay
formulation
problem.
Did you recently get a new batch of clay?
best,
Paul Herman
Great Basin Pottery
Doyle, California US
http://greatbasinpottery.com
On Apr 3, 2007, at 12:39 PM, Kathy McDonald wrote:
> Wondering if anyone else has had a whole load ruined in
this
> way
> and what do about it.
>
>
> Kathy
>
> ...who spent the morning with a hammer!! Mosaic stepping
> stones
> might become my specialty.
>
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Kathy McDonald on tue 3 apr 07
I think you are right Marcia.
This was something very different.
I don't even know what to
call it....the glazes on the outside
kind of "jumped" off the pots
and onto my shelves in melted pools.
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG]On Behalf Of
Marcia Selsor
Sent: Tuesday, April 03, 2007 5:09 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Spalling glazes: what causes it?
Doesn't sound like spalling to me. Spalling (as I know it)
usually
happens when pieces get moisture
behind the glaze where the clay is porrous (not in the
kiln) and
the glaze pops off.
Marcia
>
Marcia Selsor
http://marciaselsor.com
____________________________________________________________
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liz gowen on sun 8 apr 07
I used to get this with some glazes that contained Gerstly Borate. When they
worked they were beautiful but other times they spit off patches all over
the kiln shelves. I now only use GB in raku.
Liz Gowen
But this occurred on pretty near a whole load of pots.
Some were fine, but about 50% of them were ruined..the
glazes literally
flew off the pots and landed on shelves and other pots.
Ron Roy on mon 9 apr 07
Dear Kathy,
You must be more descriptive - some think it's a fit problem because you
did not mention the melted glaze on your shelves.
Some glazes are prone to fast shrinking when fired while still wet - and
they do fall off pots - there are two ways to deal with this.
Wait till your glazes are dry before starting to fire them fast (you said
100 per hour - F or C?). Perhaps this explains why it happened to some and
not others?
The other way is to reformulate them so this will not happen - perhaps
using ball clay instead of kaolin for instance.
RR
>I opened the kiln today to a %^$#@#$ mess!!
>Not sure what happened is indeed called spalling but it sure
>is a *&$$ off.
>
>I've had this happen to the odd piece in the e-kiln
>and chalked it up to careless glazing, poor bisque, dirt,
>etc.
>
>But this occurred on pretty near a whole load of pots.
>Some were fine, but about 50% of them were ruined..the
>glazes literally
>flew off the pots and landed on shelves and other pots.
>
>It is a Geil 20 cu ft downdraft with a DDE controller
> so I controlled the rate of temp rise to about 100 deg/hr.
>
>This ware was made of the same clay, bisqued slow to 05...
>
>washed before glazing as I normally do
>, and was dried for about a week prior to firing.
>These glazes are glazes I have used successfully in this
>kiln
>in the past in exactly the same manner.
>
>What is really strange is that the glazes literally flew off
>some pots onto
>the shelf. I seemed to affect all glazes and not just some.
>
>The other weird thing is that pots with the same clay, same
>glaze, same shape
>thrown, bisqued and glazed together were affected
>differently .
>
>There could be two identically produced pieces on the same
>shelf..one would spall
>the other would be just fine.
>
>Wondering if anyone else has had a whole load ruined in this
>way
>and what do about it.
>
>
>Kathy
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Kathy McDonald on tue 10 apr 07
Thanks to all who replied to my question.
I got some really good advice that assisted me
to process what happened in that kiln.
When I first posted that question I was upset
( lost a bunch of pots) but since then I've calmed
down and looked at it a bit more scientifically
(which is NOT my strong suit).
I seems, by all accounts, to be one offending clay a cone 6
porcelain
and a glaze called Randy's Red (which I also mix as Randy's
Blue).
It is not a glaze with gerstley borate but I know those
glazes
are kind of finicky.
They don't like one another at all. Especially in the gas
kiln.
I fired the remaining pieces made of same clay and glazed at
same time
in the electric kiln and none
had any problems.
I fire the kiln at a rate of about 80F degrees/hour for the
first 2 hours
and then switch to a 100 degree/hour increase Ron.
Seemed to be OK for some of the clays and glazes, but not
the particular
combination I used.
Thanks again
Kathy
(who finally finished grinding shelves today)
Next time all ""iffy" combinations go on one
old shelf.
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG]On Behalf Of
Ron Roy
Sent: Monday, April 09, 2007 10:54 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Spalling glazes: what causes it?
Dear Kathy,
You must be more descriptive - some think it's a fit problem
because you
did not mention the melted glaze on your shelves.
Some glazes are prone to fast shrinking when fired while
still wet - and
they do fall off pots - there are two ways to deal with
this.
Wait till your glazes are dry before starting to fire them
fast (you said
100 per hour - F or C?). Perhaps this explains why it
happened to some and
not others?
The other way is to reformulate them so this will not
happen - perhaps
using ball clay instead of kaolin for instance.
RR
>I opened the kiln today to a %^$#@#$ mess!!
>Not sure what happened is indeed called spalling but it
sure
>is a *&$$ off.
>
>I've had this happen to the odd piece in the e-kiln
>and chalked it up to careless glazing, poor bisque, dirt,
>etc.
>
>But this occurred on pretty near a whole load of pots.
>Some were fine, but about 50% of them were ruined..the
>glazes literally
>flew off the pots and landed on shelves and other pots.
>
>It is a Geil 20 cu ft downdraft with a DDE controller
> so I controlled the rate of temp rise to about 100 deg/hr.
>
>This ware was made of the same clay, bisqued slow to 05...
>
>washed before glazing as I normally do
>, and was dried for about a week prior to firing.
>These glazes are glazes I have used successfully in this
>kiln
>in the past in exactly the same manner.
>
>What is really strange is that the glazes literally flew
off
>some pots onto
>the shelf. I seemed to affect all glazes and not just some.
>
>The other weird thing is that pots with the same clay, same
>glaze, same shape
>thrown, bisqued and glazed together were affected
>differently .
>
>There could be two identically produced pieces on the same
>shelf..one would spall
>the other would be just fine.
>
>Wondering if anyone else has had a whole load ruined in
this
>way
>and what do about it.
>
>
>Kathy
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
____________________________________________________________
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