CuSO4·5H2O. (blue vitriol, bluestone) The sulphate is superior to the oxide or the carbonate for producing copper fuming effects in saggar firings, but it has limited utility otherwise due to its solubility in water and to its higher handling toxicity and firing toxicity (sulphur dioxide fumes).
For more information on copper sulphate, see the Digitalfire.com Reference Library of ceramic materials.