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Obviously cobalt, but adding tin perhaps?

No, no tin just cobalt oxide with water and dish soap experiment over a mix of coyote cone 6 glaze under the oxide and finished with a low fire clear third fire. 

 

I did splash some additional cobalt and iron oxide on the lower third but you can't really see that in the picture.

I have a crystalline recipe that uses cobalt and tin; plus a dash of rare earth oxide. The end result is similar to yours. 

Nerd, have you posted pics of that crystalline glaze?