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copper leaching from glazes--some success to report

updated wed 23 dec 98


John Hesselberth on tue 22 dec 98

Those who have been following my posts on this general subject know that
I consider copper to be one of the most difficult metals to keep in
solution in a glaze. Since I have a mat or semimat green copper glaze
that I really like, I have been examining it, with the help of Roland
Hale and the Alfred Analytical Laboratory, to see what could be done to
improve it. I'm going to lead you through a long, but at the end very
successful series of tests. If you get bored, well, you know where the
delete key is:

First, let me describe the Green Mat glaze I have been working with. It
is for Cone 6 oxidation and it is fairly sensitive, particularly to
cooling rate. I get best results when I fire it to Cone 6 at 2-3 o'clock
and then cool it slowly (2-300 degrees F/hour) to 1800. Then I turn the
kiln off. If I cool it faster than that I get this really ugly glossy
green. If I do it right I get areas that have recrystallized to give a
mat surface interspersed with small glossy areas. I get a lot of
compliments on this glaze, but it does take careful control of the kiln.
The recipe:

Green Mat (C6 oxidation)

Custer Feldspar 40
Gerstley Borate 12.3
EPK 12.2
Whiting 14.6
Talc 19.8
Bentonite 1.2


Zircopax 7.9
Copper Carbonate 4.75

The above recipe gives a unity calculation (boron NOT in unity) of:

K2O 0.103
Na2O 0.064
CaO 0.439
MgO 0.393

Al2O3 0.267
B2O3 0.115

SiO2 1.837

As you can see this glaze is low in silica and you might, therefore,
expect it to have low acid resistance. It passes the 24 hour vinegar
soak with no problems and also shows no problem in more than 2 months in
my dishwasher. However when I first submitted this glaze to Alfred
Analytical Lab 15-27 mg/l (or ppm if you prefer) of copper leached from
the surface in two separate tests. In an additional test with another
sample since then, it leached 25 ppm. I have stated in previous posts
that this is a higher level of copper leaching than I want in my glazes;
although there is no data to show this would be a hazard in normal use.
I have set a personal goal of well under 10 ppm for copper and Monona
Rossol would challenge us to come in at or below the water standard of
1.3 ppm. At any rate, I decided to see if I could lower the amount of
copper that leached from this glaze without significant deterioration in
its appearance.

In a test with a different green glaze I had tried adding zirconium
(Zircopax) which I had found in the literature would increase chemical
durability. Indeed I got a small improvement in the single test I have
tried. However the above glaze is already loaded up with zirconium so I
didn't think adding more would be of much help. Instead I decided to
overcoat this glaze with a thin coat of a transparent glaze which was
well within traditional limit formulas. My hope was that this thin coat
of (hopefully durable) transparent glaze would form a durable seal on top
of my not-so-durable Green Mat glaze. Of course it was possible it would
just mingle with the Green Mat glaze during firing and I wouldn't gain
much. However glazes, even when molton at peak temperatures are still
pretty thick and viscous so I was hoping that there wouldn't be much
comingling of the two glazes.

To select some transparent glazes to try, I just went to published
sources looking for glazes I knew people were using that, by limit
formula calculation, were within traditional limits.
I selected two that I found in Kathy Triplett's book Handbuilt Ceramics.
They are:

Kathy's Satin Clear

Custer Feldspar 26.5
Flint (325m) 23.5
Whiting 15.3
EPK 10.2
Frit 3134 22.5
Bentonite 2.0


Tin Oxide 2.0

Unity calculations are:

K2O 0.095
Na2O 0.161
CaO 0.741
MgO 0.003

Al2O3 0.275
B2O3 0.237

SiO2 3.07

This looks pretty good. A little high in calcium, but well within limits
on alumina and silica. But I chose a second one just to give the
experiment 2 different chances.

Don Davis's Clear (modified to replace F-4 with Custer)

EPK 8.0
Custer Feldspar 25.0
Flint 25.0
Dolomite 6.0
Whiting 6.0
Gerstley Borate 10.0
Frit 3124 20.0

Unity calculations are:

K2O 0.097
Na2O 0.135
CaO 0.616
MgO 0.151

Al2O3 0.322
B2O3 0.273
SiO2 3.356

Even Ron Roy would probably agree that this should be a stable glaze!

I dipped my test cups in the Green Mat glaze for my normal 5 seconds and
let them get dry to the touch. I mixed my two clear test glazes to my
normal glaze consistency and then added 50% more water. I normally mix
about 1000g dry glaze with 1 liter of water. So in this case I used 1.5
liters of water. They were fairly thin mixes. Then I took the
green-glazed test cups and poured one of the test glazes in each cup and
dumped it as quickly as I could. I wanted to get a thin, but uniform,
coat of the transparent glazes on top of my Green Mat.

Firing was to Cone 6 with the slow cooling rate described above. There
was, in my opinion, a small deterioration, in glaze appearance. The
glaze on the inside of the cups was a little smoother (glossier) and more
homogeneous looking than on the outside where I did not use the
transparent over glazes. However, for me, they were both very
satisfactory for the inside of a vessel where it is hard to see the glaze
well anyway. Off to Alfred Analytical went the cups.

Today, only 10 days later, (Alfred's service is great!) I got the results.

Green Mat over coated with Kathy's Satin Clear: 2.3 ppm of copper
Green Mat over coated with Don Davis's Clear: 1.3 ppm of copper


This is an astonishing improvement vs. the 15-27 ppm of copper I had seen
previously. Now the disclaimers and the challenge to you (you knew that
was coming didn't you?).

Will this technique of overcoating with a durable transparent glaze work
for you with your glaze? I don't have a clue.

Will this work for you with this Green Mat glaze? Maybe, but if you do
it without testing it yourself you are making a big mistake. These are
two data points. It is an interesting, exciting lead. It is not a
proven way to make a non-durable glaze durable except on the two cups I
tested and maybe, more generally, with my glaze ingredients, my way of
mixing glazes, on my clay, in my kiln, fired the way I fire it. But if
you have a glaze that leaches more metal than you are comfortable with,
you might want to try this as a technique. But you must test, test, test

And of course, please let me and Clayart know the results as I have. I
now know of a couple Clayarters who have pledged to test their glazes and
let me/us know the results when they are available. I'd be a lot happier
if I knew of 20 or 30 people working to that end.

Now back to my studio to mix some more glazes.... Happy holidays and
happy glaze testing to everyone in this great group of people.

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

"The whole aim of practical politics is to keep the populace alarmed, and
hence clamorous to be led to safety, by menacing it with an endless
series of hobgoblins, all of them imaginary." H.L. Mencken, 1925