Ian Currie on fri 1 mar 02
on 27/2/02 4:05 PM, Tom Buck at Tom.Buck@HWCN.ORG wrote:
> Vince P and Ron R:
> You both cited aspects of iron oxide behaviour at high cones.
> Vince says Iron (III) Oxide will become Iron (II) Oxide as the temperature
> goes quite high regardless of the atmosphere in the kiln.... etc...
Hi Tom
Glad I cought your post. I'm so busy getting things together for the tour
that I rarely have time right now to give Clayart the time it deserves.
I first learned about the iron oxide transition from Iron (III) to iron (II)
when I read an article in Ceramics Monthly on Oilspot Glazes (CM Oct 1980)
The facts are summarised in my book "Stoneware Glazes - A Systematic
Approach". And incidentally, the article originally attributed to Brother
Anthony of the Taize Community in France should have been attributed to
Brother Daniel, a well-known stoneware glaze researcher. Apparently Br.
Anthony was the translator.... Anyway I digress....
The gist of what Brother Daniel was saying is that the bubbling process in
the oilspot glaze is tied in with the process of lots of iron oxide in the
glaze giving up some of its oxygen as it "reduces" from Iron (III) to
Magnetite, which is I think what you termed Iron (II,III). And this is why
oxidation is required for this type of oilspot in a stoneware glaze - if
firing in reduction, the out-gassing of oxygen happens much earlier, before
the glaze has melted, and does not cause bubbling.
I have never done any tests to finally prove this theory, but have found
lots of evidence to support it. One of the more striking has come from
observing the results of many grid tiles fired at workshops. Time and again
I find in grid tiles fired to say Cone 6 or higher in oxidation evidence of
heavy pinholing over irony clay while the same set of glazes over a white
body (fired in the same firing) shows no pinholes. It is possible that it
is other things that cause the bubbling... but the fact that oxidation and
iron are present in all the cases I've noticed is suspicious. Another
example is... I get bubbling in my own shino that I am using now over an
iron slip (only where the slip is applied) when I fire it in oxidation.
This does in fact supply the essential ingredients according to Brother
Daniel - a feldspathic glaze, high iron content, and oxidation firing.
The reason I'm putting this reply is that it may explain some frustrating
pinholing that some potters experience over iron-y clay fired in oxidation
to midfire or stoneware. Plus I'd love to know more about it and find out
for sure what is the culprit.... without doing all the work!! It's not that
I'm lazy... it's just that unlike my beautiful new iBook, I can't do five
things at once. Right now I'm preparing for a tour, and ushering a batch of
white ants (Aussie termites) on to their next incarnation.
Will miss you at NCECA this year Tom ... not possible for me this time...
perhaps next year.
Incidentally, Br. Daniel has written a couple of books on stoneware glazes,
in French, and you can track them down with a search engine on the internet.
The first one he wrote is called:
"Pratique des Emaux de Gres" by Daniel de Montmollin
Regards
Ian
http://ian.currie.to/
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