Judy Musicant on thu 31 oct 02
Hello all. I need some serious help here. In recent weeks, well over 50% of my flat bottomed pieces have been cracking which only becomes visible after the glaze firing. I have never had this problem to this extent. It's not happening in bowls, or pieces with smaller flat bottoms, such as mugs and vases. But platters and bakers, even 6" flat bottomed bakers, are showing a very faint, hairline crack in the glaze that runs straight across the bottom at the widest point. I'm not throwing any differently than before. I throw primarily on wooden bats and run a wire under the pieces when they're first thrown and turn them over at a relatively soft leather hard stage and try to dry them slowly - over several days, anyway. I'm firing up slowly in both the bisque and the glaze firing. It's really discouraging when one after another $130 platter comes out cracked. Any thoughts?
Thanks much.
Judy Musicant
Christena Schafale on fri 1 nov 02
Judy,
I'm assuming you must be getting replies offlist, since I haven't seen any
on the list, but here's my take.
If the crack is hairline, and if it is showing up in the glaze firing only,
and with sharp edges, not rounded, it sounds like a cooling crack or
dunt. Does the crack go all the way through, or does it appear to be just
on the inside, glazed surface? Are you cooling any differently, or
unloading the kiln sooner? That would be the first thing to look at, I
believe.
Are you trimming the bottoms of the pots any differently than before? I
believe that trimming a foot on such pieces, especially a foot with some
openings out to the sides, can help relieve cooling stresses by allowing
the bottom of the pot to cool more evenly along with the rest of the pot,
instead of remaining hot because it is in contact with the larger thermal
mass of the kiln shelf. Then there is Vince's idea of bedding the pot on a
starburst shape of thin coils, so it is lifted off the surface of the shelf
-- same reasoning.
Other possible factors to consider -- are you using a different glaze or
clay than previously? My understanding is that situations where there is
glaze on one side and not on the other (as happens with a wide,
flat-bottomed piece) are hardest on the pot, because of the different
expansion/contraction on the glazed and unglazed surfaces. If your glaze
is expanding/contracting LESS than your pot (I think I have this right), it
could actually pull the pot apart when it cools.
Hope some of this may help. This is not a good time of year to have this
problem, I know.
Chris
At 11:55 AM 10/31/02 -0500, you wrote:
>Hello all. I need some serious help here. In recent weeks, well over 50%
>of my flat bottomed pieces have been cracking which only becomes visible
>after the glaze firing. I have never had this problem to this
>extent. It's not happening in bowls, or pieces with smaller flat bottoms,
>such as mugs and vases. But platters and bakers, even 6" flat bottomed
>bakers, are showing a very faint, hairline crack in the glaze that runs
>straight across the bottom at the widest point. I'm not throwing any
>differently than before. I throw primarily on wooden bats and run a wire
>under the pieces when they're first thrown and turn them over at a
>relatively soft leather hard stage and try to dry them slowly - over
>several days, anyway. I'm firing up slowly in both the bisque and the
>glaze firing. It's really discouraging when one after another $130
>platter comes out cracked. Any thoughts?
>
>Thanks much.
>
>Judy Musicant
>
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