Bruce Girrell on sun 20 aug 00
I promised that I'd write a note regarding the porcelain-based raku clay
that we had gotten from Great Lakes clay after we had gotten a chance to
glaze fire it. So here we go:
Throwing: This stuff was wonderful. After months of nothing but gritty old
raku clay, it was heavenly getting back into a porcelain base again. There
is no large grog in the clay, a problem that we had with other porcelain. We
may have just been lucky, but the moisture level was perfect.
Trimming: Just like regular porcelain, this clay trimmed very well.
Occasionally, I saw some drag marks from what I assume was a fine grog used
to improve thermal shock characteristics. A rubber rib was all that was
required to clean up the marks. Two of my pieces I trimmed to about 3/32"
wall thickness. Nice.
Bisque: No losses in the bisque. The fire cycle was an 8 hour candle (some
of the kiln load was leather hard), 5 hours to ^06, and cooling with both
burner ports and the exhaust left completely open. The kiln was cool enough
to unload about 7 hours after the burners were turned off.
Glaze firing: We used the pots in three different ways - normal glaze,
naked, and horsehair. We felt the whiteness and smoothness of the porcelain
made it an excellent candidate for naked raku and horsehair. Normal glaze
didn't produce any surprises. I thought that maybe the colors would be
brighter, but it didn't really seem to make a difference. We lost one pot to
cracking during the post-fire reduction. Normally, we don't lose any*.
In the naked raku technique, the porcelain-based clay did not take quite as
thick of a coat of resist slip as the standard body. We are just learning
how to handle this technique and did not get a clean white anywhere on the
pots. Certainly a thicker slip coat wouldn't have hurt, though.
Only one pot got the horsehair treatment. I pulled the pot from the kiln
using mitts and turned it while Lynne applied the hair. As we were applying
the hair we heard "tink.... tink... tink.. Snap! Pow!" Two huge cracks
appeared, one straight up the side of the pot and the other around the
entire circumference of the pot, splitting it into two pieces. Clearly, this
clay is not as tough as the regular body that we use. On the other hand,
most of the pots made it through our normal process. Horsehair raku is new
to us also. We will have to find a way to slow down the cooling of the
pieces. Slower cooling should reduce cracking and extend the surprisingly
short time window for applying the hair.
All in all, this appears to be a good commercial body. It is undoubtedly
more susceptible to cracking than Great Lakes Clay's raku body, but it does
hold up fairly well to the process.
Bruce "porcelain is just such sexy stuff" and Lynne Girrell
in northern Michigan where fall is approaching waaaaay to fast
*Wow, I guess we have made some progress somewhere. Two years ago we lost
close to 50%. I now won't load the next batch in the kiln until the
pyrometer reads below 500 oC and I leave the pieces in the reduction cans
until they are mostly cool, sometimes not quenching them at all.
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