Mike Bailey on mon 16 aug 99
In message , ginny bivaletz writes
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>mike, could you please explain more about "soaking the bisque"? thanks,
Dear Ginny,
I'll certainly have a go!
Some clays contain quite a lot of 'rubbish'; mainly due to organic
matter. In some extreme cases, for example in the U.K. we have a marl
called Wad Black, there is so much carbonaceous matter in the clay that
it's a very dark grey colour in the plastic state although it fires
white. We also have 'black' and 'blue' ball clays for the same reason.
This carbonaceous matter can also be a problem to potters
who have their kiln in the workshop and they find that they have to
reject some clays because of the unpleasant amount of 'smoke' that comes
off the clay and hangs around the room as a layer of acrid haze.
Anyway, any significant amount of the carbonates and sulphates need to
be driven out of the clay and, if it gets trapped in the clay, is a
cause of bloating. Potters that raw glaze have to be particularly aware
of this and hold the kiln around 900 - 950 deg. C. for an hour or so to
get the carbonates out before the glaze seals them in. If one is bisque
firing, some clays that are 'dirty' benefit from being 'soaked', that is
held at the top end of the firing. Typically this would be at around
1,000 deg. C. for 1 hour but each case needs its own answer. This burns
out the carbonates and prevents bloating in the glaze firing.
There is also the possibility, and this is what I had in mind re. your
pinholing problem, that some fumey clays can produce bubbles and
blisters in the glaze - hence the need to make sure that they are
removed beforehand.
Hope this helps,
Cheers,
Mike
--
Mike Bailey. Bath. U.K.
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