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cobalt flashing??

updated sat 18 dec 99


JS Firestone on fri 17 dec 99

Hi all,
My teacher built a new downdraft kiln at school, has only fired it 4 times,
still getting accustomed to it. Also testing some basic glazes. The problem
I've noticed is 3 different white matt glazes come out with lavender
blotches. This has happened each firing. Teacher said he thought it's where
glaze is thick. (?) He uses a lot of cobalt oxide, design brushed on
immediately after throwing. I read somewhere that cobalt in a glaze with
dolomite will produce lavenders/purples instead of blue. All of the 3 matt
whites we've used have dolomite. I did also test a shino that came out white
gloss, no lavender, it has no dolomite. Could it be that the cobalt is
flashing or transmigrating to the white glazed pots. The majority of the
work in any kiln load has been his so far. Thanks for any insights!