Co3O4. Cobalt Oxide is a metallic oxide that produces blue in glazes at all temperatures. Black Cobalt Oxide is the principle source of CoO used in glazes, glass, and enamels. Cobalt is the most powerful ceramic colourant and it is stable in most systems. It is also useful as a body and slip stain. Cobalt will often produce glaze specking if it is not thoroughly sieved or ball milled to distribute the particles. Cobalt carbonate tends to disperse better in glazes to give even blue coloration because it is not as powerful and produces some glaze blistering problems. A high cobalt stain is also an alternative.
Cobalt - Violet, Lilac - In magnesia glazes a colour range is from violet to lilac is possible.
Cobalt – Blue - The shade of blue can, however, be affected in many ways by the presence of different oxides. Cobalt is powerful and often less than 1% will give strong colour. If the colour needs to be toned down, additions of iron, titanium, rutile and nickel may work. Brighter blue in alkaline glazes.