Chris Schafale on thu 3 dec 98
In case anyone else is interested, I have, I think, found some
answers to a couple of questions I posted recently.
Q: Iron red glazes all seem to have very low alumina. Why don't
they run off the pot? Does the iron somehow make up for the missing
alumina?
A: According to Robin Hopper, in oxidation firing, iron in these
quantities (15-20%) does have a refractory effect, so it is in a
sense, compensating for the missing alumina. So, if you took one of
these iron red glazes that is reputed to be "just a little runny" and
you removed the iron, you would almost certainly end up with a puddle
on the kiln shelf.
I don't know, but would venture to guess that alumina does not
compensate for alumina's other contributions to the glaze (e.g.
hardness), so these glazes would also be expected to be softer,
somewhat scratchable, and prone to attack by alkaline substances like
dishwasher detergent. On the other hand, I have heard reports from
people who have dishes glazed with these glazes and have seen no
change over years of use, so maybe I'm wrong about this part.
Q: Why does Floating Blue turn cloudy green under some firing
conditions?
A: This is more of a guess, but I've come across the following
facts: Rutile plus cobalt can produce green glazes. Also, under
the right conditions (slow cooling, in particular), rutile will form
crystals which give the glaze a cloudy, opaque appearance. My
surmise is that, under slow cooling conditions, rutile crystal
formation is "swamping" the blue of the cobalt, creating the
predominantly greenish coloration. This fits with my experience that
I got the most greenish results when using a very slow firing
schedule with a long controlled cool-down. Also fits with the
reports from others and my own experience that Floating Blue works
fine at cone 6 or even 7 when fired on a fairly fast schedule (7-9
hours) and also works fine when fired somewhat slower (10-12 hours)
but only to cone 5 or 5 1/2. In either case, the glaze is spending
less time in the hypothetical crystallization zone (not sure
precisely where this is but maybe 700-1000C?) than it would in a
slow-rise, slow-cool fire to cone 6.
Any comments, additions, or other thoughts?
Chris
Light One Candle Pottery
Fuquay-Varina, NC
candle@nuteknet.com
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