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surface iron colors in wood firing

updated fri 18 jun 10


MacIntire, Matt on wed 16 jun 10

Does anyone know what happens in a wood kiln that seems to bring the iron i=
a clay body to the surface? What is the mechanism?

I've noticed that even porcelains blush to a strong tan or reddish-orange.
That rarely seems to happen in a gas reduction kiln. We put salt cups in
our kiln and this seems to enhance the surface flashin effect. However,
I've certainly seen it (a bit more subtly perhaps) in wood kilns where no
salt or soda is used. Of course, there are salts in the wood too. Grey
clays turn reddish brown. Tan/buff clays become dark brown, almost
blackish. When I break apart a porcelain pot that has flashed to an orange
color, I see that there is no color inside the body. This is totally a
surface effect. One complication is that I know people who say that their
shinos are "bleached" in a salt kiln environment and mostly come out white.
Salt glazed porcelain may flash a bit, but nothing like this. So, it may
not be the salt at all. That is why I am intrigued.

I recall reading somewhere that the chlorine combines readily with iron.
Not sure what temperature that happens. Perhaps Iron Chloride migrates to
the surface early in the firing. (Hmm, another reason to do some draw
tiles!) I'm not sure if that is the mechanism at work here. When I used t=
do raku, we sprayed Iron Chloride on the pots to get a similar sort of
effect. Perhaps this is some sort of fuming effect and the iron is
deposited on the surface, rather than migrating to the surface.

Does anyone know what is really going on?