I learned this technique from Eileen Powell. ( eileenpowell.com )
Copper oxide wash on raw clay or bisqued clay, fired to cone 06, will give a brownish look. If you put clear ^06 or white ^06 glaze over the wash, it will turn turquoise. The upper piece has several coats of oxide wash. The lower piece had oxide wash brushed on , then sponged off.
*** always wear rubber gloves as copper can leach through your skin !!!!!!
Recipe: Copper Wash
1 Tablespoon Frit 3124
2 Tablespoons Copper Carbonate
2 cups of water ( can dilute more)
keep mixing....it settles