John Johnson on sun 3 apr 05
I am using several of the glazes in Mastering Cone 6 Glazes; waterfall
brown, variegated blue, spearmint, bone, raspberry, raw sienna.
The clay body is Standard=92s 112 brown speckled stoneware ^ 4-6 which was
bisque fired to ^ 06. As I recall, I had a significant loss percentage of
bisqueware but had chalked that up to not having been sure pieces were
completely dry before bisqueing.
I fired in an electric kiln manually, using a pymometer and checking temps
every hour all the up & down. I spent at least 3 hours below 200F to assure
no moisture remained as a result of glazing. I also paid particular
attention to the advises regarding electric kiln firing and rates of temp
increase & decrease during quartz inversion periods as well as cooling from
1900F to 1500F at a rate of 100F per hour. I also soaked at 2150F for about
1 hr.
I had about a 50% loss rate when I opened the kiln at a temp under 100F.
Pieces were just fractured with no particular pattern; some spiral breaks,
some due (I=92m guessing) to glaze running onto the shelf.
3 days after removing from the kiln I am still getting little =91pinging=92
noises and noticing glaze slivers everywhere. In some cases this has been
accompanied by the pot cracking as well. I am having a grand opening this
Friday, 4/8/05 and am fearful of selling ANYTHING, since this shivering
continues and could cause injury to the owner, not to mention that their
newly purchased piece could actually crack as well.
Can anyone help me determine the cause of this problem as well as a
solution? I am planning a bisque firing for Tuesday and a glaze firing for
Wednesday so that I might have some finished ware to display & sell Friday
at the opening.
My only thoughts are to slightly decrease my firing temperature and maybe
eliminate the soaking period. Although it would seem to me that this is not
the solution. I am also considering taking the bisque firing to ^ 04 in an
effort to improve the stability of the ware prior to the glaze firing.
Any help provided would be greatly appreciated as I am sooooooooo desperate
at this point.
Ron Roy on mon 4 apr 05
Hi John,
I have run dilatometer tests on that body and there was some cristobalite
present - which would explain your result.
There have been others who have experienced the same thing with that body
and the same one without the specks.
I would find other clay that had a more normal amount of expansion/contracti=
on.
Perhaps others who read this list could email John about how they solved
this particular problem.
RR
>I am using several of the glazes in Mastering Cone 6 Glazes; waterfall
>brown, variegated blue, spearmint, bone, raspberry, raw sienna.
>
>
>
>The clay body is Standard=92s 112 brown speckled stoneware ^ 4-6 which was
>bisque fired to ^ 06. As I recall, I had a significant loss percentage of
>bisqueware but had chalked that up to not having been sure pieces were
>completely dry before bisqueing.
>
>
>
>I fired in an electric kiln manually, using a pymometer and checking temps
>every hour all the up & down. I spent at least 3 hours below 200F to assure
>no moisture remained as a result of glazing. I also paid particular
>attention to the advises regarding electric kiln firing and rates of temp
>increase & decrease during quartz inversion periods as well as cooling from
>1900F to 1500F at a rate of 100F per hour. I also soaked at 2150F for about
>1 hr.
>
>
>
>I had about a 50% loss rate when I opened the kiln at a temp under 100F.
>Pieces were just fractured with no particular pattern; some spiral breaks,
>some due (I=92m guessing) to glaze running onto the shelf.
>
>
>
>3 days after removing from the kiln I am still getting little =91pinging=92
>noises and noticing glaze slivers everywhere. In some cases this has been
>accompanied by the pot cracking as well. I am having a grand opening this
>Friday, 4/8/05 and am fearful of selling ANYTHING, since this shivering
>continues and could cause injury to the owner, not to mention that their
>newly purchased piece could actually crack as well.
>
>
>
>Can anyone help me determine the cause of this problem as well as a
>solution? I am planning a bisque firing for Tuesday and a glaze firing for
>Wednesday so that I might have some finished ware to display & sell Friday
>at the opening.
>
>
>
>My only thoughts are to slightly decrease my firing temperature and maybe
>eliminate the soaking period. Although it would seem to me that this is not
>the solution. I am also considering taking the bisque firing to ^ 04 in an
>effort to improve the stability of the ware prior to the glaze firing.
>
>
>
>Any help provided would be greatly appreciated as I am sooooooooo desperate
>at this point.
>
>___________________________________________________________________________=
___
>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.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
=46ax: 613-475-3513=20
Craig Martell on mon 4 apr 05
Hi:
Geez, I'm not sure where to start. Seems like a lot of things in
question. But the main thing is shivering and your post indicates that
this is the most severe problem.
If you've fired these glazes before on the same claybody and haven't had
any problems, the clay is in all probability, the culprit. If you are
using glazes from "Mastering Glazes", you can bet your last Quid that they
are durable and balanced and wouldn't shiver on a good clay.
The clay that you are using may have a composition problem that's allowing
cristobalite to develop. This is an ongoing thing that starts to happen
aggressively at cone 5 and 6 if the claybody is deficient in feldspar. The
longer the firing at the top end, and this includes soaking, the more
cristobalite. The problem with cristobalite is that it inverts at about
435 F. When the pots are heating, on the way up, there's no cristobalite
yet, so no problem. Upon cooling, if the dreaded cristobalite has
developed, your work goes thru two inversions. One at the quartz inversion
and then again when the crisbobalite inverts. The quartz inversion is
slower and less destructive. When cristobalite inverts it is very rapid,
sudden, and causes the glazes to go into excessive compression, which
causes the shivering and will also dunt a lot of pots. The sure way to
tell a dunt is to look at the fractures. If the edges are clean and no
glaze has run over the face of the cracks, it's a dunt. If you don't know
this already, a dunt is a cooling crack.
If you determine that this is indeed shivering and shattering due to the
possibility of cristobalite, the only thing I can recommend is to fire
quickly at high temperature and cool quickly to about 1800F. You don't
want to linger too long at the cone 5-6 range. A quicker passage here will
lessen the formation of cristobalite.
Then, when you have time, think about getting another batch of clay and
give it a good testing.
regards, Craig Martell Hopewell, Oregon
Ron Roy on wed 6 apr 05
Craig has written a very comprehensive description of what can cause
shivering - and I agree with what he has said - but -
I have been measuring clay bodies now for over 10 years and until recently
have never seen any cristobalite in a cone 6 body - and that includes all
the cone 6 bodies that Tuckers makes.
I have measured other clay bodies lately - particularly from potters who
have been having dunting and shivering problems.
If a glaze has too low an expansion you can still get dunting and shivering
- so I always check the expansion of the glazes first to see if that is
where the problem is.
I have measured some cone 6 bodies from another supplier which do show
significant amounts of cristobalite - much to my amazement.
I am assuming the cristobalite is coming with a raw material - wish I knew
more but I have not been allowed to see the recipes of those bodies.
RR
>Geez, I'm not sure where to start. Seems like a lot of things in
>question. But the main thing is shivering and your post indicates that
>this is the most severe problem.
>
>If you've fired these glazes before on the same claybody and haven't had
>any problems, the clay is in all probability, the culprit. If you are
>using glazes from "Mastering Glazes", you can bet your last Quid that they
>are durable and balanced and wouldn't shiver on a good clay.
>
>The clay that you are using may have a composition problem that's allowing
>cristobalite to develop. This is an ongoing thing that starts to happen
>aggressively at cone 5 and 6 if the claybody is deficient in feldspar. The
>longer the firing at the top end, and this includes soaking, the more
>cristobalite. The problem with cristobalite is that it inverts at about
>435 F. When the pots are heating, on the way up, there's no cristobalite
>yet, so no problem. Upon cooling, if the dreaded cristobalite has
>developed, your work goes thru two inversions. One at the quartz inversion
>and then again when the crisbobalite inverts. The quartz inversion is
>slower and less destructive. When cristobalite inverts it is very rapid,
>sudden, and causes the glazes to go into excessive compression, which
>causes the shivering and will also dunt a lot of pots. The sure way to
>tell a dunt is to look at the fractures. If the edges are clean and no
>glaze has run over the face of the cracks, it's a dunt. If you don't know
>this already, a dunt is a cooling crack.
>
>If you determine that this is indeed shivering and shattering due to the
>possibility of cristobalite, the only thing I can recommend is to fire
>quickly at high temperature and cool quickly to about 1800F. You don't
>want to linger too long at the cone 5-6 range. A quicker passage here will
>lessen the formation of cristobalite.
>
>Then, when you have time, think about getting another batch of clay and
>give it a good testing.
>
>regards, Craig Martell Hopewell, Oregon
>
>______________________________________________________________________________
>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.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Craig Martell on wed 6 apr 05
RR sez:
> - but -
Hey RR:
There's always a "but" somewhere.
I remember you saying sometime back that the presence of magnesia, probably
from talc, in midfire bodies can accelerate the development of
cristobalite. Did I get that right and is there a chance that this is what
we are seeing with the Standard body in question?
I was also reading your post to Jane where you were making the point about
the difference between shrinkage and expansion/contraction. Jane mentioned
that she used a body with 85% ball clay and that's the one that
shivered. I know that a lot of ball clays contain quite a bit of free
silica and I was wondering if that might be the cause of the shivering in
this case. I know that cristobalite will develop in bodies high in free
silica. With that much ball clay maybe the spar in the body just couldn't
assimilate all the cristobalite.
regards, Craig Martell Hopewell, Oregon
Edouard Bastarache Inc. on thu 7 apr 05
Hello Craig,
an analogy with the following processes could maybe explain this phenomenon
:
"Processing of amorphous silica by high-temperature calcining, with or
without the concomitant use of fluxing agents, alters the silica from the
benign amorphous to the pathogenic form (cristobalite), which causes lung
fibrosis.
Non-flux-calcined diatomite may contain from 20% to 30% cristobalite,
flux-calcined diatomite may contain as much as 60% cristobalite."
So, maybe flux-calcining quartz gives cristobalite in high amounts in theses
clays.
Later,
"Ils sont fous ces quebecois"
"They are insane these quebekers"
"Están locos estos quebequeses"
Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
www.sorel-tracy.qc.ca/~edouardb/Welcome.html
http://perso.wanadoo.fr/smart2000/index.htm
http://www.digitalfire.com/education/toxicity/
Craig Martell on thu 7 apr 05
Edouard commented:
>an analogy with the following processes could maybe explain this phenomenon
>:
>Non-flux-calcined diatomite may contain from 20% to 30% cristobalite,
>flux-calcined diatomite may contain as much as 60% cristobalite."
Bon Jour Edouard:
I haven't seen this before. Thanks.
I don't know why a cone 5-6 clay would contain enough cristobalite to cause
shivering or shattering unless there's some sort of activity going on that
has developed cristobalite in a mineral component of the body as your post
points out, or something else that accelerates the development at the mid
fire range.
I remember reading something about cristobalite being added to mid fire
claybodies to help with glaze fit. I can't remember where I read that. I
read too much. I should try watching more television.
thanks for the post, Craig Martell Hopewell, Oregon
Edouard Bastarache Inc. on thu 7 apr 05
Bonjour Craig,
welcome.
Being a sort of "Jack of all trades", it is rather easy for me
to transfer knowledge from one field to another one.
My way of thinking about ceramics technology has
always been influenced by my daytime job.
Later,
"Ils sont fous ces quebecois"
"They are insane these quebekers"
"Están locos estos quebequeses"
Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
www.sorel-tracy.qc.ca/~edouardb/Welcome.html
http://perso.wanadoo.fr/smart2000/index.htm
http://www.digitalfire.com/education/toxicity/
Ron Roy on fri 8 apr 05
Hi Craig,
Nothing wrong with your memory Craig - that mention was a long time ago.
There is one cone 6 body with 5% talc that shows a very small amount of
what could be cristobalite - or could it be enstatite - I don't know - but
a very small amount.
It is nowhere near the amount I found in the 225 and 212 bodies.
The 85% ball clay body - fired to what cone? I would not be surprised to
see cristobalite in such a body if there was not enough spar to eat up the
cristobalite as it is generated - if fired at cone 10.
If it is fired at cone 6 and there is a dunting problem then we have to
measure it - I would be surprised to see any cristobalite - unless there is
some in the ball clay to start with. Could there be cristobalite in some
grogs that are not calcined high enough?
I need a fired sample - tell who ever is having the problem to get in tough
with me and I'll send instructions how to make the samples - it's easy.
I have read about some low fired techniques - adding cristobalite to bodies
to help prevent crazing.
I am beginning to think there is a material available that does have some
cristobalite in it - and it is being used in some clay bodies. We need to
find out what that material is as soon as possible.
RR
>There's always a "but" somewhere.
>
>I remember you saying sometime back that the presence of magnesia, probably
>from talc, in midfire bodies can accelerate the development of
>cristobalite. Did I get that right and is there a chance that this is what
>we are seeing with the Standard body in question?
>
>I was also reading your post to Jane where you were making the point about
>the difference between shrinkage and expansion/contraction. Jane mentioned
>that she used a body with 85% ball clay and that's the one that
>shivered. I know that a lot of ball clays contain quite a bit of free
>silica and I was wondering if that might be the cause of the shivering in
>this case. I know that cristobalite will develop in bodies high in free
>silica. With that much ball clay maybe the spar in the body just couldn't
>assimilate all the cristobalite.
>
>regards, Craig Martell Hopewell, Oregon
>
>______________________________________________________________________________
>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.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
| |
|
