Carol Baker on sun 11 jul 99
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
Michael Banks on mon 12 jul 99
What you've encountered Carol, is probably delayed crazing due to an overly
porous (underfired) body. This is a body problem, not a glaze one. Water
entering the overly-porous parts of the body is jacking it up, expanding it
beyond the tensile strength of the glaze to withstand.
Because the crazing is occurring at, or near the base of your pots indicates
that the parts of your ware in contact with the shelves is lagging behind in
temperature. I would advise lengthening the soak period to ensure that the
shelf temperature catches-up with the rest of the kiln before the firing is
terminated.
It is important that all the kiln contents be given sufficient time to reach
the same top temperature (equilibrate). Too-rapid firing to top temperature
exacerbates the problem and requires an even longer top-soak to counteract
it. Quick firings may be attractive, but there are no free lunches in
potting :)
Michael Banks,
Nelson,
New Zealand
> ----------------------------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. (snip)..... , usually on the bottom of a plate or bowl, (snip)
> Carol and the Blue Cat in Arizona
Craig Martell on mon 12 jul 99
Hello Carol:
I didn't put your glaze into the seger formula to look at the molecular
structure but it looks as though you might be able to squeeze in some more
silica to lower the expansion a bit.
Another thing you may want to check is the vitrification of your clay.
Delayed crazing is most often caused by an unvitrified body absorbing
moisture over a long period of time and eventually starting craze fractures
in a glaze that may be a "close" fit. You might also want to try placing
cones at several different locations in the kiln to see if there are cool
spots where the clay isn't getting the heat work it needs to totally mature.
Perhaps some of the delayed crazing is being caused by these cool areas in
the kiln.
Do an absorbtion test on the clay to check vitrification. I usually do at
least 5 different samples and average. My preference for stoneware clay is
about 2.5% absorbtion and no more. For porcelain, it should be closer to
zero. I shoot for not more than 0.5% with porcelain. Place cones by your
samples too, if you choose to go to this trouble, and make sure they hit the
target temp.
Fast cooling doesn't cause crazing unless it's going to happen at some point
anyway. There is an incompatible expansion differential between the clay
and glaze and this will exist no matter how fast or slowly you cool the
kiln. A quick cool only hastens the inevitable. You can see this happen
too by freezing samples overnight and plunging them straight into boiling
water. If the glaze and clay have a good fit, this freeze-boil activity
cannot cause crazing. This is a good test when working with a clay and
glaze to gain good fit.
regards, Craig Martell in Oregon
Phyllis E. Tilton on mon 12 jul 99
Hi Carol: I use B-Mix ^5, also, with underglazes. Since I do not have space
to store supplies, I use commercial glazes. I had an Amaco clear glaze that
had been ok on other clays that I used, but crazed on B-mix. I tried the
Laguna Clear Bright glaze and it is perfect. It must have been formulated to
fit B-Mix because it is beautiful and an end to my frustration with the
crazing.
I might as well get into the conversation about the Grip. For me it is a time
saver to use-particularly when I have a bunch of mugs to trim. I still use
the plugs of clay to anchor some forms that are asymmetrical and have other
differences that the grip doesn't work that well with. I use it to band with
color, carve specific lines to carve or facet in, and, use the triangular
grooves to mark pots either for feet or for holes or attachments for hanging
pots. It is just another tool-as is my electric can opener, my convection
oven,my big Kitchen Aid Mixer, my microwave, my dishwasher, my garbage
disposer, my air conditioner,my computer, my cd player that will take 5
discs,--all tools and I love the technology.I am old enough to have seen all
this develop. We had a Victrola that we had to wind up when I was a child,
and none of the things I just listed. My grandmother lost her hearing and
used an 'ear phone'. I am to be fitted with hearing aids that are state of
the art. All this discussion about the pros and cons of using the grip has
been very interesting to read. It is the USA !!! We can say what we think. We
all have different thoughts and learn from the ideas and backgrounds of the
discussers.( What about the cussers? -just a thought).
We are so blessed to have this Listservice!!!!!!!!!!!!!!
Phyllis Tilton
Daisypet@aol.com
Ron Roy on wed 14 jul 99
Hi Mei,
I think it is possible that your Bmix is not vitrified sufficiently for
what ever reason - however - perhaps trying this glaze reformulated with a
lower expansion is worth a try.
WHITING............. 16.50
Gers Borate 9/97.... 18.00
G 200 SPAR.......... 32.00
EPK................. 14.00
SILICA.............. 19.50
----------
Total.....................100.00
RATIO 7.34 (org is 7.36)
EXPAN 510.16 (org is 550.30
WEIGHT 270.28
After firing freeze for 24 hours and dip in boiling water - do that 5 times
- then leave in water for a week. If it crazes after or during the freezing
and boiling then it's the clay/glaze fit. If it crazes from being soaked
the clay is not vitrified enough - try firing higher.
RR
>----------------------------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
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
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