LizzardOL@aol.com on wed 16 oct 96
I'm not an expert in toxicology but I do work with hazardous materials quite
a bit at my "real" (non-pottery) job. It seems extremely unlikely that any
heavy metal oxide (or carbonate or other metallic salt) applied and fired as
you describe would be food safe because particles may be abraded from the
surface during use and swallowed. In a glaze with a crystalline structure
the concern is that toxic chemicals may leach out of the glaze into the food
during use (which might also happen with the procedure you describe). There
are tests you could run to better quantify the risk - I recall we have an
expert on this newsgroup who periodically surfaces with lots of useful
information; perhaps she will also respond.
You post makes me wonder about how extensive the testing has been of the
leachability of metals in "regular" glazes at various temperatures and
formulas. Everyone has been warned about barium and lead; why do we think
cobalt et al. is okay in glazes?