Kim Marie on thu 11 apr 02
I'm still getting lots of pots with cracks in them when I pit fire. I've
tried the following with noted results. Pieces were polished terra sig
bisque fired to either 018 or 04. When ^04 I lined with commercial ^04
clear glaze. pit was 3' x 3' cement block above ground.
Laguna eastern 10T - Works great on mini to medium size pieces. Seems to
encourage orange flashes when using copper sulfate. Not good for large
pieces
Laguna eastern #55 - unfavorable results all around
Laguna eastern #45 - unfavorable
Standard #112 - ok results still seeing cracking in large pieces
Standard Raku - I was surprised but lots of cracked pots - these were large
however.
Standard #153 - Very nice results but still a low % of cracking
Laguna eastern #200 raku - this is the heavier grogged raku clay. tough on
the hands when throwing. Will be pit fired tomorrow.
Can anyone see why I might have such a high breakage rate? I am beginning
to notice if I have a real ragin' fire I get more cracks but I'm still
surprised at how many crack. When I did Bob Compton's workshop, none of the
pots cracked and we had one impressive fire ragin' through the night!! I
can't figure why the difference. I am wondering if some forms lend
themselves to breakage more than others.
Any help appreciated,
Kim
In sunny Spafford NY where it's still cold but cha' sure don't have to
shovel cold so I'm a happy mud puppy!
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Lori Leary on thu 11 apr 02
Kim,
I forgot to add: as far as shapes go, you will have much better luck with
rounded even walled pots (the heat is distributed more evenly)....although I
have found that Vince's clay recipe I mentioned in my previous post is very
forgiving of other shapes.
Lori
lleary@epix.net
> Can anyone see why I might have such a high breakage rate? I am beginning
> to notice if I have a real ragin' fire I get more cracks but I'm still
> surprised at how many crack. When I did Bob Compton's workshop, none of
the
> pots cracked and we had one impressive fire ragin' through the night!! I
> can't figure why the difference. I am wondering if some forms lend
> themselves to breakage more than others.
Jeff Tsai on fri 12 apr 02
I am about to be very unscientific, so bare with me.
These are just things that I have noticed, more or less, in pit fires I've
done.
The best surviving shapes have always been, for me, rounder. I don't have a
clue why. Cylindrical and flared forms have cracked more often for me, I
have
no answer why.
Thicker work cracks more easily, and pieces exceeding 10" in diameter.
Here is a completely baffling one, Almost every high grog stoneware clay
I've
ever used has had significant cracking. However, I have never lost a
porcelain or bee-mix pot in a pit fire to cracks. I also use a fine particle
earthenware (mostly talc and ball clay) with little to no loss of work (I
figure the talc in the eathenware helps this). This goes against everything
I
have been taught is true, but I also tend to throw thinner with my porcelain
and eathenware clays.
Here is a little bit more scientific of an answer. Liner glazes can,
especially if thickly applied, cause cracking. When only the interior of the
work has glaze, there is some tension caused by the thermal expansion of the
glaze during firing. Since the same tension doesn't exist on the exterior of
the clay, this can cause small cracks to form, usually around the rim.
Again,
this is not scientificly based (at least not for me) it is something I have
experienced through experimentation. It tends to happen with my low fire pit
work, but, oddly enough, my high fire soda or wood work that uses only liner
glazes don't have this problem, why, I don't know.
Hope you figure it out, and maybe get a few more informative responses.
-jeff
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