These pots are being fired with the Raku method, which involves them being exposed to flames before being covered. Doing so exhausts the oxygen from the air and results in metallic colours in different hues appearing in the glaze. It's an unpredictable process, so the potter has very little control of the end result.
I should warn you that I have quite a lot of images like this and they are all different, so they could be become a feature of this season's competitions if I start getting desparate for something to put in.
