Chun Blue is one of the interesting but sometimes elusive glaze effects. The blue isn’t from any pigments in the glaze; it’s an “optical” colour, like the colours in a rainbow. The glaze forms tiny bubbles in the firing process that are the right size to refract blue light, but for this to happen, the glaze chemistry, firing conditions and glaze thickness all have to be right. If any of these are lacking the glaze comes out in the olive green seen in this photo where the glaze coat is thinner.
My Opalescent glazes also rely on light refraction for their colour, but in their case the refraction comes from tiny crystals that form as the glaze is cooling. Either fast cooling or reducing the amount of titanium (which encourages crystal formation) will result in the glaze being amber instead of the vibrant purples and blues.
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