daniel on sun 5 dec 04
Hi all,
I've been working on a clear glaze to fit a specific claybody - Glacia
Porcelain. I've had considerable help from Ron Roy on this (thanx again Ron)
and am finally making progress. I've seen a lot of crazed/shivered samples
along the way. None of my shivered samples have actually had glaze peel
away.
So here's a simple question. How do you tell the difference between
shivering and crazing ?
I have now established with some certainty the expansion of the glaze that
would fit this claybody. I will have to do a couple more tests to nail it
down but I know about where it is. As a consequence I know that I have made
glazes that should have shivered and some that would have crazed. The tiles
across the blends look (from what should be crazing to what should be
shivering) just look crazed. Well, except for two, two tests which either
crazed very little or shivered very little - the almost successes.
Harry Fraser, in his "Ceramic Faults and their remedies", explains that
shivering can look like crazing and that glaze may not peel off. Checking
Taylor and Bull, Ceramics Glaze Technology, a number of tests are suggested,
the only practical one for me seemingly, is the sharp blow with a needle
point and examine the fracture patterns. Results were inconclusive. RR and
JH in their book suggest freeze/boil tests and these have failed to cause
glaze to come free. The only test remaining seems to be to use test pieces
with a very thin edge and apply the blends thickly to such pieces. Hopefully
they can be induced to peel away. RR suggested that the edge of the craze
lines can be felt with a hard point if its due to compression. I have read
this also. In these tests this was inconclusive.
So, to get to the point. This raises a number of questions.
1) Is shivering where the glaze actually peels away, only likely to be seen
where there is a very dramatic mismatch in expansion ? This would accord
with what is said about a glaze's ability to withstand compression better
than tension. But it would suggest also that is should be possible to get a
compression fractured glaze with some ease. It is said in a number of
sources that crazing is much more common. hmmm......
2) If that is true, then one wonders what this compression fracturing is
that I'm seeing. It seems, from my tests, that the range of glaze COE that
fits this body between crazing and shivering is (at least in this case) very
short. I had expected that one could use a fair range of COEs, increasing
compression, without causing fracture. I appear to be wrong - again :)
Taylor and Bull do note that the range of perfect is pretty tight.
3) It would seem likely that the composition of the glaze would impart a
greater or lesser resistance to compression fracture. Can anyone point me to
works that might discuss glaze composition in this light ?
4) It is definitely true that the thicker parts of the tests failed (crazed
or shivered) more substantially, and sooner. This accords well with
conventional wisdom. I also saw delayed stress relief occurring in one of
the tests. On first examination it was very good. Next day it had many more
lines. I believe this piece is in compression. So this would be delayed
shivering.
I'm not strictly sure what I'm asking here, but perhaps people who've
played with such things might offer some comments.
Thanx
D
Belmont, California, USA
(ex terra australis)
David Hendley on sun 5 dec 04
I'm confused and I think others may be also.
Michael, what is the desired result of this test?
Do you want the glaze to slightly open the broken ring or hold
it tightly together?
I can't come up with the answer even when trying to logically follow
what is happening.
David Hendley
Working hard to make sure there is no mug left behind
david@farmpots.com
http://www.farmpots.com
----- Original Message -----
> David Waln uses porcelain and he showed me a wonderful way to determine
> whether a glaze is in compression or tension.
> 1. throw some cylinders as thin as practical.
> 2. cut the cylinders into 1" tall rings and let dry.
> 3. bisque fire the rings. then glaze some of them on the inside with the
> glazes in question.the rest are used to refine the data.
> glaze fire the rings on bisque cookies of the same clay to keep them round
> (use alumina if needed to prevent sticking)
> 4. This is the tricky part: break the fired ring at one place so that it
> remains one ring with no pieces missing.
> 5. examination analysis: if the ring has a gap after it is broken, the
> inside glaze is under compression and stretches the ring. If the ring
stays
> tightly closed, the glaze is under tension and keeps the crack tight.
> His approach allows you to zero in on a glaze that puts your body into
> modest tension while being sure it will nor see delayed crazing later.
> Pick two glazes that are near the ideal expansion, one above and one
below,
> and line blend them until you find the fit you want. Then record the blend
> and recalculate the glaze using the percentages of the ingredients needed
to
> total to 100 %. retest the blend whenever you get new batches of
ingredients
> to be sure it is still correct since materials change.
> This method also works well for dark glazes where it is hard to see if the
> glaze has crazed.
> Regards,
> Michael Wendt
Michael Wendt on sun 5 dec 04
Daniel,
David Waln uses porcelain and he showed me a wonderful way to determine
whether a glaze is in compression or tension.
1. throw some cylinders as thin as practical.
2. cut the cylinders into 1" tall rings and let dry.
3. bisque fire the rings. then glaze some of them on the inside with the
glazes in question.the rest are used to refine the data.
glaze fire the rings on bisque cookies of the same clay to keep them round
(use alumina if needed to prevent sticking)
4. This is the tricky part: break the fired ring at one place so that it
remains one ring with no pieces missing.
5. examination analysis: if the ring has a gap after it is broken, the
inside glaze is under compression and stretches the ring. If the ring stays
tightly closed, the glaze is under tension and keeps the crack tight.
His approach allows you to zero in on a glaze that puts your body into
modest tension while being sure it will nor see delayed crazing later.
Pick two glazes that are near the ideal expansion, one above and one below,
and line blend them until you find the fit you want. Then record the blend
and recalculate the glaze using the percentages of the ingredients needed to
total to 100 %. retest the blend whenever you get new batches of ingredients
to be sure it is still correct since materials change.
This method also works well for dark glazes where it is hard to see if the
glaze has crazed.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, Idaho 83501
USA
wendtpot@lewiston.com
www.wendtpottery.com
daniel on sun 5 dec 04
Hi Michael,
Thanx for this. this is doc'd in Taylor and Bull as well though I think
they suggest using the glaze on the outside. Like you said step 4 is the
trick. Need to find someone with a diamond saw or is there a better way ? Is
it perhaps better to try to crack it with a blow from something like a cold
chisel so there is no gap from the saw cut ? I guess I'll try some stuff
out.
Thanx again
D
> Daniel,
> David Waln uses porcelain and he showed me a wonderful way to determine
> whether a glaze is in compression or tension.
> 1. throw some cylinders as thin as practical.
> 2. cut the cylinders into 1" tall rings and let dry.
> 3. bisque fire the rings. then glaze some of them on the inside with the
> glazes in question.the rest are used to refine the data.
> glaze fire the rings on bisque cookies of the same clay to keep them round
> (use alumina if needed to prevent sticking)
> 4. This is the tricky part: break the fired ring at one place so that it
> remains one ring with no pieces missing.
> 5. examination analysis: if the ring has a gap after it is broken, the
> inside glaze is under compression and stretches the ring. If the ring stays
> tightly closed, the glaze is under tension and keeps the crack tight.
> His approach allows you to zero in on a glaze that puts your body into
> modest tension while being sure it will nor see delayed crazing later.
> Pick two glazes that are near the ideal expansion, one above and one below,
> and line blend them until you find the fit you want. Then record the blend
> and recalculate the glaze using the percentages of the ingredients needed to
> total to 100 %. retest the blend whenever you get new batches of ingredients
> to be sure it is still correct since materials change.
> This method also works well for dark glazes where it is hard to see if the
> glaze has crazed.
> Regards,
> Michael Wendt
> Wendt Pottery
> 2729 Clearwater Ave
> Lewiston, Idaho 83501
> USA
> wendtpot@lewiston.com
> www.wendtpottery.com
>
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Thanx
D
Belmont, California, USA
(ex terra australis)
Michael Wendt on mon 6 dec 04
David Waln explains it like this:
If at room temperature the ring opens up, that means the glaze is larger
than the clay body as wanted and is able to stretch the "thin" porcelain
ring. When the glaze is larger than the clay, the clay is in tension and the
glaze is in compression which we want to prevent crazing.
Notice he puts glaze only on the inside since if both faces are coated with
glaze, they counteract each other.
If you put glaze on the outside only, the result is reversed so that glazes
in compression keep the cracked ring closed while glazes in tension will
stretch the ring open if the clay is thin enough.
Thanks for asking. I hope it makes more sense.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, Idaho 83501
USA
wendtpot@lewiston.com
www.wendtpottery.com
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