This produces an amazing metallic green and oil-slick-like luster.

Raku Copper Luster Glaze

Gerstley borate or Colemanite – 80
Cornish stone – 20
Copper carbonate – 2
(or Frit 510 at 105% instead of the Gerstly borate)

The vessel was fired to around 950 deg C, whipped out the kiln and thrown into a bin of sawdust. An awesome and somewhat scary experience. I’ve found this glaze fades over time somewhat. I last fired with this glaze in the late 80’s.