Janet H Walker on tue 1 dec 98
...the blue bleeds a little into the glaze...
Maybe someone else has said this already (I'm behind on reading
ClayArt again). Hamer has an extensive discussion of cobalt in the
Potters Dictionary of Materials and Techniques. Apparently cobalt
ions migrate around a lot during firing. I don't recall whether
there was any discussion of how to prevent this, if there is a way.
Recently I had a piece with cobalt glaze under a clear glaze. The
blue pattern happened to have an incised line around it. The cobalt
was not able to swim up the hump and over the moat! So the only
place where the pattern blurred was at the very bottom where there
was no line in the surface.
For what it's worth -- try that along with all the glazes you've already
tried. The solution may turn out to be physical rather than chemical.
Cambridge MA USA