Ingeborg Foco on sun 2 apr 06
Hi Clay People,
I have tested a glaze copied from CM Dec 2005 called Blue ^10 Reduction. It
would appear to be the standard Leach 4321 glaze plus cobalt carb and
Manganese Diox . The results are a very stable, glossy royal blue. It
seemed to be fine, passed a variety of tests but then I noticed the edges
and rims were a bit rough - but not on all pieces. Some of them actually
appear to have chipped just sitting there I think. The fact that it doesn't
happen on everything is a bit befuddling.
The recipe is as follows:
Feldspar Potash 40
Silica 30
Whiting 20
Kentucky Ball clay 10
add: Cobalt Carb 2%
Manganese Diox 2 %
Has anyone tried this particular recipe with better results? I should
mention I am using Highwater Clay "Phoenix" and fire to cone 10 Reduction.
It is a shame, as the glossy royal blue is a very nice addition to the rest
of my glazes but the rough edges and potential chipping makes it not
terribly functional. Does anyone have any ideas?
Thanks in advance for any suggestions and or answers.
Ingeborg
the Potter's Workshop & Gallery
P.O. Box 510
3058 Stringfellow Road
St. James City, Florida 33956
239-283-2775
Ingeborg Foco on mon 3 apr 06
Hi Paul,
I suspected that was the problem but for me it is usually crazing not
shivering. I can't recall ever having a shivering problem. I thought the
glaze was wonderful since it didn't craze and passed all of the
lemon/vinegar tests. I thought wow...nice but what a disappointment to see
it chip on the rims.
Since I buy my clay and just bought a ton, changing the clay body is not an
option. I think I will try to substitute kaolin for ball clay and see what
happens. Would I substitute it exactly 10 ball clay for 10 kaolin? I guess
if that doesn't work I will need to look for another royal blue or strike
out for something totally different.
Ingeborg
the Potter's Workshop & Gallery
P.O. Box 510
3058 Stringfellow Road
St. James City, Florida 33956
239-283-2775
> Hi Ingeborg,
>
> I've used that glaze for many years, but with kaolin instead of ball
> clay. It sounds like you have "shivering" from your description of
> the little chips on the edges. It's the opposite of crazing, meaning
> the glaze is in so much compression that it has to give somewhere.
>
>
Paul Herman on mon 3 apr 06
Hi Ingeborg,
I've used that glaze for many years, but with kaolin instead of ball
clay. It sounds like you have "shivering" from your description of
the little chips on the edges. It's the opposite of crazing, meaning
the glaze is in so much compression that it has to give somewhere.
You can fix it by increasing the thermal expansion of the glaze,
decreasing the expansion of the clay body, or using a different body.
I've never had shivering from this glaze, and have used it on lots of
different clays. I even added kaolin and silica to stop it's crazing.
Because of that, I suspect your clay body is the problem.
Best,
Paul Herman
Great Basin Pottery
Doyle, California US
http://greatbasinpottery.com
On Apr 2, 2006, at 7:05 PM, Ingeborg Foco wrote:
> I noticed the edges
> and rims were a bit rough - but not on all pieces. Some of them
> actually
> appear to have chipped just sitting there I think. The fact that
> it doesn't
> happen on everything is a bit befuddling.
>
> The recipe is as follows:
>
> Feldspar Potash 40
> Silica 30
> Whiting 20
> Kentucky Ball clay 10
>
> add: Cobalt Carb 2%
> Manganese Diox 2 %
>
> Has anyone tried this particular recipe with better results? I should
> mention I am using Highwater Clay "Phoenix" and fire to cone 10
> Reduction.
Bonnie Staffel on tue 4 apr 06
I don't recall which glaze I was using when I had a shivering problem, =
but
it could have been this one, being a Leachite those many years ago. I =
found
that if I lowered the final temperature by one cone that the problem =
could
be solved. Did that with success. I was mixing my own clay at the time =
and
was glad that I didn't have to reformulate the clay. I have used =
Phoenix
for many years with no problems, however, I fire in an electric kiln now =
to
Cone 9.
Bonnie Staffel
http://webpages.charter.net/bstaffel/
DVD Throwing with Coils and Slabs
DVD Beginning Processes
Charter Member Potters Council
Ron Roy on tue 4 apr 06
Hi Ingeborg,
It would help if I knew which spar and which clay - the expansion of this
glaze is not all that low.
The glaze is a little short of alumina and silica for a cone 10 glaze -
which may explain the running away from edges.
Some glazes are like that and rims need to be formed in such a way as to
avoid any sharp edges.
I suspect some cristobalite in the body which is increasing the
expansion/contraction to be putting the glaze under too much compression -
which would explain the shivering (chipping on sharp edges.)
If you would like me to measure some of that clay to see if that is the
problem let me know and I will tell you have to prepare a sample - not hard
at all by the way.
RR
>I have tested a glaze copied from CM Dec 2005 called Blue ^10 Reduction. It
>would appear to be the standard Leach 4321 glaze plus cobalt carb and
>Manganese Diox . The results are a very stable, glossy royal blue. It
>seemed to be fine, passed a variety of tests but then I noticed the edges
>and rims were a bit rough - but not on all pieces. Some of them actually
>appear to have chipped just sitting there I think. The fact that it doesn't
>happen on everything is a bit befuddling.
>
>The recipe is as follows:
>
>Feldspar Potash 40
>Silica 30
>Whiting 20
>Kentucky Ball clay 10
>
>add: Cobalt Carb 2%
> Manganese Diox 2 %
>
>Has anyone tried this particular recipe with better results? I should
>mention I am using Highwater Clay "Phoenix" and fire to cone 10 Reduction.
>
>It is a shame, as the glossy royal blue is a very nice addition to the rest
>of my glazes but the rough edges and potential chipping makes it not
>terribly functional. Does anyone have any ideas?
>
>Thanks in advance for any suggestions and or answers.
>
>
>Ingeborg
>the Potter's Workshop & Gallery
>P.O. Box 510
>3058 Stringfellow Road
>St. James City, Florida 33956
>
>239-283-2775
>
>______________________________________________________________________________
>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
Leonard Smith on wed 5 apr 06
With 2% iron I used this glaze for many years as a Celadon, a little
on the grey side of green, and most of the time it had excellent fit
on the bodies I was using. l still have a 35 year old set of bowls
in regular use with out a chip or mark.
I received a new batch of clay and suddenly the glaze started to
shiver. The clay was already quite siliceous but this batch had been
wrongly mixed, the supplier wouldn't accept responsibility, and as it
was such a lovely fine body with a light crusty blush in reduction I
used a Harry Davis technique of Crash Cooling from 1300oC to 1000oC
to solve the problem. I was using blown oil, so I just left the
blower on at the end of the firing. None of the other glazes was
unduly effected, but I was not using anything that relied on crystal
development for its surface quality.
As Ron says below the body is probably too high in silica/
cristobalite and the solution is to use another body or modify it, or
to increase the silica in the glaze, or use crash cooling. BTW Harry
used a very siliceous body in New Zealand and the crash cooling was
essential to the durability of his pots. Still a lot of his 40 year
old pots around doing good service. Wish I had a few.
Best wishes
Leonard
>
Leonard Smith
Rosedale Street Gallery
2a Rosedale Street
Dulwich Hill NSW 2103 Australia
leonard@rosedalestreetgallery.com
http://www.rosedalestreetgallery.com
On 04/04/2006, at 4:28 PM, Ron Roy wrote:
> Hi Ingeborg,
>
> It would help if I knew which spar and which clay - the expansion
> of this
> glaze is not all that low.
>
> The glaze is a little short of alumina and silica for a cone 10
> glaze -
> which may explain the running away from edges.
>
> Some glazes are like that and rims need to be formed in such a way
> as to
> avoid any sharp edges.
>
> I suspect some cristobalite in the body which is increasing the
> expansion/contraction to be putting the glaze under too much
> compression -
> which would explain the shivering (chipping on sharp edges.)
>
> If you would like me to measure some of that clay to see if that is
> the
> problem let me know and I will tell you have to prepare a sample -
> not hard
> at all by the way.
>
Ron Roy on wed 5 apr 06
Hi Leonard,
Good points,
The reason crash cooling helps when cristobalite is involved - cristobalite
forms starting about 1100C but does not really get going until after cone 6
- and it keep forming all the way up and all the way down till it passes
1100C - so crash cooling lessens the amount that forms on the way down to
1100C
Peter Sohngen did the definitive research on this - it was published in
Studio potter vol 28 #1.
Essentially he says - and I did the dilatometery to show he is right - have
at least 10% spar in the body and avoid silica with microfines - which is
mostly what we get now.
He searched and eventually found properly graded silica - and proved that
if the silica was not microfine - it did not get easily converted to
cristobalite.
The real answer is to have enough KNaO around to melt the super fine silica
generated during mullite production - as it is formed.
Many dark reduction bodies - because the iron is a flux - just can't have
enough KNaO to stop the cristobalite - but they are not the only clay
bodies that can have cristobalite.
You will never see any cristobalite in porcelains - because they have way
more than 10% spar - even though they have lots of free silica - the spar
does the job.
Cristobalite dramatically increases the expansion/contraction of any clay -
if a clay has a high expansion rate - uneven heating will lead to problems.
If the glaze is low expansion the problem will be magnified.
All this is not hard to test for by the way - a 4" by 4" cylinder only
glazed on the inside - clay a little thinner than usual and glaze a little
thicker - freeze it for 24 hours - then- if not cracked - put in a sink (in
case it cracks) and pour in boiling water. If your clay glaze combination
cannot take that you and your customers have a problem.
RR
>With 2% iron I used this glaze for many years as a Celadon, a little
>on the grey side of green, and most of the time it had excellent fit
>on the bodies I was using. l still have a 35 year old set of bowls
>in regular use with out a chip or mark.
>
>I received a new batch of clay and suddenly the glaze started to
>shiver. The clay was already quite siliceous but this batch had been
>wrongly mixed, the supplier wouldn't accept responsibility, and as it
>was such a lovely fine body with a light crusty blush in reduction I
>used a Harry Davis technique of Crash Cooling from 1300oC to 1000oC
>to solve the problem. I was using blown oil, so I just left the
>blower on at the end of the firing. None of the other glazes was
>unduly effected, but I was not using anything that relied on crystal
>development for its surface quality.
>
>As Ron says below the body is probably too high in silica/
>cristobalite and the solution is to use another body or modify it, or
>to increase the silica in the glaze, or use crash cooling. BTW Harry
>used a very siliceous body in New Zealand and the crash cooling was
>essential to the durability of his pots. Still a lot of his 40 year
>old pots around doing good service. Wish I had a few.
>
>Best wishes
>
>Leonard
>>
>Leonard Smith
>Rosedale Street Gallery
>2a Rosedale Street
>Dulwich Hill NSW 2103 Australia
>leonard@rosedalestreetgallery.com
>http://www.rosedalestreetgallery.com
>
>
>On 04/04/2006, at 4:28 PM, Ron Roy wrote:
>
>> Hi Ingeborg,
>>
>> It would help if I knew which spar and which clay - the expansion
>> of this
>> glaze is not all that low.
>>
>> The glaze is a little short of alumina and silica for a cone 10
>> glaze -
>> which may explain the running away from edges.
>>
>> Some glazes are like that and rims need to be formed in such a way
>> as to
>> avoid any sharp edges.
>>
>> I suspect some cristobalite in the body which is increasing the
>> expansion/contraction to be putting the glaze under too much
>> compression -
>> which would explain the shivering (chipping on sharp edges.)
>>
>> If you would like me to measure some of that clay to see if that is
>> the
>> problem let me know and I will tell you have to prepare a sample -
>> not hard
>> at all by the way.
>>
>
>______________________________________________________________________________
>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
Ingeborg Foco on wed 5 apr 06
Hi Ron,
The clay used was OM4 and the spar G 200. The claybody is Highwater
Phoenix a ^10 body. Yes I would like to test the clay body just tell me what
to do.
I will be out of town Fri thru Tues but then back to finish the end of the
season.
Sincerely,
Ingeborg
the Potter's Workshop & Gallery
P.O. Box 510
3058 Stringfellow Road
St. James City, Florida 33956
239-283-2775
> Hi Ingeborg,
>
> It would help if I knew which spar and which clay - the expansion of this
> glaze is not all that low.
>
> The glaze is a little short of alumina and silica for a cone 10 glaze -
> which may explain the running away from edges.
>
>
Ron Roy on thu 6 apr 06
Hi Leonard,
What a great attitude - you have my admiration - what a consept! Make sure
what you make does what it's supposed to do.
RR
>Ron
>
>As usual your response is well informed.
>
>As this clay was always a little "tender" I would always test my
>teapots with three rounds of ice water followed by boiling water
>before I would sell them. It is a potter's nightmare to sell a teapot
>that splits and burns some one. In the end I couldn't live with the
>risk and started to make my own clay bodies and yes they had 15%
>feldspar as a minimum.
>
>Never had a problem after that. (Quality control was a lot more
>complex than that of course including testing new materials and still
>always testing cooking and tea pots and as a last resource product
>liability insurance.)
>
>Leonard Smith
>Rosedale Street Gallery
>2a Rosedale Street
>Dulwich Hill NSW 2103 Australia
>leonard@rosedalestreetgallery.com
>http://www.rosedalestreetgallery.com
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Leonard Smith on thu 6 apr 06
Ron
As usual your response is well informed.
As this clay was always a little "tender" I would always test my
teapots with three rounds of ice water followed by boiling water
before I would sell them. It is a potter's nightmare to sell a teapot
that splits and burns some one. In the end I couldn't live with the
risk and started to make my own clay bodies and yes they had 15%
feldspar as a minimum.
Never had a problem after that. (Quality control was a lot more
complex than that of course including testing new materials and still
always testing cooking and tea pots and as a last resource product
liability insurance.)
Leonard Smith
Rosedale Street Gallery
2a Rosedale Street
Dulwich Hill NSW 2103 Australia
leonard@rosedalestreetgallery.com
http://www.rosedalestreetgallery.com
On 06/04/2006, at 6:02 AM, Ron Roy wrote:
> All this is not hard to test for by the way - a 4" by 4" cylinder only
> glazed on the inside - clay a little thinner than usual and glaze a
> little
> thicker - freeze it for 24 hours - then- if not cracked - put in a
> sink (in
> case it cracks) and pour in boiling water. If your clay glaze
> combination
> cannot take that you and your customers have a problem.
>
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