David Hewitt on tue 13 jul 99
Hello Carol,
I know that I do not have a nice neat answer to your problem, but
perhaps what I add will be of help.
Firstly, it is clear that you have a problem of glaze fit with your body
and, while it does not show immediately in all cases, I doubt if you
would find a permanent solution just by altering your firing cycle. the
solution is going to lie with a change of recipe (to give a lower
coefficient of expansion) or clay body (to give a higher coefficient of
expansion).
Analysing your recipe from the data that I have available, it looks as
follows:-
K2O .0507
Na2O .1289
CaO .8204
Al2O3 .4141
B2O3 .2584
SiO2 3.0135
TiO2 .0015
Coefficient of Expansion 4.476 English & Turner x10-6/oC linear
The ^6 clear transparent glaze that I use has a coefficient of expansion
of 2.67, which is clearly a lot less. I do not have crazing problems
even on porcelain with this glaze.
I could give you the recipe for this, but it contains a low expansion
fritt and I do not know of an exact ferro or pemco equivalent.
The coefficient of expansion for your recipe would be reduced by
increasing relatively the amount of B2O3 and SiO2 as these oxides have
the lowest expansion coefficients. GB, which adds the B2O3, also adds
other oxides and, when you analyse this recipe, adding more GB has
little effect on the resultant coefficient of expansion. Adding silica,
on the other hand does reduce the coefficient significantly. An
additional 32 parts of silica reduces the coefficient to 3.406. You
could try testing by making progressive additions of 4 parts silica up
to this amount of 32 and seeing if somewhere along this progression the
crazing is eliminated and the glaze is still clear and shiny enough.
David
In message , Carol Baker writes
>----------------------------Original message----------------------------
>Hi clay lovers!
>As a beginner I've learned so much from you, but now I really need your help.
> I've started selling my work and have done very well, then the crazing
>started. Some of these are pieces that were fired 10 months ago. I'm afraid
>irate customers will start calling soon. Would you look at the glaze recipe
>to see if there is a problem that I don't understand? I've researched
>crazing and have Jeff Zamek's book. I think I understand some possible
>solutions, but if I made changes how would I know that in 10 more months the
>problem wouldn't happen again? Oh, the joy and wonder of clay! I'm willing
>to test but was hoping that you could save me some frustration and time.
>
>Here are the details: clay- Laguna B-mix ^5, glaze- Mei Ming's Clear
>^5-7. I want to stay with this clay because of the color. I fire electric
>and use underglazes and stains as watercolors. Glaze is thin and works
>beautifully with colors. Bisque is to ^03 small cone in sitter. Glaze is to
>^6 small cone in sitter. Visual cone 5 down. Schedule: 3 hours low, 2
>medium, then to high, fire-down for 1/2 hour at med and 1 hour low, cool to
>touch. Total about 8-9 hours.
>
>The crazing: fine network of lines 1/4 to 1/2 inch apart, usually on the
>bottom of a plate or bowl, running across the face. These are large slab
>plates with manipulated and cut rims. I had some that crazed a few days
>after firing, but I think that was because I was cooling too fast. Most have
>crazed months later and were cooled slowly. (I'm learning). I have tested
>some in the dishwasher for months, also microwave, oven, and freezer. All
>was fine until this week.
>
>The glaze: Mei Ming Clear ^5-7
>
>Whiting 16
>Gerstley Borate 18
>G-200 Feldspar 40
>EPK 10
>Flint 325 16
> 100
>
>Well? How does it look? Could there be an obvious solution? I'd appreciate
>any help. By the way, did you notice that I tried to included all the
>details? I told you that you all have taught me a lot. Sure wouldn't want
>to leave out a cone #! You would send the kiln gods after me! The "crazies"
>are enough right now. You're great! Thanks for any help.
>
>Carol and the Blue Cat in Arizona
>CBaker866@aol.com
>
--
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
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
Mike Bailey on tue 13 jul 99
In message , Carol Baker writes
Dear Carol,
Re your crazing problem. This can be one of the most difficult problems
to sort out. However, you've obviously got a good understanding of the
problem as the important variables are listed so clearly!
I can only discuss this in a general was as the clays and glazes that
you mention aren't available in the U.K. but here's some thoughts.
The glaze when it has cooled down is too small for the body and to
correct this something has got to be changed.
This could be 1. The glaze
2. The clay
or 3. The firing.
1, Altering the glaze - if only there was a magic ingredient that we
could add to a glaze to cut down the thermal expansion figure! But there
isn't. Boric oxides comes closest but the reality is that as soon as you
start adjusting the glaze's expansion you alter the glaze itself.
However, as you seem to be just using a clear glaze why not try other
glazes - I'll tell you how to test at the end.
2. Changing the clay. This is the easiest and quickest solution.
Normally one goes for a higher silica clay. Could you contemplate using
another clay?
3. Altering the firing. The explanation about what is happening is quite
complicated. The usual way to correct crazing is to increase heatwork,
which makes more cristobalite, which makes your clay shrink a bit more -
so the glaze isn't so tight. The danger is over-firing.
So to summarise, first choice alter the clay, second choice use a lower
expansion glaze, third choice alter the firing.
Now then, how do you get over the fact that your glaze comes out of the
kiln uncrazed and the crazing develops later. There is an empirical test
that you can do. You could even use some test tiles of your present clay
& glaze as a standard against which to measure improvements. The idea is
based upon 'torturing' the clay/glaze interface and goes like this.
Have about 5 tiles of the glaze/clay being tested - heat up to 130 deg c
and drop into cold water, examine for crazing. Next heat them up to 140
deg. C. and drop in cold water - examine for crazing. Continue this
regime to 150 deg C. and 160 deg C.
The idea is that if it crazes from being dropped in cold water from 130
deg C it would only last a week or two without crazing anyway.
from 140 deg C. and if crazes = craze developing from 1 month to 6
months.
from 150 deg C. and if crazes = craze developing in 6 months to a year.
from 160 deg C. probably o.k for a year plus.
It would be interesting to try this regime with your present clay/glaze
and see where the crazing starts. New clays and glazes could be tested
against this.
Hope this is of some help.
Mike Bailey.
>----------------------------Original message----------------------------
>Hi clay lovers!
>As a beginner I've learned so much from you, but now I really need your help.
> I've started selling my work and have done very well, then the crazing
>started. Some of these are pieces that were fired 10 months ago. I'm afraid
>irate customers will start calling soon. Would you look at the glaze recipe
>to see if there is a problem that I don't understand? I've researched
>crazing and have Jeff Zamek's book. I think I understand some possible
>solutions, but if I made changes how would I know that in 10 more months the
>problem wouldn't happen again? Oh, the joy and wonder of clay! I'm willing
>to test but was hoping that you could save me some frustration and time.
>
>Here are the details: clay- Laguna B-mix ^5, glaze- Mei Ming's Clear
>^5-7. I want to stay with this clay because of the color. I fire electric
>and use underglazes and stains as watercolors. Glaze is thin and works
>beautifully with colors. Bisque is to ^03 small cone in sitter. Glaze is to
>^6 small cone in sitter. Visual cone 5 down. Schedule: 3 hours low, 2
>medium, then to high, fire-down for 1/2 hour at med and 1 hour low, cool to
>touch. Total about 8-9 hours.
>
>The crazing: fine network of lines 1/4 to 1/2 inch apart, usually on the
>bottom of a plate or bowl, running across the face. These are large slab
>plates with manipulated and cut rims. I had some that crazed a few days
>after firing, but I think that was because I was cooling too fast. Most have
>crazed months later and were cooled slowly. (I'm learning). I have tested
>some in the dishwasher for months, also microwave, oven, and freezer. All
>was fine until this week.
>
>The glaze: Mei Ming Clear ^5-7
>
>Whiting 16
>Gerstley Borate 18
>G-200 Feldspar 40
>EPK 10
>Flint 325 16
> 100
>
>Well? How does it look? Could there be an obvious solution? I'd appreciate
>any help. By the way, did you notice that I tried to included all the
>details? I told you that you all have taught me a lot. Sure wouldn't want
>to leave out a cone #! You would send the kiln gods after me! The "crazies"
>are enough right now. You're great! Thanks for any help.
>
>Carol and the Blue Cat in Arizona
>CBaker866@aol.com
>
--
Mike Bailey
Michael Banks on thu 15 jul 99
I beg to differ David. The firing cycle can be critical. In my experience
when delayed crazing develops in the basal portions of pots which previously
did not exhibit this problem, then any changes in firing cycle are usually
found to be the culprit.
Carol has since indicated to me off-list that her regular firing cycle was
changed prior to the delayed crazing appearing. Usually when one parameter
is changed, subsequent problems are usually due to that change.
Delayed crazing almost always means that the clay body is under-mature and
underfired. Mature stoneware usually has a few percent (normally kept less
than 2%) indicated porosity. It is well established that when this porosity
is too high (higher than a certain critical value which varies from body to
body), pore water pressure causes a gradual expansion of the ware, causing
eventual crazing. This effect is due to under-maturity of the ceramic body
only and all else being equal, is unrelated to the overall glaze fit on a
mature body, which is a separate issue. Water ingress crazing is a very
common problem here, constituting about 90% of the crazing faults reported
to me professionally and (contrary to some other opinions) is easy to
correct. The offending pottery merely has to be fired closer to its
zero-porosity vitrification point.
If the ware continues to exhibit a crazed glaze after it is established (by
water absorbsion testing) that the body is fully mature, then the overall
glaze/body fit should be reviewed. But, it is absolutely critical that body
maturity is confirmed before fooling with the glaze or body recipes.
Potters often encounter water crazing in the base of their ware when they
omit to hold a soak temperature or speed-up their firings. This is caused by
a porosity gradient from top to bottom of the pot due to the kiln shelves
lagging behind in temperature during the firing and not given sufficient
time to reach the maturation temperature of the body. Often the top of the
pot is mature and the foot ring several cones less mature. Sillimanite and
alumina kiln shelves are very poor conducters of heat and the portion
directly beneath the pot is often the coolest part of the kiln. Quicker
firings and shorter soak periods often cost the potter in the long-run
because cool spots and ware do not equilibriate properly.
Michael Banks,
Nelson,
New Zealand
> ----------------------------Original message----------------------------
> Hello Carol,
> I know that I do not have a nice neat answer to your problem, but
> perhaps what I add will be of help.
> Firstly, it is clear that you have a problem of glaze fit with your body
> and, while it does not show immediately in all cases, I doubt if you
> would find a permanent solution just by altering your firing cycle
(snip)...
> David Hewitt
------------------------------Original
Message-------------------------------------------
> Carol Wrote
>Hi clay lovers!
>As a beginner I've learned so much from you, but now I really need your
help.
> I've started selling my work and have done very well, then the crazing
>started. Some of these are pieces that were fired 10 months ago. I'm
afraid
>irate customers will start calling soon. Would you look at the glaze
recipe
>to see if there is a problem that I don't understand? I've researched
>crazing and have Jeff Zamek's book. I think I understand some possible
>solutions, but if I made changes how would I know that in 10 more months
the
>problem wouldn't happen again? Oh, the joy and wonder of clay! I'm
willing
>to test but was hoping that you could save me some frustration and time.
>
>Here are the details: clay- Laguna B-mix ^5, glaze- Mei Ming's Clear
>^5-7. I want to stay with this clay because of the color. I fire electric
>and use underglazes and stains as watercolors. Glaze is thin and works
>beautifully with colors. Bisque is to ^03 small cone in sitter. Glaze is
to
>^6 small cone in sitter. Visual cone 5 down. Schedule: 3 hours low, 2
>medium, then to high, fire-down for 1/2 hour at med and 1 hour low, cool
to
>touch. Total about 8-9 hours.
>
>The crazing: fine network of lines 1/4 to 1/2 inch apart, usually on the
>bottom of a plate or bowl, running across the face. These are large slab
>plates with manipulated and cut rims. I had some that crazed a few days
>after firing, but I think that was because I was cooling too fast. Most
have
>crazed months later and were cooled slowly. (I'm learning). I have tested
>some in the dishwasher for months, also microwave, oven, and freezer. All
>was fine until this week.
>
>The glaze: Mei Ming Clear ^5-7
>
>Whiting 16
>Gerstley Borate 18
>G-200 Feldspar 40
>EPK 10
>Flint 325 16
> 100
>
>Well? How does it look? Could there be an obvious solution? I'd
appreciate
>any help. By the way, did you notice that I tried to included all the
>details? I told you that you all have taught me a lot. Sure wouldn't want
>to leave out a cone #! You would send the kiln gods after me! The
"crazies"
>are enough right now. You're great! Thanks for any help.
>
>Carol and the Blue Cat in Arizona
>CBaker866@aol.com
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