Copper Oxide Black –  COPPER OXIDE up to 5% will give an apple green colour under neutral or oxidising conditions. Yellow-Greens in POTASH glaze, Blue-Greens in BARIUM glaze. If used in alkaline glazes it will give a turquoise blue colour. Fired under reduction it creates a copper red or sang de-boeuf. The carbonate is weaker but gives less risk of speckle. NOTE – Be careful if adding to low solubility glazes, it causes the solubility of the LEAD to be greatly increased. Strong Flux.