David Beumee on fri 11 oct 02
Dear Susan,
It is possible to determine if a clay body is of high or low expansion by using Jim Robinson's Glaze Test Series, fired to
cone 10. GTS #1 has an expansion of 8.4 ( very high ), and GTS#10 has an expansion of 5.05 (very low), and all the others are
in between, ( 7.84- 5.02). I have permission from Studio Potter to reprint the article, and I'd be happy to send you a copy if you'll
send me your address.
If your clay body is of high expansion, then it will fit the high expansion glazes in the GTS, numbers 1, 2, 3, and 4, but
since there are much lower expansion glazes in the GTS, shivering may occur and you may see the lower expansion glazes in the
GTS popping off the tests. In extreme cases, dunting can occur and you will find your tests shattered.
If your clay body is of low expansion, then it will fit the low expansion glazes in the GTS, numbers 7,8,9, and 10, but
since there are also much higher expansion glazes in the series, many of your tests will be crazed, certainly #1 and 2, and
probably numbers 3,4,5, and sometimes even GTS numbers 6 and 7.
The goal is to first adjust the body so that it fits a majority of the glazes in the GTS, numbers 3 - 10, then use your glaze
software to adjust your favorite glazes accordingly. I can help you with advice on how to proceed with testing of the body.
It is puzzling that glazes of very nearly the same expansion are both crazing and not crazing over your clay body. The
best advice I can give you is to follow the directions in Jim's article and do the work necessary to know that you have both done
your best to elimimate growth of cristobalite, and added enough silica to your clay body to fit a wide range of glazes.
Some clays are more unstable and batches can vary widely, causing lots of problems with glaze fit. I can help you
review the choices you are making.
David Beumee
Earth Alchemy Pottery
10/11/02 6:30:26 AM, Susan Giddings wrote:
>I am confused about some of the reading and research I have done and how to
>USE this information.
>
>I work out of a co-op and we make out own clay body from raw ingredients.
>The search started because i wanted to determine the coefficient of thermal
>expansion of our clay body.
>I understand that it is not possible to determine the expansion of the body
>from the recipe. Further, (and here's where I may not be understanding
>things) you can determine the expansion from analyzing glaze fit on the
>body. Well, I am also in the process of having all our glazes sent out for
>lab testing. So I have Insight software and have run all glaze recipes
>through it in order to send the glaze formula with a sample piece to the
>lab.
>
>Each recipe report shows the expansion percent of the glazes. Some of the
>glazes have a strong tendency to craze, most don't. But the expansion rate
>is not either consistently high or low for those that craze. I'm showing,
>for example, that glazes that craze run
>7.56, 7.36, and 6.77. Glazes that do not show any evidence of crazing (even
>after a lot of time and notmal household abuse) run 7.43, 7.58, 5.13, 7.17
>(all are glossy glazes.) Is there another factor involved in this?
>
>I should note, that the glazes that craze are 2 - clear and 1 - celadon,
>the glazes that don't are 1- white, the others with colorant oxides and the
>colorants I added to the glaze recipe in Insight. All are ^10 gas reduction
>fired.
>
>Can someone help me to understand either what I am doing wrong or how I can
>interpret these results?
>
>Thanks very much in advance!
>
>Susan Giddings
>Expressions Pottery Workshop
>East Granby, CT
>
>______________________________________________________________________________
>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.
>
Susan Giddings on fri 11 oct 02
I am confused about some of the reading and research I have done and how to
USE this information.
I work out of a co-op and we make out own clay body from raw ingredients.
The search started because i wanted to determine the coefficient of thermal
expansion of our clay body.
I understand that it is not possible to determine the expansion of the body
from the recipe. Further, (and here's where I may not be understanding
things) you can determine the expansion from analyzing glaze fit on the
body. Well, I am also in the process of having all our glazes sent out for
lab testing. So I have Insight software and have run all glaze recipes
through it in order to send the glaze formula with a sample piece to the
lab.
Each recipe report shows the expansion percent of the glazes. Some of the
glazes have a strong tendency to craze, most don't. But the expansion rate
is not either consistently high or low for those that craze. I'm showing,
for example, that glazes that craze run
7.56, 7.36, and 6.77. Glazes that do not show any evidence of crazing (even
after a lot of time and notmal household abuse) run 7.43, 7.58, 5.13, 7.17
(all are glossy glazes.) Is there another factor involved in this?
I should note, that the glazes that craze are 2 - clear and 1 - celadon,
the glazes that don't are 1- white, the others with colorant oxides and the
colorants I added to the glaze recipe in Insight. All are ^10 gas reduction
fired.
Can someone help me to understand either what I am doing wrong or how I can
interpret these results?
Thanks very much in advance!
Susan Giddings
Expressions Pottery Workshop
East Granby, CT
Susan Giddings on sun 13 oct 02
Thank you - all of you who have responded to this.
YES! I have every intention of making the results public!
I've been in tough with John Hesselberth (and Ron Roy) and they have agreed
to help me through the process.
John's Website has postings of lab test results of glazes that have been
tested where the information was shared with him. I am hoping that he will
include on his site our results.
Ron asked me to post 4 recipes - 2 of glazes that crazes and 2 of glazes
that don't.
Here they are:
OOPS! Note that colorants and additions are listed as part of the base
glaze! I can't figure out to have Insight show them separately unless as a
notation to the glaze. If I new more about this I'd write my own program to
do it (I am an application developer by trade!)
******************
~~~~~~~~~~~~~~
GT Clear Glossy clear glaze with a greenish tint ^10 reduction fired
(crazes on our clay body)
Whiting 20%
Custer Feldspar 40%
Ball Clay 25%
Flint 15%
Si:AL 6.23
Expan 7.56
~~~~~~~~~~~~~~~~~
Chinese Clear Glossy Clear glaze, sometimes with a pale yelloe/green tint
^10 reduction fired (crazes on our claybody)
Whiting 20.59%
Magnesium Carbonate 2.93%
Custer Feldspar 23.81%
Kaolin 18.26%
Flint 30.37%
Bentonite 3.03%
Epsom Salts 1.01%
Si:Al 7.75
Expan 6.77
****************************
Glazes that don't seem to show any crazing
~~~~~~~~~~~~~~~~~~~~
Volcanic Ash Glossy - Semi Glossy White glaze ^10 reduction
Volcanic Ash 30.30%
Ball Clay 30.30%
Flint 15.15%
Dolomite 16.67%
Zircopax 7.58%
Si:Al 9.03
Expan 5.13
~~~~~~~~~~~~~~~~~~~
Navy Blue Gloss Glossy transparent blue ^10 Reduction
Neph Sy 23.21%
Whiting 23.76%
EPK Kaolin 13.57%
Flint 36.92%
Red Iron Oxide 2.04%
Cobalt Carbonate .50%
Si:AL 8.75
Expan 7.43
I'll post the clay body recipe when I get to the studio to get it!
Thanks again to everyone for your comments and help!
Susan
Expressions Pottery Workshop
East Granby, CT
David Hewitt on sun 13 oct 02
Susan,
Your experience raises some very pertinent questions and it will be most
interesting to know what results you get from the lab tests on your
glazes. Are you going to have each glaze and your clay body measured for
expansion coefficient? If so I hope that you will be good enough to
share the results with us.
I see that David Beumee has given you a method on ' grading' your clay
body.
I personally don't worry too much about the absolute value of the
calculated coefficient of expansion, but if a glaze crazes and I want to
avoid this, then I will adjust that recipe to give a lower figure and
re-test.
David
In message , Susan Giddings writes
>I am confused about some of the reading and research I have done and how =
>to
>USE this information.
>
>I work out of a co-op and we make out own clay body from raw ingredients.
>The search started because i wanted to determine the coefficient of therm=
>al
>expansion of our clay body.
>I understand that it is not possible to determine the expansion of the bo=
>dy
>=66rom the recipe. Further, (and here's where I may not be understanding
>things) you can determine the expansion from analyzing glaze fit on the
>body. Well, I am also in the process of having all our glazes sent out fo=
>r
>lab testing. So I have Insight software and have run all glaze recipes
>through it in order to send the glaze formula with a sample piece to the
>lab.
>
>Each recipe report shows the expansion percent of the glazes. Some of the
>glazes have a strong tendency to craze, most don't. But the expansion rat=
>e
>is not either consistently high or low for those that craze. I'm showing,
>for example, that glazes that craze run
>7.56, 7.36, and 6.77. Glazes that do not show any evidence of crazing (ev=
>en
>after a lot of time and notmal household abuse) run 7.43, 7.58, 5.13, 7.1=
>7
>(all are glossy glazes.) Is there another factor involved in this?
>
>I should note, that the glazes that craze are 2 - clear and 1 - celadon,
>the glazes that don't are 1- white, the others with colorant oxides and t=
>he
>colorants I added to the glaze recipe in Insight. All are ^10 gas reducti=
>on
>fired.
>
>Can someone help me to understand either what I am doing wrong or how I c=
>an
>interpret these results?
>
>Thanks very much in advance!
>
>Susan Giddings
>Expressions Pottery Workshop
>East Granby, CT
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP18 3DQ, UK. Tel:- +44 (0) 1633 420647
FAX:- +44 (0) 870 1617274
Web site http://www.dhpot.demon.co.uk
Alistair Gillies on sun 13 oct 02
>>Each recipe report shows the expansion percent of the glazes. Some of the
>>glazes have a strong tendency to craze, most don't. But the expansion rat=
>>e
>>is not either consistently high or low for those that craze. I'm showing,
>>for example, that glazes that craze run
>>7.56, 7.36, and 6.77. Glazes that do not show any evidence of crazing (ev=
>>en
>>after a lot of time and notmal household abuse) run 7.43, 7.58, 5.13, 7.1=
>>7
>>(all are glossy glazes.) Is there another factor involved in this?
Hi,
A glaze crazes because it does not 'fit' the clay body. That is, do both
of these elements expand and contract to the same extent during the
firing.
On cooling if a glaze contracts more than the clay body it will be under
tension, like a piece of clothing that is too small, and it will have
the tendency to release this pressure by cracking [crazing].
Occasionally [rarely] a clay body will contract much more than the glaze
and it is possible for the glaze to be so 'loose' that it is pushed off
the pot - can be seen on rims [shivering].
The important thing is not how much the body or glaze expands or
contracts but whether they do it together and 'fit'.
All the best,
Alistair Gillies,
Shropshire, England
Ron Roy on thu 17 oct 02
Hi Susan,
I have added my expansion calculation under each glaze as well
Glaze #1 = 521.78 (I used OM#4 for ball clay)
Glaze #2 = 438.3
Glaze #3 = 344.13 (I used Vol ash (Mt. St. Helens type - there are two
kinds - and it depends how long it has weathered for both types)
Glaze #4 = 486.42
So listed from High to low:
Glaze #1 = 521.78
Glaze #4 = 486.42
Glaze #2 = 438.3 <<<<< does not make sense!
Glaze #3 = 344.13
You say #1 & 2 are crazing - so I am assuming there is something wrong with
the calculated expansion of that one - I am sure the bentonite is not
needed by the way and that is way too much Epsom salts for a glaze - first
step is to fire it without those two - it will not settle out fast. I'm
going to guess and say it's the salts that are making it craze.
I also think Glaze one could be adjusted to not craze - add 5 dolomite and
lower Ball to 15 - add EPK till Al2O3 is up to original amount (0.50) then
top up silica - expansion is now 475 - lower than #4.
Let me know if you have any questions - RR
>YES! I have every intention of making the results public!
>
>I've been in tough with John Hesselberth (and Ron Roy) and they have agreed
>to help me through the process.
>
>John's Website has postings of lab test results of glazes that have been
>tested where the information was shared with him. I am hoping that he will
>include on his site our results.
>
>Ron asked me to post 4 recipes - 2 of glazes that crazes and 2 of glazes
>that don't.
>
>Here they are:
>OOPS! Note that colorants and additions are listed as part of the base
>glaze! I can't figure out to have Insight show them separately unless as a
>notation to the glaze. If I new more about this I'd write my own program to
>do it (I am an application developer by trade!)
>
>******************
>~~~~~~~~~~~~~~
>GT Clear Glossy clear glaze with a greenish tint ^10 reduction fired
>(crazes on our clay body)
#1
>
>Whiting 20%
>Custer Feldspar 40%
>Ball Clay 25%
>Flint 15%
>
>Si:AL 6.23
>Expan 7.56
RR expansion = 521.78 for #1
>
>~~~~~~~~~~~~~~~~~
>Chinese Clear Glossy Clear glaze, sometimes with a pale yelloe/green tint
> ^10 reduction fired (crazes on our claybody)
#2
>Whiting 20.59%
>Magnesium Carbonate 2.93%
>Custer Feldspar 23.81%
>Kaolin 18.26%
>Flint 30.37%
>Bentonite 3.03%
>Epsom Salts 1.01%
>
>Si:Al 7.75
>Expan 6.77
RR expansion - 438.3 for #2
>
>
>****************************
>Glazes that don't seem to show any crazing
>~~~~~~~~~~~~~~~~~~~~
#3
>Volcanic Ash Glossy - Semi Glossy White glaze ^10 reduction
>Volcanic Ash 30.30%
>Ball Clay 30.30%
>Flint 15.15%
>Dolomite 16.67%
>Zircopax 7.58%
>
>
>Si:Al 9.03
>Expan 5.13
RR expansion = 344.13 for #3
>
>~~~~~~~~~~~~~~~~~~~
#4
>Navy Blue Gloss Glossy transparent blue ^10 Reduction
>Neph Sy 23.21%
>Whiting 23.76%
>EPK Kaolin 13.57%
>Flint 36.92%
>Red Iron Oxide 2.04%
>Cobalt Carbonate .50%
>
>Si:AL 8.75
>Expan 7.43
RR Expansion = 486.42 for #4
>
>
>I'll post the clay body recipe when I get to the studio to get it!
>
>Thanks again to everyone for your comments and help!
>
>Susan
>Expressions Pottery Workshop
>East Granby, CT
>
>______________________________________________________________________________
>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
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