When I started wood firing some 30 years ago I had a problem with the kiln so I wrote a letter to ask advice from Steve Harrison in The Land of OZ. 6 months later I got an answer, tried that, wrote back, and all told it took about 2 years to get the answer. Then I joined Clayart Discussion group and answers came in the next morning. One of those people on the list was Craig Martell. Steve Harrison was too. They
Here is the brilliant answer of Craig Martell.
Craig Martell The
term for this lovely, pleasant phenomenon is: "bi-refringence. It's
the same deal as oil slicks and rainbow lustre on chrome tailpipes. The
light is reflecting from both the surface and the underside of the
glaze and the crossing wave lengths cancel out some of the others and
create this effect. I think one factor is to glaze thin along with
having the magic recipe. I got this info some years back from Michael
Banks a potter and mineralogist from New Zealand. Not a real solid
explanation from me but maybe you can do a search on the net and get
some real focused info. Beautiful cup dude! I hope you can make more.