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pit firing and cracking

updated fri 26 feb 99


Kayo Parsons-Korn on thu 25 feb 99

In response to Claudia's questions (see below). You say you are pit
firing but you're talking about a kiln? It kind of sound like you are
sawdust firing inside a barrel?

Well anyway, I started out using earthenware clay and found it doesn't
work well for pit firing. Raku clays can take the shock better. Try
using Coleman's Grogless Raku, you can burnish that. Tacoma Clayart
Center carries it, but Geoergie's may as well. That is where I take it
you are getting the Warm Springs Red. And you can always apply a terra
sig to clays that don't burnish well. I have been using a clay from
Tacoma Clay Art called BC6 (no sand). Its wonderful to hand build with
and burnishes beautifully. Haven't had any cracks yet, but I wouldn't
pit fire in the conditions we have had recently. Too wet and windy.
You can also cut down on your breakage by warming the pieces up first
before you put them into the pit. Put them in the oven at 500. Or if
they are too big, into an electric kiln. (I'm assuming you're bisquing
with an electric, I may be wrong.)

But boy, if you are already bisquing to Cone 06, its hard to believe
you're getting cracks. I'm only bisquing to Cone 018. If these were big
flat pieces, I could see the problem. But you say these are vessels. If
they are coil built, you may be getting cracks along the coil edges. You
have to really work them together to avoid this.

Hope this helps. Definitely change clays.


Hi all...
I just unloaded another pit firing and lost 3 out of 6 vessels. I am
not happy with those percentages, and wonder...can we talk!
I am using Warm Springs Red...a typical low fire earthenware, which I
first bisque to ^06. These vessels are burnished, so this is as high as
I dare to bisque. My fuel is a mixture of sawdust, wood pellets, and
various combustibles from around the landscape. When I used Raku clay,
I had very little breakage (but I could not burnish), and I realize that
this red clay is tighter and more prone to disaster than that Raku
clay. Still...I should be able to do better. Should I fire fewer
pieces at one time...I fire 6 usually, three above, three below. The
breaks are not consistent as to position in the kiln, so I do not have
problems with the vessels falling into each other. I have not been
diligent about filling the insides and wonder if this could be part of
the problem. We have been having a lot of wind, but the outside
temperature is not awfully cold here these days...around 50 degrees. I
did not open the kiln until it was totally cold....just to prevent the
cracks! Of course, it was the larger vessels that cracked...which could
also be part of the problem. I just wonder if anyone out there has had
this problem and has minimized it in any way. Thank you!