All these bowls are examples of raku fired copper bowls.
Raku is a Japanese technique for ceramics. The glazed object is burnt in an open fire (instead of heated in the kiln) in a closed container or in a hole in the ground, and left there to cool. Because at some point there is no more oxygen in the closed space, it is extracted from the enamelled surface. With ceramics he result is a craquelé effect and very beautiful special glaze. With vitreous enamel the raku processresults in a spectacular glaze and copper-related colours like green or copper-red. If silver nitrate is painted on the bowl before the raku process, the silver nitrate lines turn into actual silver.