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hairline cracks

updated sat 16 jan 99

 

Janet H Walker on fri 15 jan 99

...hairline cracks that run from the rim...vertically...
...turned it off and left it overnight. In the morning I
turned it on again, fired to 900oC...

Lyn -- I'd be surprised if those particular cracks were readsorption
cracks since your description didn't mention any particular period
or circumstance during which the pieces could have taken up water
again. I don't think they would readily take up the water being
released by the still damp pots. If you left your pots in the kiln
overnight and they were still warm when you started it up again, it
seems unlikely that they got rewetted. Unless you're really awash
in summer humidity down there?

To get readsorption, you've got to expose the fully dried pieces to
good high humidity for awhile. (No, I don't know how high or how
long. I know what I did when it happened to me and it involved an
unusual set of circumstances. Funny how so few texts ever define
those relative terms that are so crucial to actual success. I guess
that's in the nature of the diff between theory and practice!)

Another interesting source of diagnoses for cracks is Harry Fraser's
Ceramic Faults and Their Remedies. He talks about some very fine
hairline cracks that are caused by very rapid bisque firing for
example. He also covers readsorption, suggesting that bodies with
low green strength are more susceptible.

I wonder whether the formulation of the clay has changed enough in
the time since you last used it that its working characteristics are
now different and hence your problems. i know the clay I was using
changed enough every batch that I had to basically relearn each time
what its working range was, how it liked to be joined, and how
quickly it could dry! (So I switched manufacturers and now have to
learn absolutely everything all over again... But hopefully just
this once. Unless it goes on forever like this.)

One further thought. Are you saying that you did your bisque only
to 900oC? That is a pretty soft bisque actually. (Fraser: "Most
craft wares are biscuited at 1000oC and this should be considered
the minimum.") Why? "rapid biscuiting ... produces greater
cracking, greater distortion, risk of bloating, and lower fired
strength." But I am probably digressing. Again, what is "rapid"?
In this case, Fraser has distinct definitions -- 100oC per hour in
the range of 600-1000. If you fire with a pyrometer, you'll know
roughly how fast the temp is going up. Again, there are plenty of
people who fire bisque fast and it works for them because of the
kinds of clay they use and/or the kinds of ware they make. When you
have something perplexing going on, you have to find out more about
the "it depends" subtle parts of the business!

Good luck.
Jan

Lyn Packer on fri 15 jan 99

After discussing my problem of the hairline cracks with a pottery colleague
yesterday, and after reading Tom's message, I had a thought. I round up
each mug after the handle has been attached. I wonder if I've been a bit
rough when I put the rounder in and turn it. It was also suggested that I
fill the glazed mug with water and boil it in the microwave for 20mins. This
will 'make or break it'.

Lyn
lyn.packer@clear.net.nz