Tom Buck on thu 29 aug 96
TH: Sorry, but you made an assumption about Cobalt Oxide that is
unfortunately not so. You suggest that cobalt oxide is CoO or Co(II)O and
that it is the article of commerce; not so. What dealers sell to potters
as black cobalt oxide is a mix of Co(III)O aka Co2O3 and Co(II)O aka CoO.
My chem dictionary cites cobalto-cobaltic oxide as Co3O4 which can only be
Co2O3.CoO but the the amount of each oxide in the mix is somewhat variable
due to plant processes.
If the potter wants reliable cobalt oxide content, he would use
cobalt carbonate, more properly 2Co(II)CO3.3Co(II)(OH)2.H2O since this is
the compound of commerce sold as cobalt carbonate, the familiar violet
crystals. This material is of high purity with a steady analysis over
time. The so-called oxide would not be as uniform over time. So, the best
one can hope for as equivalent between the "carbonate" form and the
"oxide" form is 1 part oxide = 1.3 parts carbonate. Any idea of further
precision flies against commerical practice.
Cheers TomB Hamilton ON Canada URL http://digitalfire.com/magic/tombuck.htm