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copper leaching from glazes (not boring to me)

updated wed 22 sep 99


Michael Banks on tue 21 sep 99

I wish to add my comments to Gavin's and applaud your efforts John. But
like him, I would like to know what glaze base you were using so that we
may compare it to other compositions.

It is clear from the geological literature that copper is more soluble in
silicate melts (and glasses) which have a high proportion of octahedral
sites compared to tetrahedral sites. Octahedral sites are spaces in the melt
structure which allow atoms to be in 6 fold co-ordination (unlike other
liquids, e.g. water which are essentially structureless, silicate melts are
full of chains and networks of silica tetrahedra and alumina octahedra).
Melts with a high alumina/alkali,alkali earth ratios have been found to have
the highest proportions of octahedral sites and natural copper contents
(ref: Feiss P.G. 1978, Economic Geology. V23, pp397-404, "Magmatic Sources
of copper in Porphyry Copper Deposits".

I believe these data are directly applicable to stoneware glazes (excluding
boron and phosphorous glazes). The problem for potters is that copper in
high alumina glazes is rendered a dull green colour, versus the bright
blues, turquoise and reds possible in high alkali, low alumina glazes. But
unfortunately alkaline glazes are probably even more prone to leaching
(apart from limited copper solubility) because they are inherently less
durable (softer) and craze prone (high surface area available for leaching).

There is little geological information regarding copper stability in melts
and glasses high in lithium, boron, phosphorous, barium or fluorine. Until
potters get stuck-in and report the results of their investigations.

Michael Banks,
New Zealand

----------------------------Original message----------------------------
>As many of you know I have been puzzled about why copper seems to be so
>difficult to keep in a glaze, at least at the levels for 4-5% where I
>have tested it. I have formed a working hypothesis about this and have
>set about testing it.

>Hypothesis: Colorant oxides are stable within a "good glass" base glaze
>up to a certain level depending on the exact structure of the glaze
>matrix and the characteristics of each particular oxide (i.e. if there
>are "holes" in the glaze matrix into which the colorant oxide will fit,
>t will be stable in the glaze until all the holes are filled--then, at
>higher levels, it will leach quite easily). Each colorant oxide will
>behave differently in a given base glaze depending on things like it
>molecular or ionic size, its electrical characteristics, and probably

>For an initial test of this hypothesis I took one of my standard base
>glazes and made sample cups with 5 different levels of copper: 0.5, 1.0,
>2.0, 3.5 and 5.0% copper carbonate. I sent the samples to Alfred
>Analytical Lab and just got the results back.

>Copper Level, % Copper Leached, mg/L

0.5 0.07
1.0 0.17
2.0 0.14
3.5 4.73
5.0 14.37

At 2 % or less copper, almost none leaches; these numbers are barely
above the detectable limit of 0.05% and may not even be significantly
different from each other. This particular glaze is very, very stable to
copper leaching at low levels of copper in the base glaze. Somewhere
above 2 % (straight line extrapolation would predict it is about 2.7%)
significant amounts of copper begin to come out of this glaze. By the
way, at 2% it gives a very attractive light-medium green color at cone 6

So, at least in a first test, I have added credibility to the hypothesis
above. I won't be so bold as to claim the hypothesis is proven on the
basis of a single test like this; however this does lead me to want to
test more and gives me a hint that we may eventually be able to predict
leaching propensity. Some of the questions that come to my mind are:

1. Will I get similar results for other colorant oxides? Do they all
have a "limit of solubility" in a given base glaze? Are those limits
dramatically different for the various colorant oxides?

2. We know that different base glazes behave substantially differently in
their ability to hold colorant oxides. What are the important factors?
Is being within traditional limits and making "good glass" one of them?
Is there a simple way to predict which are stable?

3. Is there a ranking of the "limits of solubility" of colorant oxides
in glazes, e.g. is iron always more "soluble" than cobalt which in turn
is always more "soluble" than copper.

4. Would adding a second or third colorant oxide affect the above results
on copper or do the oxides behave essentially independently of each
other, each fitting into their own "holes" in the glaze matrix?

For those who seem to want to read more into my leaching results than
should be read, I must add the following caution: These results do not
translate to your base glaze! Maybe someday a few years from now we will
know how to translate them, but not yet. If I hear anyone suggest that
it has now been proven that using copper at 2% or less gives a stable
result, you will hear my scream no matter what part of the world you are

What I am interested in from Clayart members are your thoughts, comments
and suggestions on the above. I will continue to follow up on this work
as I can afford it and have the time; however I would like my experiments
to be as efficient as possible. I know there are some brilliant minds
out there and I am wide open as to where to go next. Please let me hear
from you if you have an interest in helping me figure out how to design
the next experiment(s) or if you think you have a good mechanistic
explanation of the above results or whatever.

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

"It is time for potters to claim their proper field. Pottery in its pure
form relies neither on sculptural additions nor on pictorial decorations.
but on the counterpoint of form, design, colour, texture and the quality
of the material, all directed to a function." Michael Cardew in "Pioneer