Paul Herman on sat 11 oct 03
Hello Ron,
I have a question that has been bothering me about claybodies.
In a post to Snail you said:
> If it's cone 10 and a high iron body then I would guess yes - but
> it is possible to formulate bodies at that temperature that will not have a
> cristobalite problem.
I've noticed you mention this before, about the difficulties with ^10
clays that are higher in Iron. My question is, what can a claymaker do
to discourage cristobolite development in the higher Iron bodies?
When I switched to woodfiring, I experienced dunting broblems with my
old brown clays. The wares stay the kiln (at above 1100C) for literally
days, having plenty of time to cook up cristobolite. I've switched to
some new light colored bodies, which work well, but it's still bugging
me. I would like to be able to use some darker clays, and hope you can
point me in some possible direction.
My understanding is that the Iron rich melt will not properly dissolve
cristobolite, and if you supply enough Feldspar, then it fuses too much.
What's a Potter to do?
Many thanks in advance,
Paul Herman
Great Basin Pottery
423-725 Scott Road
Doyle, California 96109 US
potter@psln.com
----------
>From: Ron Roy
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Re: Ron: 'Glaze Freeze' question
>Date: Sat, Oct 11, 2003, 11:33 AM
>
Ron Roy on sun 12 oct 03
Hi Paul,
There are essentially four problems here.
1. Iron acts as a big time flux in cone 10 reduction - so you cannot have
enough KNaO to gobble up the cristobalite seeds that start forming about
1100C - the solution to that is - use refractory clay and then use more
KNaO to help melt it.
2. Any addition of micro sized silica to the body should be avoided as it
converts easily to cristobalite. See Peter Sohngens article in Studio
Potter - vol 28 #1 for the scoop - I did the dilatometer work for it.
3. Iron has the reputation of being a catalyst for cristobalite production
- I have no scientific evidence for this one but I have seen and heard it
several times. MgO and CaO have a similar reputation but I suspect it is
because they displace the KNaO which have a voracious appetite for melting
SiO2.
4. Cristobalite starts to form above about 1100C so the longer the time
between 1100C and top temperature, soaking time and cooling down to 1100C
will result in more - unless of course you have enough KNaO to stop it.
You are right in that wood fired kilns stay hotter longer - because the
coals cannot be turned off - crash cooling down to 1100C will help
somewhat.
So I have been able to make high fire cone 10 bodies with high iron content
that do not have any cristobalite.
Keeping in mind - as Peter found out - you need at least 10% spar with at
least 13% KNaO content to counter the cristobalite - even in the absence of
micro fine silica.
Here is a starting point for such a body (with 12% spar) - not a great body
to throw with but it can be altered - and the shrinkage is probably too
high - grog will help but better to avoid the crushed used fire brick type
which can be full of cristobalite.
Hawthorn - 15.0
FHC - 20.0
G200 - 12.0
EPK - 50.0
Red iron - 2.5 (perhaps black iron would work say at 2.0)
Total 99.5
Beware the suggestion of using an iron slip over a white body - I'm not
saying it isn't possible but the slip could have cristobalite after firing
and having a glaze and two kinds of clay fighting each other can be a
nightmare.
Proper fit testing for compatibility of the body, slip and glaze are mandatory.
RR
>I have a question that has been bothering me about claybodies.
>In a post to Snail you said:
>> If it's cone 10 and a high iron body then I would guess yes - but
>> it is possible to formulate bodies at that temperature that will not have a
>> cristobalite problem.
>I've noticed you mention this before, about the difficulties with ^10
>clays that are higher in Iron. My question is, what can a claymaker do
>to discourage cristobolite development in the higher Iron bodies?
>
>When I switched to woodfiring, I experienced dunting broblems with my
>old brown clays. The wares stay the kiln (at above 1100C) for literally
>days, having plenty of time to cook up cristobolite. I've switched to
>some new light colored bodies, which work well, but it's still bugging
>me. I would like to be able to use some darker clays, and hope you can
>point me in some possible direction.
>
>My understanding is that the Iron rich melt will not properly dissolve
>cristobolite, and if you supply enough Feldspar, then it fuses too much.
>What's a Potter to do?
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Louis Katz on sun 12 oct 03
Hi Ron,
Do you think that high iron clays that crack upon cooling and are
described as "brittle" are cracking because of cristobalite?
What does it cost to have a sample run in your dilatometer? Is there a
special size or shape you need/prefer?
I have a claybody, fires gorgeous at cone ten in our soda kiln. It is
edgy on many fronts, but on occasion it cracks on cooling or when hot
water is added to a cool cup. I recognize that hot water is not even
close to the cristobalite transition.
Louis
On Sunday, October 12, 2003, at 12:59 PM, Ron Roy wrote:
> Hi Paul,
>
> There are essentially four problems here.
>
> 1. Iron acts as a big time flux in cone 10 reduction - so you cannot
> have
> enough KNaO to gobble up the cristobalite seeds that start forming
> about
> 1100C - the solution to that is - use refractory clay and then use more
> KNaO to help melt it.
>
> 2. Any addition of micro sized silica to the body should be avoided as
> it
> converts easily to cristobalite. See Peter Sohngens article in Studio
> Potter - vol 28 #1 for the scoop - I did the dilatometer work for it.
>
> 3. Iron has the reputation of being a catalyst for cristobalite
> production
> - I have no scientific evidence for this one but I have seen and heard
> it
> several times. MgO and CaO have a similar reputation but I suspect it
> is
> because they displace the KNaO which have a voracious appetite for
> melting
> SiO2.
>
> 4. Cristobalite starts to form above about 1100C so the longer the time
> between 1100C and top temperature, soaking time and cooling down to
> 1100C
> will result in more - unless of course you have enough KNaO to stop it.
> You are right in that wood fired kilns stay hotter longer - because the
> coals cannot be turned off - crash cooling down to 1100C will help
> somewhat.
>
> So I have been able to make high fire cone 10 bodies with high iron
> content
> that do not have any cristobalite.
>
> Keeping in mind - as Peter found out - you need at least 10% spar with
> at
> least 13% KNaO content to counter the cristobalite - even in the
> absence of
> micro fine silica.
>
> Here is a starting point for such a body (with 12% spar) - not a great
> body
> to throw with but it can be altered - and the shrinkage is probably too
> high - grog will help but better to avoid the crushed used fire brick
> type
> which can be full of cristobalite.
>
> Hawthorn - 15.0
> FHC - 20.0
> G200 - 12.0
> EPK - 50.0
> Red iron - 2.5 (perhaps black iron would work say at 2.0)
> Total 99.5
>
> Beware the suggestion of using an iron slip over a white body - I'm not
> saying it isn't possible but the slip could have cristobalite after
> firing
> and having a glaze and two kinds of clay fighting each other can be a
> nightmare.
>
> Proper fit testing for compatibility of the body, slip and glaze are
> mandatory.
>
> RR
>
>
>> I have a question that has been bothering me about claybodies.
>> In a post to Snail you said:
>
>>> If it's cone 10 and a high iron body then I would guess yes - but
>>> it is possible to formulate bodies at that temperature that will not
>>> have a
>>> cristobalite problem.
>
>> I've noticed you mention this before, about the difficulties with ^10
>> clays that are higher in Iron. My question is, what can a claymaker do
>> to discourage cristobolite development in the higher Iron bodies?
>>
>> When I switched to woodfiring, I experienced dunting broblems with my
>> old brown clays. The wares stay the kiln (at above 1100C) for
>> literally
>> days, having plenty of time to cook up cristobolite. I've switched to
>> some new light colored bodies, which work well, but it's still bugging
>> me. I would like to be able to use some darker clays, and hope you can
>> point me in some possible direction.
>>
>> My understanding is that the Iron rich melt will not properly dissolve
>> cristobolite, and if you supply enough Feldspar, then it fuses too
>> much.
>> What's a Potter to do?
>
> 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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>
Ron Roy on wed 15 oct 03
Hi Louis,
I have never seen any reference to iron making bodies brittle - at least
not from sources I respect. I have the feeling that - because it's always
in reference to reduction firing - that is because the body is over fluxed.
I work with a high iron body in reduction and have never noticed the clay
being overly "brittle."
I do think the cristobalite - when present in a glassified body - will do a
lot of damage - as will beta to alpha quartz.
Cost for a dilatometer run is $75 US - I need a sample - fired to glaze
temp - about 3" long by about 0.5" in diameter - smooth, even and straight
- if there are bumps and unevenness I will have to do a lot of grinding to
make the rods even - that costs another 25 US - wrecks my finger nails and
aggravates my arthritis.
It would be most accurate if you did both a bisque and a glaze fire but I
don't usually.
You do not say if there is any glaze involved - if the glaze is already
compressed by the cristobalite squeeze - pouring hot water in will make the
glaze expand a little before the body and it will crack. You probably don't
know how many tea pots crack each year because the inside glaze is under
too much compression but it must be in the thousands - not to mention
casseroles.
Keep this in mind - the quartz inversion - up and down - is in the order of
1% volume change - the cristobalite - at about 200C - is in the order of a
3% volume change.
RR
>Hi Ron,
>Do you think that high iron clays that crack upon cooling and are
>described as "brittle" are cracking because of cristobalite?
>What does it cost to have a sample run in your dilatometer? Is there a
>special size or shape you need/prefer?
>I have a claybody, fires gorgeous at cone ten in our soda kiln. It is
>edgy on many fronts, but on occasion it cracks on cooling or when hot
>water is added to a cool cup. I recognize that hot water is not even
>close to the cristobalite transition.
>Louis
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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