cobalt oxide (coo or co3o4) is a consistent, reliable oxide and a common colorant in pottery. it is available in powdered form and is blackish in color. it is a very strong oxide, and using very small quantities yields bright, intense blues. in ceramics, cobalt oxide is used mainly in slips, washes and glazes and can be applied through,copper oxide red the ceramic shop,red copper oxide (cu 2 o) is a reddish powder that is a reduced form of black copper oxide that produced by heat-treating the black form. it is used as a colorant in many ceramics applications, including the preparation of slips and glazes. red copper oxide will maintain its structure and produce reds in reduction firings.