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the color of cobalt

updated fri 31 may 96


Cathy Nelson Hartman on wed 22 may 96

I would like to develope a blue overglaze for cone 10. My base glaze is:
colemanite 1200
dolomite 2000
soda spar 4000
kaolin 150
bentonite 150
silica 4300

Satin crystalline matt. Goes on thick. Most oxides or overglazes bleed
destroying any line quality. This is due to the sheeting action of the
magnesia glaze. However when I use the same base for the overglaze ,
everything moves together and I end up with the design I started
with-just down a little from where I put it on. Comprende? So what is the
question? Does anyone have a suggestion for an addition to the glaze to
produce a dark blue on the green side? Cobalt and what? Straight cobalt
oxide wash interacts with the silica in the base to produce the purple
cobalt silicate color. According to Hamer's book I need to form cobalt
aluminate to stay in the green area of blue.
Yeah I know I'm splitting hairs but I 'll sure appreciate a good one.

walter hartman

Brian P. Jones x2712 on thu 23 may 96

What about just adding Copper carb? Adding a small
percentage should give your bule a green tint.


June Perry on thu 23 may 96

You can try either copper or chrome to give you more of a blue green cast. I
would try about two or three per cent copper or 1 per cent chrome as a
starting point.

June Perry