John Hesselberth on fri 2 apr 99
After 25 years of making pots, I thought I had found all the
problems--well, I found one I've never encountered over the last month.
Clay continues to humble me.
I was gearing up to fill all the orders I took at the Rosen show in
Philadelphia. Bought another lot of my usual clay, made a couple of fine
tuning adjustments on my glaze firing cycle to try to get rid of a few
pinholes and went to work.
I fire to cone 6 in a computer controlled L&L kiln (great kiln, by the
way--went to it after using a Skutt for several years and I'll be
sticking with L&L from now on) using Standard 306 clay. This is a tan
stoneware body that is rated at cones 4-10.
I opened the kiln after a glaze firing in early March and found 4 pots
with classic cooling dunt cracks. Since that was a small percentage of
the load I didn't think too much about it. Next load was worse--nearly
every pot of any size had cracked. Only a few small ones survived.
Since then I have been in the crisis mode.
My first thought was the new batch of clay. My problem seemed to
coincide exactly with cleaning out the old batch and starting to use the
new. Talked to my supplier (Del-Val) and Standard and was assured that
no one else had reported this problem and they had made no changes in
this lot vs. the previous one I had been using. I decided, though, to
try two Miller Clays (40 and 55) to test the clay hypothesis. Bottom
line--both Miller Clays dunted at least as badly as my Standard Clay!
What is going on???? I concluded it wasn't the clay.
I hand build from slabs rolled on a Bailey slab roller. I texture the
slabs on the outside and leave the iside smooth. I pour glaze on the
inside and finish the outside by brushing on multiple slips and glazes.
The bottoms of my pots have multiple slips, but no glaze so I can set
them directly on the shelf without waxing. I got to wondering if the
lack of glaze on a fairly broad bottom coupled with the surface contact
of the pot and the shelf, which would give uneven cooling, was causing my
problem. I glazed the bottoms of a few pots and set them on stilts.
Bottom line--no progress--every large pot cracked. This pretty much
convinced me that what I was seeing was not a result of the silica
transition that occurs at 573 deg. C, but possibly was the result of
cristobalite formation. But could cristobalite really be forming at Cone
6???
At this point I hit the books another time to really study what is known
about dunting, cristobalite formation etc. Hamer and Hamer say that it
takes 10 days to convert 100% of silica to cristobalite at 1300 deg. C.
The lowest practical temperature for any cristobalite conversion is 1100
deg. C. and "at this temperature a long time is required". It seems like
at cone 6 (let's just call it 1200 deg. C for simplicity) it would be
possible but would take a long, long time.
Then I read on and noted that both calcium and magnesium are catalysts
for the conversion of silica to cristobalite. One of my glazes is a high
calcium glaze and another has a fair amount of magnesia. But they are
only on the surface, not in the clay. Also, those minor changes I made
in my firing cycle were in the wrong direction, e.g. I increased the soak
time at temperature from 20 to 30 minutes and increased the end point
temperature by 10 deg. C. (from a cone 6 at 3 o'clock to cone 6 nearly
full down.)
The hypothesis that was forming in my mind was as follows:
Even though cristobalite formation should be slow at the temperatures I
was operating at, it could be happening at/near the clay/glaze interface
because of the presence of a highly mobile calcium and magnesium
molecules in my molten glazes. This was potentially aggravated by the
longer soak and slightly higher temperature that I had recently switched
to.
Experiment to test the hypothesis: Take the temperature back down and
eliminate the 30 minute soak. I opened the kiln this morning. I only
fired 3 pots, but there were NO COOLING DUNTS. A small success, but it
gives me the courage to try a larger number of pots.
Now some questions for Clayart members while my next load is firing:
1. Does this hypothesis/potential solution make sense? Can this really
have been happening the way I describe above? Or did I just get lucky
with the last 3 pots and the real problem is waiting to bite me again
when I fire a full load?
2. This clay is supposed to be good to cone 10. It would surely see a
lot more heat in getting it to Cone 10 than it sees by soaking it for 30
minutes at Cone 6. Does anyone take this clay to Cone 10? I am assured
by my supplier that people do without any problems.
3. There have been comments on Clayart about the questionable practice
of making/using clays that have a large maturity range (cones 4-10 in my
case). Does anyone know how clay suppliers typically accomplish this?
Is it, by any chance, by using a high silica recipe or by using talc in
the recipe thereby making cristobalite formation more likely?
4. What other experiments or advice would you have to get me away from
the edge of this cliff I have apparently been dancing along?
Ahhh, isn't clay wonderful. It certainly keeps you learning throughout
your life. It doesn't let the gray cells get rusty!
Any and all input/ideas/comments/encouragement will be most welcome. If
my current hypothesis turns out to be incorrect I am absolutely at wits
end.
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is time for potters to claim their proper field. Pottery in its pure
form relies neither on sculptural additions nor on pictorial decorations.
but on the counterpoint of form, design, colour, texture and the quality
of the material, all directed to a function." Michael Cardew in "Pioneer
Pottery"
Steven Roberts on fri 2 apr 99
I have a photocopy of an excellent article on cristobalite from an old
Studio Potter (I don't know which issue) called "Commentary on the Malone
Report," by James McKinnell. He gives the lowest temperature possible for
cristobalite formation as 950 Centigrade. Another figure I found in my
notes is 980 Centigrade. These are both below cone six!
McKinnell lists the causes of most cristobalite formation as 1) too
little feldspar in the body; he suggests a minimum of 10%. 2) too little
ball clay; ball clay adds fluxes. 3) too much iron or iron-bearing clay.
4) too slow cooling; cristobalite growth takes time. 5) too long
firing cycle; same effects as slow cooling. 6) high percentages of the
wrong fireclays (he lists Lincoln and Sutter as cristobalite-promoting
clays.
It seems that if the free silica in your fired body is dissolved in a
glass, it cannot assume a crystalline form like cristobalite. Sufficient
feldspar will create enough glass to dissolve your free silica. The worst
instance of cristobalite I ever saw was an entire kiln load of porcelain
that a student had accidentally mixed without feldspar. The pots shattered
and crumbled into tiny pieces when the door was opened! Quite a sight.
Anyway, John, if lowering the firing temperature eliminated your problem
that may be the solution; but, there is no excuse for a clay to be that
sensitive. Either a non-standard batch was mixed accidentally (these
things can happen despite the manufacturer's good intentions) or it's a
very sensitive clay. You might want to look into a clay with more
tolerance for slight overfiring!
Steve Roberts
>----------------------------Original message----------------------------
>After 25 years of making pots, I thought I had found all the
>problems--well, I found one I've never encountered over the last month.
>Clay continues to humble me.
>
>I was gearing up to fill all the orders I took at the Rosen show in
>Philadelphia. Bought another lot of my usual clay, made a couple of fine
>tuning adjustments on my glaze firing cycle to try to get rid of a few
>pinholes and went to work.
>
>I fire to cone 6 in a computer controlled L&L kiln (great kiln, by the
>way--went to it after using a Skutt for several years and I'll be
>sticking with L&L from now on) using Standard 306 clay. This is a tan
>stoneware body that is rated at cones 4-10.
>
>I opened the kiln after a glaze firing in early March and found 4 pots
>with classic cooling dunt cracks. Since that was a small percentage of
>the load I didn't think too much about it. Next load was worse--nearly
>every pot of any size had cracked. Only a few small ones survived.
>Since then I have been in the crisis mode.
>
>My first thought was the new batch of clay. My problem seemed to
>coincide exactly with cleaning out the old batch and starting to use the
>new. Talked to my supplier (Del-Val) and Standard and was assured that
>no one else had reported this problem and they had made no changes in
>this lot vs. the previous one I had been using. I decided, though, to
>try two Miller Clays (40 and 55) to test the clay hypothesis. Bottom
>line--both Miller Clays dunted at least as badly as my Standard Clay!
>What is going on???? I concluded it wasn't the clay.
>
>I hand build from slabs rolled on a Bailey slab roller. I texture the
>slabs on the outside and leave the iside smooth. I pour glaze on the
>inside and finish the outside by brushing on multiple slips and glazes.
>The bottoms of my pots have multiple slips, but no glaze so I can set
>them directly on the shelf without waxing. I got to wondering if the
>lack of glaze on a fairly broad bottom coupled with the surface contact
>of the pot and the shelf, which would give uneven cooling, was causing my
>problem. I glazed the bottoms of a few pots and set them on stilts.
>Bottom line--no progress--every large pot cracked. This pretty much
>convinced me that what I was seeing was not a result of the silica
>transition that occurs at 573 deg. C, but possibly was the result of
>cristobalite formation. But could cristobalite really be forming at Cone
>6???
>
>At this point I hit the books another time to really study what is known
>about dunting, cristobalite formation etc. Hamer and Hamer say that it
>takes 10 days to convert 100% of silica to cristobalite at 1300 deg. C.
>The lowest practical temperature for any cristobalite conversion is 1100
>deg. C. and "at this temperature a long time is required". It seems like
>at cone 6 (let's just call it 1200 deg. C for simplicity) it would be
>possible but would take a long, long time.
>
>Then I read on and noted that both calcium and magnesium are catalysts
>for the conversion of silica to cristobalite. One of my glazes is a high
>calcium glaze and another has a fair amount of magnesia. But they are
>only on the surface, not in the clay. Also, those minor changes I made
>in my firing cycle were in the wrong direction, e.g. I increased the soak
>time at temperature from 20 to 30 minutes and increased the end point
>temperature by 10 deg. C. (from a cone 6 at 3 o'clock to cone 6 nearly
>full down.)
>
>The hypothesis that was forming in my mind was as follows:
>
>Even though cristobalite formation should be slow at the temperatures I
>was operating at, it could be happening at/near the clay/glaze interface
>because of the presence of a highly mobile calcium and magnesium
>molecules in my molten glazes. This was potentially aggravated by the
>longer soak and slightly higher temperature that I had recently switched
>to.
>
>Experiment to test the hypothesis: Take the temperature back down and
>eliminate the 30 minute soak. I opened the kiln this morning. I only
>fired 3 pots, but there were NO COOLING DUNTS. A small success, but it
>gives me the courage to try a larger number of pots.
>
>Now some questions for Clayart members while my next load is firing:
>
>1. Does this hypothesis/potential solution make sense? Can this really
>have been happening the way I describe above? Or did I just get lucky
>with the last 3 pots and the real problem is waiting to bite me again
>when I fire a full load?
>
>2. This clay is supposed to be good to cone 10. It would surely see a
>lot more heat in getting it to Cone 10 than it sees by soaking it for 30
>minutes at Cone 6. Does anyone take this clay to Cone 10? I am assured
>by my supplier that people do without any problems.
>
>3. There have been comments on Clayart about the questionable practice
>of making/using clays that have a large maturity range (cones 4-10 in my
>case). Does anyone know how clay suppliers typically accomplish this?
>Is it, by any chance, by using a high silica recipe or by using talc in
>the recipe thereby making cristobalite formation more likely?
>
>4. What other experiments or advice would you have to get me away from
>the edge of this cliff I have apparently been dancing along?
>
>Ahhh, isn't clay wonderful. It certainly keeps you learning throughout
>your life. It doesn't let the gray cells get rusty!
>
>Any and all input/ideas/comments/encouragement will be most welcome. If
>my current hypothesis turns out to be incorrect I am absolutely at wits
>end.
>
>John Hesselberth
>Frog Pond Pottery
>P.O. Box 88
>Pocopson, PA 19366 USA
>EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
>
>"It is time for potters to claim their proper field. Pottery in its pure
>form relies neither on sculptural additions nor on pictorial decorations.
>but on the counterpoint of form, design, colour, texture and the quality
>of the material, all directed to a function." Michael Cardew in "Pioneer
>Pottery"
Louis Katz on sat 3 apr 99
Dear John,
1. I have had what I assume are cristobalite problems. This year I have
assumed that it was because of some lowfire talc containing scrap that made
its way into our clays. We have also however been using Goldart and
Greenstripe fireclay in fairly substantial quantities to dry out the slop.
I would ask your suppliers if your bodies contain Goldart. Perhaps it is the
culprit.
2. Is the dunting caused by too rapid cooling or glaze fit;(just a question
whose answer might provide some insight)?
Louis
John Hesselberth wrote:
> ----------------------------Original message----------------------------
> After 25 years of making pots, I thought I had found all the
> problems--well, I found one I've never encountered over the last month.
> Clay continues to humble me.
>
> I was gearing up to fill all the orders I took at the Rosen show in
> Philadelphia. Bought another lot of my usual clay, made a couple of fine
> tuning adjustments on my glaze firing cycle to try to get rid of a few
> pinholes and went to work.
>
> I fire to cone 6 in a computer controlled L&L kiln (great kiln, by the
> way--went to it after using a Skutt for several years and I'll be
> sticking with L&L from now on) using Standard 306 clay. This is a tan
> stoneware body that is rated at cones 4-10.
>
> I opened the kiln after a glaze firing in early March and found 4 pots
> with classic cooling dunt cracks. Since that was a small percentage of
> the load I didn't think too much about it. Next load was worse--nearly
> every pot of any size had cracked. Only a few small ones survived.
> Since then I have been in the crisis mode.
>
> My first thought was the new batch of clay. My problem seemed to
> coincide exactly with cleaning out the old batch and starting to use the
> new. Talked to my supplier (Del-Val) and Standard and was assured that
> no one else had reported this problem and they had made no changes in
> this lot vs. the previous one I had been using. I decided, though, to
> try two Miller Clays (40 and 55) to test the clay hypothesis. Bottom
> line--both Miller Clays dunted at least as badly as my Standard Clay!
> What is going on???? I concluded it wasn't the clay.
>
> I hand build from slabs rolled on a Bailey slab roller. I texture the
> slabs on the outside and leave the iside smooth. I pour glaze on the
> inside and finish the outside by brushing on multiple slips and glazes.
> The bottoms of my pots have multiple slips, but no glaze so I can set
> them directly on the shelf without waxing. I got to wondering if the
> lack of glaze on a fairly broad bottom coupled with the surface contact
> of the pot and the shelf, which would give uneven cooling, was causing my
> problem. I glazed the bottoms of a few pots and set them on stilts.
> Bottom line--no progress--every large pot cracked. This pretty much
> convinced me that what I was seeing was not a result of the silica
> transition that occurs at 573 deg. C, but possibly was the result of
> cristobalite formation. But could cristobalite really be forming at Cone
> 6???
>
> At this point I hit the books another time to really study what is known
> about dunting, cristobalite formation etc. Hamer and Hamer say that it
> takes 10 days to convert 100% of silica to cristobalite at 1300 deg. C.
> The lowest practical temperature for any cristobalite conversion is 1100
> deg. C. and "at this temperature a long time is required". It seems like
> at cone 6 (let's just call it 1200 deg. C for simplicity) it would be
> possible but would take a long, long time.
>
> Then I read on and noted that both calcium and magnesium are catalysts
> for the conversion of silica to cristobalite. One of my glazes is a high
> calcium glaze and another has a fair amount of magnesia. But they are
> only on the surface, not in the clay. Also, those minor changes I made
> in my firing cycle were in the wrong direction, e.g. I increased the soak
> time at temperature from 20 to 30 minutes and increased the end point
> temperature by 10 deg. C. (from a cone 6 at 3 o'clock to cone 6 nearly
> full down.)
>
> The hypothesis that was forming in my mind was as follows:
>
> Even though cristobalite formation should be slow at the temperatures I
> was operating at, it could be happening at/near the clay/glaze interface
> because of the presence of a highly mobile calcium and magnesium
> molecules in my molten glazes. This was potentially aggravated by the
> longer soak and slightly higher temperature that I had recently switched
> to.
>
> Experiment to test the hypothesis: Take the temperature back down and
> eliminate the 30 minute soak. I opened the kiln this morning. I only
> fired 3 pots, but there were NO COOLING DUNTS. A small success, but it
> gives me the courage to try a larger number of pots.
>
> Now some questions for Clayart members while my next load is firing:
>
> 1. Does this hypothesis/potential solution make sense? Can this really
> have been happening the way I describe above? Or did I just get lucky
> with the last 3 pots and the real problem is waiting to bite me again
> when I fire a full load?
>
> 2. This clay is supposed to be good to cone 10. It would surely see a
> lot more heat in getting it to Cone 10 than it sees by soaking it for 30
> minutes at Cone 6. Does anyone take this clay to Cone 10? I am assured
> by my supplier that people do without any problems.
>
> 3. There have been comments on Clayart about the questionable practice
> of making/using clays that have a large maturity range (cones 4-10 in my
> case). Does anyone know how clay suppliers typically accomplish this?
> Is it, by any chance, by using a high silica recipe or by using talc in
> the recipe thereby making cristobalite formation more likely?
>
> 4. What other experiments or advice would you have to get me away from
> the edge of this cliff I have apparently been dancing along?
>
> Ahhh, isn't clay wonderful. It certainly keeps you learning throughout
> your life. It doesn't let the gray cells get rusty!
>
> Any and all input/ideas/comments/encouragement will be most welcome. If
> my current hypothesis turns out to be incorrect I am absolutely at wits
> end.
>
> John Hesselberth
> Frog Pond Pottery
> P.O. Box 88
> Pocopson, PA 19366 USA
> EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
>
> "It is time for potters to claim their proper field. Pottery in its pure
> form relies neither on sculptural additions nor on pictorial decorations.
> but on the counterpoint of form, design, colour, texture and the quality
> of the material, all directed to a function." Michael Cardew in "Pioneer
> Pottery"
--
Louis Katz
lkatz@falcon.tamucc.edu
NCECA Director of Electronic Communication and Webmaster
Texas A&M-CC Division of Visual and Performing Arts
Visit the NCECA World Ceramics Image Database Online
Looking for a school or a class? Visit NCECA Ceramics Educational Programs
Database Online
Coastal Bend Arts Calendar Webmaster
Craig Martell on sat 3 apr 99
John:
Just a quick note this time, but I'll give it some more thought and try to
pass along some more suggestions if I think of anything pertinent.
IF, you are developing cristobalite that is causing your pots to shatter,
you would more than likely see some shivering in the glazes. Shattering is
the worst case scenario of shivering. Normally, if a claybody contains
enough feldspar, any cristobalite that is developed will be taken into the
body glass and neutralized as fused silica, which has a lower COE than
crystalline.
Perhaps the trouble emanates from clay-glaze incompatability. Since you are
pouring a thicker coat of glaze on the inside and just slipping and lightly
glazing the outside of the pots, there may be stresses developing between
the clay and glaze that can only be relieved by breakage. Here is the usual
question, has there been a materials change, or new batch of one raw
material, or another?
Have you tried glazing the inside and outside of a pot to see what happens
then.
good luck, Craig Martell in Oregon
| |
|
