David Hendley on thu 24 dec 98
"Clay and Glazes for the Potter" by Daniel Rhodes is really
maddening. In the "Reduction glazes" chapter Rhodes gives
typical molecular formulas for these reduction glazes:
celadon
iron saturate
copper red
high magnesia
All of them include zinc oxide.
But in the text, that goes on for many pages, there is never any
reference to why the zinc is included.
I did my own little zinc oxide experiment by putting about 40 grams
of zinc oxide in the bottom of a broken cup with an irregular top.
I covered the top with a piece of kiln shelf, but it was far from
air-tight because of the broken top of the cup.
This was fired to cone 10 in my wood kiln, with early and long
reduction. The result:
There was a tiny bit of a rough black substance on the bottom
of the cup, and the inside surface of the cup was covered with
a smooth white flashing. Very nice, actually.
Next time I'll try it without a lid to see what I get.
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com
At 10:40 AM 12/23/98 EST, you wrote:
>----------------------------Original message----------------------------
>Thanks to Tom for helping on this - Hamer says ZnO is easily reduced - even
>slight reduction above 925C will eliminate it from the glaze. To answer the
>question - will it stay in a reduced cone 6 (1200C) glaze - the answer must
>be no. Besides this it is not an active flux below 1085C. From this it
>could be surmised that it is not useful as a glaze flux in reduction at any
>temperature.
>
>It makes sense to me that if it is not reduced until the end of a firing
>that goes above 1085C that it will help the melt and therefore will have
>some effect - I also have to assume that if reduced at the end at least
>some of it will be lost and this will have some effect on the durability
>and the expansion of the glaze.
>
>It is an easy point to prove - mix up any reduction glaze with and without
>ZnO and fire them together in a reduction atmosphere.
>
>I wonder how much all that Zinc, in all those reduction glazes, over all
>the years, cost? The current price for ZnO up here is about $5 per lb.
>
>I just did a quick survey of my books and found only 4 references to this.
>Hamer - as I have stated already is where I first found it - Out of the
>Earth Into the Fire by Mimi Obstler mentions the Hamer reference, Cardew in
>Pioneer Pottery says it happens and Ian Currie in his Stoneware Glazes says
>on page 39 "When the zinc set is fired in strong reduction, zinc oxide
>seems to disappear without a trace."
>
>I recommend all of the above books The Hamer, Currie and Obstler books are
>still in print.
>
>The Cardew book is a gem but out of print - there is talk of having it
>legally reprinted - anyone interested in a copy or copies should let me
>know and I will advise the cost per copy when we know more -
>
>I have a lot of books - about 50 and this aspect of ZnO is simply not
>included in great majority of them - makes me wonder - what else is
>missing? Can anyone else find any references?
>
>Ron Roy
>93 Pegasus Trail
>Scarborough, Ontario
>Canada M1G 3N8
>Tel: 416-439-2621
>Fax: 416-438-7849
>
>Web page: http://digitalfire.com/education/people/ronroy.htm
>
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