Cobalt oxide is a very powerful colorant to get blues. Using less than 1% of the material will yield a very strong blue color in glazes. However, in most firings, there is not enough time for the material to properly disperse itself in the glaze, making it splotchy and speckled. To fix this issue, cobalt carbonate can be used instead.
Information gathered from The Potter's Dictionary of Materials and Techniques by Frank Hamer.
