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copper glaze safety question

updated wed 31 jul 96


Robert Wilt on sun 28 jul 96

I'd like some information and/or opinions about the potential
hazards of using glazes with high amounts of copper for food
surfaces. By "high amounts" I mean enough to cause areas
of gray or black crystallization. For example, I sometimes
see this occur when we're scraping the bottom of a bucket of
Oribe; it also happens with some glazes if you apply them too
thickly, or at the inside bottom of the pot where the glaze

On a related note - this may or may not be the same phenomenon:
Here's an analysis of a glaze we are contemplating using in
an educational setting. It's a semi-matt cone 6 glaze. The
analysis includes the additions 5% rutile and 1% black copper

Na2O 0.07 Al2O3 0.38 SiO2 2.06
K2O 0.15 Fe2O3 0.01 TiO2 0.14
CaO 0.48
CuO 0.03
ZnO 0.27
Alumina:Silica ratio is 1.00 : 5.44

On most areas of the tested pots this glaze is various shades of
light green, with interesting little crystals from the rutile,
but in areas where this glaze is thick the crystals are sparkly
gray and matt. So, not knowing much about this, we are somewhat
concerned about copper leaching from the glaze. Is this a legitimate
concern, or is copper leaching not a problem?

Thanks in advance for any advice.

bob wilt
in boston
where the cicadas are joyously singing
and the birds are joyously eating them
and the cats are swishing their tails

Ron Roy on tue 30 jul 96

For Bob in Boston,

Re the oribe glaze with the high copper. From what I read there may be some
problems with copper for some people but the danger seems slight - however
any addition of copper to a glass will make it less durable - read increase
in solublity. If there are other materials in the glaze which are
problematic these will leach out at a faster rate. It would be helpful to
know the recipe or the molecular formula in order to make a better
prediction in this case.

On the semi mat cone 6 glaze the SiO2 is way below 2.50 which, according to
the limit formulas I use, is minimum for a durable glass at this
temperature. I would expect this glaze to change during use - a tile left
in the dishwasher and occasionally compared to one left out might be an
appropriate test.

It would surprise me if small amounts of copper would be a significant
hazard - after all most of our water comes through copper - but then again
I don't have any current information.

Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849

Dave Spangenberg on wed 31 jul 96

Adding to the comments by Bob in Boston and Ron Roy in Toronto.

F & J Hamer, "The Potters Dictionary of Materials and Techniques" under the
heading "copper oxides" says in part:
"..... Five percent gives a black which so fills the glaze that the surface is
metallic. The black copper oxide will often come off the surface of the
finished glaze onto the fingers when a black copper glaze is touched. Since
this oxide is soluble in fruit juices, glazes strongly stained by copper oxide
should not be used for food because the copper is poisonous. ......."

I have used some copper green glazes with copper carbonate or oxide additions in
the order of 5% where black spotting begins to occur producing a nice mottled
appearance. An overnight soaking in either vinegar or hydrochloric acid will
modify the green color and reduce the black areas. I suspect that the copper is
also weakening the bonding of these cone 6 glazes to produce a more soluble
glaze. I do not see black finger tips after rubbing the glazes. I have not
considered these glazes food safe based on Hamer's statement. I have made no
attempt to have the solubility measured.

I also have cooked in copper pans that corrode to what I think is black copper
oxide. Is the apple sauce tainted? Is Hamer correct, and to what degree?