JJHerb@aol.com on fri 18 oct 96
For LINDA BLOSSOM and her wondering about conversions: There are several
hydrated copper carbonate minerals/chemical compounds. Some of the mineral
varieties are intimately intermixed and probably the proportions change as
mining progresses. If the people you talked to at the supplier could give
you a chemical formula or a mineral name, an attempt at conversion is
possible. Copper oxide (all that really stays in the glaze) weighs 79.5
units per molecule. Add a Carbon dioxide molecule, weighing 44 units, we get
a copper carbonate molecule weighing 123.5 units. Each water added to the
basic copper carbonate molecule weighs 18 units. Notice that if you have two
waters and one carbon dioxide along with the copper oxide (159 units),
almost exactly half (80 units) leaves the kiln as gas. Knowing the full
chemical composition of your material allows substitution of one material for
another and the calculation (shudder) of cost per copper oxide molecule of
the various materials available to you. One famous copper mineral that is
also a carbonate is malachite - Cu2CO3(OH)2 - now there s a glaze component!
>> For LizzardOL and the lead/colorant question: A major difference between
colorant additions to glaze and the cadmium/lead that we all talk about is
the amount in the glaze. Some glazes are half lead - the metal - by weight.
Some cadmium glaze recipies call for 30 to 50 parts cadmium carbonate per
100. There is a LOT of these metals in some glaze recipies. A good blue
from cobalt might take 1 part per 100. A major difference. I agree that
eating metal oxides (other than iron perhaps) is generally bad. I might
suggest that any pottery that made you feel bad about touching the surface
shouldn t be eaten from. A further point about the content of glazes, as
many here have noted, a well composed glaze with a balance of glass formers
and fluxes makes a glass that is insoluble in normal use. Insoluble means
that none of the parts come out in to water, food, or whatever. Just
brushing an oxide on a clay surface and hoping for the best is a treatment
for a decorative object or wall hanging, not a vessel of use. I lied about
the solubility product constant.
Joseph Herbert
JJHerb@aol.com
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