"Cobalt green is made in various ways. Some manufacturers mix a pale shade of ultramarine blue which is known commercially as artificial ultramarine green with a mixture of oxide of zinc. Sometimes it is made by grinding oxide of zinc and zaffer which is a native oxide of cobalt. In any case the cobalt green which has been examined is apparently very permanent, but lacks very much in opacity, which, however, is no detriment, because the color is principally used as a glazing color. It appears to be a very expensive color when made from the salt of genuine cobalt, and as such is permanent under any and all conditions. It may also be a composition of genuine cobalt blue mixed with chromate of zinc or zinc yellow, in which case it would also be permanent, but if it is a mixture of ultramarine blue and chromate of zinc, it is not permanent, and has sometimes been known to decompose in the tube."