iandol on mon 18 nov 02
Dear Tom Buck,=20
Great Post. Good research. Well done.
The Seger unity formula you quote is almost equivalent to the =
composition of Seger Cone 02a, just minus the 0.1 Fe2O3. Is this a =
Celadon at Cone 10?.
Accepting the given reason, the precipitation of Cristobalite, as being =
plausible. Is it also possible that, if Kaolin is included the recipe, =
the decomposition of metakaolin will release Mullite and silica which =
are not dissolving into the vitreous solution. Hence the potential for =
Cristobalite to form via a second route.
The addition of Boric oxide would increase the ability of the melt to =
dissolve more silica. It is a good solvent flux.
I am sure the original glaze could be reformulated using Wollastonite =
or, if it can be found, Anorthite. This would serve to eliminate free =
silica and, in the case of the latter mineral, Kaolin.
Regarding shortening the firing cycle. I am suspicious about this. The =
assumption here is that all of the more insoluble materials will have =
dissolved in the melt. This is unproven in this instance. Lengthening =
the firing cycle, moving up a cone or two or having a longer dwell time =
at top temperature would allow time for solvation of silica and Mullite =
to become complete. Vitreous silica is, so I understand, a relatively =
stable material.
The original posting, which I think came from Linda Knapp was in =
relation to tests using the Ian Currie method. So there were some very =
new and complex inter-relationships being introduced into the picture.
An interesting topic.
Best regards,
Ivor Lewis. Redhill, South Australia
David Beumee on tue 19 nov 02
John,
In general it would be very difficult for cristobalite to form in a porcelain body which contains 25% feldspar,
but I would be very hesitant naming a specific pecentage. Growth of cristobalite is certainly a greater
possibility in stoneware bodies where much less spar is needed to lower the absorption percentage
sufficiently. With the amount of flux in glazes at cone 10, I wouldn't think it possible that cristobalite
could form. Perhaps others know more than I.
David Beumee
Earth Alchemy Pottery
Lafayette, CO
11/19/02 7:43:52 AM, John Britt wrote:
>Ian,
>
>Correct me if I am wrong here, but I though that cristobalite will not form
>in a body with more that 25% flux. That is why porcelains have less chance
>to develop cristobalite. And therefore certainly glazes do not form
>cristobalite with or without kaolin? Am I mistaken here?
>
>Also, I missed the Tom Buck reference to iron in the unity formula? Did I
>miss a post?
>
>All I saw was the folowing:
>
>"Norton wrote: The basic porcelain glaze firing at cone 8 and
>9 (1225 to 1250 oC) is expressed by:
> 0.3K2O 0.4Al2O3 4.0SiO2
> 0.7CaO
>(Note: This Seger Formula could yield a batch recipe made of potash, kaolin,
>silica sand, and limestone/whiting. Glzcalc could suggest other mixes)."
>
>I thought that this is the formula for a Seger cone 4 but I did not see the
>reference to iron. Where was that?
>
>In need of clarification,
>
>John Britt
>
>
>
>
>
>On Mon, 18 Nov 2002 16:57:36 +1030, iandol wrote:
>
>>Dear Tom Buck,
>>
>>Great Post. Good research. Well done.
>>
>>The Seger unity formula you quote is almost equivalent to the composition
>of Seger Cone 02a, just minus the 0.1 Fe2O3. Is this a Celadon at Cone 10?.
>>
>>Accepting the given reason, the precipitation of Cristobalite, as being
>plausible. Is it also possible that, if Kaolin is included the recipe, the
>decomposition of metakaolin will release Mullite and silica which are not
>dissolving into the vitreous solution. Hence the potential for Cristobalite
>to form via a second route.
>>
>>The addition of Boric oxide would increase the ability of the melt to
>dissolve more silica. It is a good solvent flux.
>>
>>I am sure the original glaze could be reformulated using Wollastonite or,
>if it can be found, Anorthite. This would serve to eliminate free silica
>and, in the case of the latter mineral, Kaolin.
>>
>>Regarding shortening the firing cycle. I am suspicious about this. The
>assumption here is that all of the more insoluble materials will have
>dissolved in the melt. This is unproven in this instance. Lengthening the
>firing cycle, moving up a cone or two or having a longer dwell time at top
>temperature would allow time for solvation of silica and Mullite to become
>complete. Vitreous silica is, so I understand, a relatively stable material.
>>
>>The original posting, which I think came from Linda Knapp was in relation
>to tests using the Ian Currie method. So there were some very new and
>complex inter-relationships being introduced into the picture.
>>
>>An interesting topic.
>>
>>Best regards,
>>
>>Ivor Lewis. Redhill, South Australia
>>
>>___________________________________________________________________________
>___
>>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.
>
John Britt on tue 19 nov 02
Ian,
Correct me if I am wrong here, but I though that cristobalite will not form
in a body with more that 25% flux. That is why porcelains have less chance
to develop cristobalite. And therefore certainly glazes do not form
cristobalite with or without kaolin? Am I mistaken here?
Also, I missed the Tom Buck reference to iron in the unity formula? Did I
miss a post?
All I saw was the folowing:
"Norton wrote: The basic porcelain glaze firing at cone 8 and
9 (1225 to 1250 oC) is expressed by:
0.3K2O 0.4Al2O3 4.0SiO2
0.7CaO
(Note: This Seger Formula could yield a batch recipe made of potash, kaolin,
silica sand, and limestone/whiting. Glzcalc could suggest other mixes)."
I thought that this is the formula for a Seger cone 4 but I did not see the
reference to iron. Where was that?
In need of clarification,
John Britt
On Mon, 18 Nov 2002 16:57:36 +1030, iandol wrote:
>Dear Tom Buck,
>
>Great Post. Good research. Well done.
>
>The Seger unity formula you quote is almost equivalent to the composition
of Seger Cone 02a, just minus the 0.1 Fe2O3. Is this a Celadon at Cone 10?.
>
>Accepting the given reason, the precipitation of Cristobalite, as being
plausible. Is it also possible that, if Kaolin is included the recipe, the
decomposition of metakaolin will release Mullite and silica which are not
dissolving into the vitreous solution. Hence the potential for Cristobalite
to form via a second route.
>
>The addition of Boric oxide would increase the ability of the melt to
dissolve more silica. It is a good solvent flux.
>
>I am sure the original glaze could be reformulated using Wollastonite or,
if it can be found, Anorthite. This would serve to eliminate free silica
and, in the case of the latter mineral, Kaolin.
>
>Regarding shortening the firing cycle. I am suspicious about this. The
assumption here is that all of the more insoluble materials will have
dissolved in the melt. This is unproven in this instance. Lengthening the
firing cycle, moving up a cone or two or having a longer dwell time at top
temperature would allow time for solvation of silica and Mullite to become
complete. Vitreous silica is, so I understand, a relatively stable material.
>
>The original posting, which I think came from Linda Knapp was in relation
to tests using the Ian Currie method. So there were some very new and
complex inter-relationships being introduced into the picture.
>
>An interesting topic.
>
>Best regards,
>
>Ivor Lewis. Redhill, South Australia
>
>___________________________________________________________________________
___
>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.
iandol on wed 20 nov 02
Dear John Britt,
I pointed out that the unity formula given was the same as that of Seger =
Cone 02a with the iron oxide subtracted.
Best regards,
Ivor Lewis. Redhill, South Australia
John Britt on wed 20 nov 02
Ivor,
Correct me if I am wrong here, but I though that cristobalite will not form
in a body with more that 25% flux. That is why porcelains have less chance
to develop cristobalite. And therefore certainly glazes do not form
cristobalite with or without kaolin? Am I mistaken here?
Also, I missed the Tom Buck reference to iron in the unity formula? Did I
miss a post?
All I saw was the folowing:
"Norton wrote: The basic porcelain glaze firing at cone 8 and
9 (1225 to 1250 oC) is expressed by:
0.3K2O 0.4Al2O3 4.0SiO2
0.7CaO
(Note: This Seger Formula could yield a batch recipe made of potash, kaolin,
silica sand, and limestone/whiting. Glzcalc could suggest other mixes)."
I thought that this is the formula for a Seger cone 4 but I did not see the
reference to iron. Where was that?
In need of clarification,
John Britt
Ron Roy on fri 22 nov 02
Hi John,
You are right - I have dilatometered many porcelains - never any
cristobalite. I would not expect it with only 15% spar either - with most
bodies.
Almost non existant in cone 6 bodies as well - probably because it just
does not have the time needed to form - some with a talc fluxed body but
hardly visable.
Cristobalite does not form in glazes - not enough clay I suspect and way
too much flux. Melts the quartz and any cristobalite when ever it starts to
form.
Some times cristobalite is added to low fire bodies as an anti craze -
gotta be careful though - keeping the amount the same each time.
RR
>Correct me if I am wrong here, but I though that cristobalite will not form
>in a body with more that 25% flux. That is why porcelains have less chance
>to develop cristobalite. And therefore certainly glazes do not form
>cristobalite with or without kaolin? Am I mistaken here?
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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