Tom Buck on sun 8 aug 99
Ray A:
You raise the issue of copper's solubility in two regimes,
standard municipal water supply; and orange juice, or vinegar, or other
acidic liquid food.
Copper metal and copper oxide, Cu(II)O, are essential inert in a
water environment. However, if the municipality's waterworks adds to the
water a bactericide, usually an excess of elemental chlorine (a gas at
standard temperature and pressure), then the water could become slightly
acidic. As a result, there will be some copper dissolved, particularly if
the chlorinated water sits stagnant in the household pipes for 10-12 hours
overnight. Some people say that there could then exist in the water up to
3 parts per million of copper ion, Cu++ (3 milligrams per litre (ie, a US
quart)). For most of us, this would not be harmful. But there are children
who have a dysfunction in their copper metabolism and they could be
"troubled" by this level of copper ion. So, in some cities, there is a
reccomendation that the pipes be flushed in the morning for at least a
minute or two, perhaps longer if pipe runs are extensive.
Now, onto the copper in glaze. If all potters used one glaze,
fired to one particular cone (heatwork), then this question could be
addressed in a straight-forward way. As all know, such is far from the
case. There are almost as many glaze recipes as there are potters.
Consequently, there is no definitive answer to the question: How much
copper is held in a fired glaze?
When the glaze powders turn into molten glass on the surface of
the claypot, there is formed a "random network polymer" or an
alumino-silicate structure of Al2O3 molecules combined with SiO2
molecules. In this network there "exists" a specific number of "holes"
ready to receive molecules of co-melters (possibly including any of the
following: CaO, K2O, Na2O, MgO, ZnO, B2O3). The colourant oxides, CuO, CoO
Fe2O3, Cr2O3, others, will compete for a place in these holes. And if it
is bound into the polymer, then this molecule becomes inert to chemical
attack (unless the whole polymer is attacked).
However, the chief flux oxides will find holes in preference to
the colourant oxides (a general statement not always true). So, then the
colourant oxides will either combine into tiny crystals which become
dispersed throughout the molten glass, or stay differentiated as molecules
mostly and "dissolve" in the melt, that is, find places to rest alongside
the network polymer strings, and while not being chemically bound the
colourant oxides are weakly held in place by other forces.
So how much CuO goes into holes, how much dissolves, how much
forms tiny crystals? In my readings, albeit limited, I have not seen any
specific data that would lead to an answer to these queries. Perhaps
Michael Cardew's comment on iron oxide solubility in stoneware glazes
gives a clue. He says (Page 140, Pioneer Pottery) that up to 5% of Fe2O3
can be held in solution by rapid cooling of the molten glaze, leading to a
bright clear black glaze. (This is the basis of the tenmoku glaze).
Since glaze coloured with Copper (II) Oxide seldom has more than
5% of the substance in the mix, it is likely that the copper will be held
within the glaze when its solidifies. This assumes the alumino-silcate
polymer is properly generated from the glaze powders. Which is what Limit
Formulas attempt to predict.
But your glaze may not meet the criteria of "balanced good glass"
and the only way you can be sure that your pot doesn't release excessive
amounts of copper to acidic liquid foods is to have it tested by Prof.
Hale as John Hesselberth urges all to do.
Tom Buck ) tel: 905-389-2339 (westend Lake Ontario,
province of Ontario, Canada). mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
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