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firing copper red glazes (fwd)

updated fri 31 jan 97


Tom Buck on thu 16 jan 97

Marshall Talbott says the following may be of wide interest, and asks that
it be posted to the list.

Cheers TomB Hamilton ON Canada URL

---------- Forwarded message ----------
Date: Tue, 14 Jan 1997 11:50:06 -0500 (EST)
From: Tom Buck
To: Talbott
Subject: Re: firing schedules for copper red glazes

Marshall: Kiln operation is the key to successful copper reds
consistently. So say most authors who have written about the topic. First,
have a glaze mix that is molten\fluid at the cone you finish at.
Second, start reducing at C08-C06, lightly then more strongly
until your final cone. But be wary of firing much above C9 since at C10
well bent copper metal starts to boil.
Third, go back to oxidative conditions for up to 30 minutes (soak
time). Then shutdown, and seal the kiln; don't open til it is below 100 C.
The reasons one does these steps is 1) to get copper (II) oxide
(black) to copper (I) oxide (red). This gas\liquid-solid reaction at the
glaze surface is a slow one; it just takes time to bring about the change
from black to red. BTW, copper carbonate basic (green material) converts
to copper oxide black well before C08.
If you over-reduce the red will go to metal and darken
eveything, an unpleasant colour; so 2) re-oxidize the surface slightly to
take the metal back to red. If you find you tend to overfire
regularly, you'll have to try several saok times at
slightly different temperatures to get results you're happy with.
There is quite a bit of literature on copper reds. Try to access
back copies of American Ceramic Society reviews and bulletins, Studio
Potter (early 1980s), Ceramic Monthly (various), Ceramic Review (1990 and
earlier). Also, Robert Tichane wrote a book: Reds, Reds, copper reds.
Years earlier Joseph Grebanier gave a a section on reds in his book
Chinese Stoneware Glazes.

Cheers TomB Hamilton ON Canada URL