Paul on tue 27 jan 04
Hello,
i recently bought some fine grog from a different supplier than usual and
the bag said "mulcoa mulgrain" by C-E minerals. From what i have found on
there website, this is the same thing as mullite, which i understand is
commonly used as grog in porcelain and stonewares. I read in a clayart post
from the archives that this is also the same thing as kyanite, but the
mullite is calcined.
The question i have is that i was considering adding kyanite to my cone 10
stoneware body in place of the 200 mesh silica (10%) i have been using, as a
way to minimize the risk of crystobalite causing functional ware to break.
To my knowledge, this has not been a problem so far in my pottery but i want
to be a safe as possible. From what i understand, the kyanite serves a
similar function as silica in that it helps with glaze fit but is better in
that it does not contribute to crystobalite formation. Is this correct? If
the body already has around 8% mullite, is there any problems with adding
another 10% kyanite (assuming that i read correctly that these really are
the same materials)? thanks for any info,
Paul B
falmouth, Ky
David Beumee on wed 28 jan 04
> The question i have is that i was considering adding kyanite to my cone 10
> stoneware body in place of the 200 mesh silica (10%) i have been using, as a
> way to minimize the risk of crystobalite causing functional ware to break
Paul,
The best way to minimize the possibility of cristobalite growth is to make
sure you have a sufficient quantity of spar in your body to soak up the
ejected silica from the decomposition of clay in the firing and the super fine
silica dust that comes with the 200 mesh silica. My article in the Dec. 2003
Studio Potter explains how to test your body for absorption. If it turns out
you have 1% absorption or less at cone 10, you should have plenty enough spar
content to take care of any potential cristobalite.
Also, as Ron has said, the longer you leave the firing soaking at top
temperature, the greater your chances of forming cristobalite in the body.
David Beumee
Earth Alchemy Pottery
Lafayette, CO
> Hello,
> i recently bought some fine grog from a different supplier than usual and
> the bag said "mulcoa mulgrain" by C-E minerals. From what i have found on
> there website, this is the same thing as mullite, which i understand is
> commonly used as grog in porcelain and stonewares. I read in a clayart post
> from the archives that this is also the same thing as kyanite, but the
> mullite is calcined.
.
> To my knowledge, this has not been a problem so far in my pottery but i want
> to be a safe as possible. From what i understand, the kyanite serves a
> similar function as silica in that it helps with glaze fit but is better in
> that it does not contribute to crystobalite formation. Is this correct? If
> the body already has around 8% mullite, is there any problems with adding
> another 10% kyanite (assuming that i read correctly that these really are
> the same materials)? thanks for any info,
> Paul B
> falmouth, Ky
>
> ______________________________________________________________________________
> 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/
>
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Craig Edwards on wed 28 jan 04
Hello Paul; You are right. If you are having problems with crystobalite
I would get rid of the silica and replace it with Kyanite, sillmanite or
andalusite, all of which are natural forms of mullite.
Not knowing what your claybody is and how you fire it, this input is "
shooting from the hip" but I hope that I helps.
Craig Edwards
New London MN
Paul wrote:
>Hello,
>i recently bought some fine grog from a different supplier than usual and
>the bag said "mulcoa mulgrain" by C-E minerals. From what i have found on
>there website, this is the same thing as mullite, which i understand is
>commonly used as grog in porcelain and stonewares. I read in a clayart post
>from the archives that this is also the same thing as kyanite, but the
>mullite is calcined.
>The question i have is that i was considering adding kyanite to my cone 10
>stoneware body in place of the 200 mesh silica (10%) i have been using, as a
>way to minimize the risk of crystobalite causing functional ware to break.
>To my knowledge, this has not been a problem so far in my pottery but i want
>to be a safe as possible. From what i understand, the kyanite serves a
>similar function as silica in that it helps with glaze fit but is better in
>that it does not contribute to crystobalite formation. Is this correct? If
>the body already has around 8% mullite, is there any problems with adding
>another 10% kyanite (assuming that i read correctly that these really are
>the same materials)? thanks for any info,
>Paul B
>falmouth, Ky
>
>______________________________________________________________________________
>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 fri 30 jan 04
Dear Friends,
Technically this is an interesting discussion.
In my opinion substituting Mullite, 3Al2O3.2SiO2, Kyanite or
Sillimanite, Al2O3.SiO2 for a mass of Quartz in any body seems like a
good ploy. But if you think about the consequences it may be counter
productive. It just might increase the refractoriness of the body in
question and help to prevent consolidation during sintering because of
insufficient liquid phase to conclude the vitrification process.
I am not sure that the alternative of adding a further proportion of
Felspar to the original recipe, as suggested by David Baumee, would be
beneficial either. Here again there may be problems if it is intended
that such free silica as may be precipitated during the destruction of
Metakaolin, as well as that in the Clay recipe, should dissolve in the
additional melt. Now there could be an excess of fluid in the system
which would lower the degree of refractoriness of the clay body.
Without testing, who would know? But thinking about the problem gives
us insight into the possibilities
Best regards,
Ivor Lewis. Redhill, South Australia
David Beumee on fri 30 jan 04
> I am not sure that the alternative of adding a further proportion of
> Felspar to the original recipe, as suggested by David Baumee, would be
> beneficial either.
I did not suggest making an addition of feldspar to the original recipe. I
suggested testing the absorption of the fired clay body to find out if the
body was at a level of absorption that would eliminate any formation of
cristobalite and preclude the necessity of making any addition of any kind.
The idea is to come up with the most workable and plastic clay body possible,
if it to be used on the wheel. The original makeup of the body stated that an
addition of kyanite or mullite had already been added to the mixture. There's
no reason to add more unless it'd needed.
David Beumee
Earth Alchemy Pottery
Lafayette, CO
> Dear Friends,
> Technically this is an interesting discussion.
> In my opinion substituting Mullite, 3Al2O3.2SiO2, Kyanite or
> Sillimanite, Al2O3.SiO2 for a mass of Quartz in any body seems like a
> good ploy. But if you think about the consequences it may be counter
> productive. It just might increase the refractoriness of the body in
> question and help to prevent consolidation during sintering because of
> insufficient liquid phase to conclude the vitrification process.
> I am not sure that the alternative of adding a further proportion of
> Felspar to the original recipe, as suggested by David Baumee, would be
> beneficial either. Here again there may be problems if it is intended
> that such free silica as may be precipitated during the destruction of
> Metakaolin, as well as that in the Clay recipe, should dissolve in the
> additional melt. Now there could be an excess of fluid in the system
> which would lower the degree of refractoriness of the clay body.
> Without testing, who would know? But thinking about the problem gives
> us insight into the possibilities
> 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.
Ivor and Olive Lewis on sat 31 jan 04
Dear David Beumee,
Sorry, I misread the posting.
Best regards,
Ivor Lewis. Redhill, South Australia
.
Ron Roy on sat 31 jan 04
Hi Paul,
Subbing Kyanite or mullite does help to prevent cristobalite but - you
loose some of the anticrazing that comes from free quartz. You can also
think of cristobalite as an anticraze but I don't think making a body have
more expansion at ovenware temps a smart thing to do.
Better to adjust your glazes to properly fit your clay.
It would be to your advantage to read Peter Sohngens article - Studio
Potter Magazine - vol 28 #1.
The main thrust is - use at least 10% spar - add kaolin if you have to - to
get the refractoriness you need.
Limit free micro fine silica ( silica sand - even fine mesh - does not
produce cristobalite.) The problem is finding real 150 or 200M silica
without the fines - you can't tell from what they print on the bag anymore.
RR
>i recently bought some fine grog from a different supplier than usual and
>the bag said "mulcoa mulgrain" by C-E minerals. From what i have found on
>there website, this is the same thing as mullite, which i understand is
>commonly used as grog in porcelain and stonewares. I read in a clayart post
>from the archives that this is also the same thing as kyanite, but the
>mullite is calcined.
>The question i have is that i was considering adding kyanite to my cone 10
>stoneware body in place of the 200 mesh silica (10%) i have been using, as a
>way to minimize the risk of crystobalite causing functional ware to break.
>To my knowledge, this has not been a problem so far in my pottery but i want
>to be a safe as possible. From what i understand, the kyanite serves a
>similar function as silica in that it helps with glaze fit but is better in
>that it does not contribute to crystobalite formation. Is this correct? If
>the body already has around 8% mullite, is there any problems with adding
>another 10% kyanite (assuming that i read correctly that these really are
>the same materials)? thanks for any info,
>Paul B
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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