Sharon Miranda on sun 8 dec 96
I have followed this thread with great interest. I don't mean to
resuscitate it but either I missed something or no one wrote about how to
get rid of crazing. Last summer, as I prepared for a show, the last firing,
where all the important pieces were, came out crazed. (and me, too).
I switched my glaze to Laguna (clear,06, on white earthenware) and am STILL
getting crazing. I stopped peeking at the pots while they were cooling
(can't even warm my hands on my kiln much less melt a flashlight!) - I now
open the kiln at 150 degrees.
I bisque to 05 at a medium speed, brush on the (really well mixed)glaze,
(2 light coats), glaze fire at a medium speed. And still one or two pieces
come out crazed.
Could this be caused by recycling scraps? (I try to be very careful and
only recycle the same clay always)
Could this be a too fast bisque?Should I try a hotter bisque?
I am also using laguna clay (em210). Any thoughts would be welcome.
(BTW I've read every book I own on this)
Thanks - Sharon
*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*
Sharon LaRocca-Miranda *
92 Morgan Street *
Oberlin, Ohio 44074 *
Sharon.Miranda@oberlin.edu *
. . . . . . . . . . . . . . . .*
* * * * * * * * * * * * * * * * *
Christine Davis on tue 10 dec 96
When I started bisquing at cone 04 instead of 05 or 06, I got less
crazing (didn't eliminate it entirely, but it did reduce the incidence of
it.)
Cummings Pottery on tue 10 dec 96
Hi list,
Time for my thoughts on crazing. When I bisque ^03, glaze fire ^05,
on white earthenware I also battle delayed crazing. I made a discovery that
has helped. When I bisque fire, I let it cool very slowly, until it is
cold. For some reason pots bisqued that way don't seem to craze, even
delayed. I still tend to rush cooling my glaze firings, and that doesn't
seem to matter.
And when a customer mentions a pot crazing, I just tell them to
bring it by, and I stick it in a bisque firing, and voila, good as new.
Pat....
Monday morning after 3 day studio sale, sick of smiling!!!!!
Date: Sun, 8 Dec 1996 21:52:15 EST
From: Sharon Miranda
Subject: crazing - causes?
----------------------------Original message----------------------------
I have followed this thread with great interest. I don't mean to
resuscitate it but either I missed something or no one wrote about how to
get rid of crazing. Last summer, as I prepared for a show, the last firing,
where all the important pieces were, came out crazed. (and me, too).
I switched my glaze to Laguna (clear,06, on white earthenware) and am STILL
getting crazing. I stopped peeking at the pots while they were cooling
(can't even warm my hands on my kiln much less melt a flashlight!) - I now
open the kiln at 150 degrees.
I bisque to 05 at a medium speed, brush on the (really well mixed)glaze,
(2 light coats), glaze fire at a medium speed. And still one or two pieces
come out crazed.
Could this be caused by recycling scraps? (I try to be very careful and
only recycle the same clay always)
Could this be a too fast bisque?Should I try a hotter bisque?
I am also using laguna clay (em210). Any thoughts would be welcome.
(BTW I've read every book I own on this)
Thanks - Sharon
.. . . . . .
Craig Martell on tue 10 dec 96
Hi Sharon: Crazing is caused by an expansion differential between the clay
body and glaze that is putting the glaze uner too much tension. For glazes
to fit properly they must be under slight compression. I was taught to first
examine the clay to determine if the feldspar and silica were amounts were
balanced. Too much feldspar will cause crazing and too little silica will
cause crazing. If you are using a commercial body, your only course of
action is to alter the glaze. Additions of silica to the glaze can produce a
better fit. Sometimes you have to look at the fluxes and lower the high
expansion fluxes and raise the low expansion fluxes. A bit of reading in
books that cover this sort of thing will help a lot. In your case, there is
really nothing you can do outside of contacting Laguna and asking them to
help you with the problem. When you are using both commercial clay and glaze
mixes, you don't know what the formulas are and Laguna won't tell you.
Kind Regards, Craig Martell-Oregon
Robert Kittel on wed 11 dec 96
At 01:42 PM 12/10/96 +0000, you wrote:
>----------------------------Original message----------------------------
>Hi list,
> Time for my thoughts on crazing. When I bisque ^03, glaze fire ^05,
>on white earthenware I also battle delayed crazing. I made a discovery that
>has helped. When I bisque fire, I let it cool very slowly, until it is
>cold. For some reason pots bisqued that way don't seem to craze, even
>delayed. I still tend to rush cooling my glaze firings, and that doesn't
>seem to matter.
> And when a customer mentions a pot crazing, I just tell them to
>bring it by, and I stick it in a bisque firing, and voila, good as new.
> Pat....
>Monday morning after 3 day studio sale, sick of smiling!!!!!
>
>
>
>Date: Sun, 8 Dec 1996 21:52:15 EST
>From: Sharon Miranda
>Subject: crazing - causes?
>
>----------------------------Original message----------------------------
>I have followed this thread with great interest. I don't mean to
>resuscitate it but either I missed something or no one wrote about how to
>get rid of crazing. Last summer, as I prepared for a show, the last firing,
>where all the important pieces were, came out crazed. (and me, too).
>I switched my glaze to Laguna (clear,06, on white earthenware) and am STILL
>getting crazing. I stopped peeking at the pots while they were cooling
>(can't even warm my hands on my kiln much less melt a flashlight!) - I now
>open the kiln at 150 degrees.
>I bisque to 05 at a medium speed, brush on the (really well mixed)glaze,
>(2 light coats), glaze fire at a medium speed. And still one or two pieces
>come out crazed.
>Could this be caused by recycling scraps? (I try to be very careful and
>only recycle the same clay always)
>Could this be a too fast bisque?Should I try a hotter bisque?
>I am also using laguna clay (em210). Any thoughts would be welcome.
>(BTW I've read every book I own on this)
>Thanks - Sharon
>. . . . . .
Im sure that this thread has been beat into the ground but doesnt anyone
adjust their glaze when they have crazing problems? How about adding silica,
make the glaze fit properly. Frit your glaze, use ferro frit 3249, start
your test with a 1% addition. one percent of the total mixed wt of the
glaze. do your test to a maximum addition of 10%. Also on the silica side
Try micron ground msil A25, addition to total wt of mixed glaze not to
exceed 7 1/2 %. The ferro frit works well with low and high fire glazes.
Hotter bisques in my opinion are not the answer, the coefficient of
expansion is still there no matter what you do. Probably the net result of
raising bisque temps is a lose of good glaze interface.
ZALT@aol.com on sat 14 dec 96
Crazing will be with us till the cows come home. We can adjust the clay,
adjust the Glaze, adjust the firing time, adjust the thickness, adjust the
bisque time, adjust the cooling time, adjust the oxides, adjust the recyling
weights, adjust the drying time, adjust the drafts around the greenware,
adjust the sunlight entering and stricking the fired piece, adjust the
humidity in the display cabinate, adjust the temperature, or adjust the
throwing and trimming methods. Why not adjust our thinking and accept
crazing for what it is. A fact of life.
Terrance F. Lazaroff
St Hubert, Quebec, Canada !!!!
Bill Amsterlaw on mon 16 dec 96
Hi Clayart:
Many glazes have a fine network of crazing that is hard to see. The real test
is to mark a glazed area with a marking pen, rub off the excess, and then
examine the area with a hand lens. If you try this test, you will be amazed
at how many apparently craze-free glazes actually are crazed.
Sometimes crazing is beautiful. Certainly, if your work is not intended to
contain food or drink there is no need to consider anything but the visual
impact of the work. True, a lot of crazed utilitarian work exists. In some
cases, the crazing is so beautiful, any negative features of crazing is offset
by the visual contribution of the crazing. But what does crazing do for your
work if you need a handlens to see it? This kind of crazing does not
contribute to the visual impact of your work in any way.
You might ask: If a surface is pleasing to the unaided eye, if the pot looks
"good", why should anyone go looking for "flaws" with a magnifier? The idea
that food and bacteria get trapped in the cracks doesn't bother me. I know
that bacteria are everwhere - in our mouths, on our skin, in our intestinal
tracks, on every surface we touch; who cares if they are also in the cracks of
a glaze? But the idea that a crazed surface is less durable and makes the
clay weaker does bother me. If you have a glaze in which you WANT the crazing
effect for visual impact, then that may be sufficient reason to use it.
Otherwise, if you are making things that will be banged around in everyday
use as a food container, used in ovens and microwaves ... then there is reason
to making the effort to reformulate a glaze to eliminate crazing.
I am no glaze expert - but I have patiently done the job of reformulating a
glaze to get it to stop crazing on the claybody I was using. It took quite a
few test tiles until I was happy with the new glaze. I understand how the old
glaze evolved into the new glaze both in terms of composition and appearance -
and I admit that the craze-free version is quite different in appearance from
the original. (I understand a quote fromKen Stevens: "When you change the
glaze, you change the glaze.")
I like the new glaze. I like its appearance, and I also like the improvements
in its physical properties. I can now remove pots from the kiln when the
pyrometer reads nearly 500 degrees F and they do not ping. I can distinuish
the old glaze from the new by its sound. If you flick a bowl glazed with the
old glaze with your finger, you get a kind of "clunk" sound; a bowl glazed
with the new glaze will ring like a bell. I had some pieces glazed in the old
glaze which cracked in the microwave; I think the cause may have been
vaporization of water which penetrating into the clay.
The existence of glazed surfaces that have remained craze-free after hundreds
of years disproves the notion that "all glazes craze eventually". It seems
probable to me that if a glaze fits properly (ie, is in slight compression on
the claybody) it will not craze even after years of use ... even if the piece
is used for cooking in an oven. It seems just as probable to me that if a
glaze is in contraction on the claybody, it is UNSTABLE and has to craze
sooner or later. If your glaze fits poorly, no matter how cautious you are
about cooling your kiln, it is going to craze, anyway: If the glaze doesn't
fit, it doesn't fit.
I think there is no reason to hesitate taking ware from a kiln when it is
still too hot to touch with bare hands. If the glaze doesn't fit, it is going
to craze sooner or later, regardless. If the glaze DOES fit, nothing will
happen to the glaze: Taking a hot pot out of the kiln is no different from
taking a hot cassarole dish out of an oven.
- Bill Amsterlaw (wamster@msn.com)
Plattsburgh, NY
Terrance F. Lazaroff wrote:
>>
Crazing will be with us till the cows come home. We can adjust the clay,
adjust the glaze, adjust the firing time, adjust the thickness, adjust the
bisque time, adjust the cooling time, adjust the oxides, adjust the recyling
weights, adjust the drying time, adjust the drafts around the greenware,
adjust the sunlight entering and striking the fired piece, adjust the humidity
in the display cabinet, adjust the temperature, or adjust the throwing and
trimming methods. Why not adjust our thinking and accept crazing for what it
is. A fact of life.
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