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copper leaching from glazes

updated fri 24 sep 99


Tom Buck on wed 22 sep 99

June (others):
In effect, if you lay down a coat of a fluid glaze at C-6 or C-10
that contains enough copper compound to produce a red colour in reducing
conditions, and then you lay down a second glaze coat, this time a less
fluid clear, you will generate/create a new copper red glaze as the pot
reaches maturity since the high energy input will make molecules mobile.
This molecular activity will likely cause the two coats to inter-diffuse
(merge) and become one. Now the question becomes: is the combination
liquidus sufficiently fluid (ie, low enough viscosity) to allow CuO
(black) molecules to come into contact with the reducing gas molecules and
be changed to Cu2O (red) and perhaps some Cu (metal)? If so, the cooled
pot (after an oxidative polish) will show as red; if the molten glaze is
too viscous, CuO reduction will be inhibited and most of the pot may show
It seems that a more practical approach would be to analyze the
two glaze mix, and adjust its viscosity, and lay down a thicker coat of
this than one usually coats a biscuit.
But why would you want to change a successful copper red glaze? At
most it contains slightly over 1% copper carbonate basic, and as has been
stated by John H., a low level of copper in a glaze is held firmly in the
glass matrix and very little released in a leach test?
Regards. TTYL. Tom.

Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada

On Tue, 21 Sep 1999, June Perry wrote:

> ----------------------------Original message----------------------------
> If a thin coating of clear glaze can keep copper reds from being fugitive it
> might work to keep the higher copper content glazes from leaching. Have you
> tried this?
> Warm regards,
> June

June Perry on thu 23 sep 99

Hi Tom:

Someone mentioned a while back that someone does this (puts a clear, thin
coat over the copper red glaze) to keep the copper from becoming fugitive.
That's why I asked the poster if he had tried it.

Warm regards,

John Hesselberth on thu 23 sep 99

Tom Buck wrote:

> as has been
>stated by John H., a low level of copper in a glaze is held firmly in the
>glass matrix and very little released in a leach test?

Hi Tom,

I did not say what you credit me with above. I said IN MY BASE GLAZE
this is what happens. I have no idea what happens in a copper red glaze
with 1 % copper. Please don't extrapolate my data to other glazes.

John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: web site:

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