Cobalt oxide (CoO).  A powerful colourant and flux. Tends to speck glazes and not to disperse as well as the Cobalt carbonate.  Insoluble in water. 
Melts at low‐fire temperatures.  Gives strong blue colors, transparent if dilute. Ultramarine with alkaline fluxes. Purple with Manganese.  Green with Titanium. Screen glazes containing Cobalt well to avoid spotting. Concentrated use of Cobalt spits in firing, leaving blue halos on kiln shelves and adjacent wares.