Cobalt is the strongest colourant in common use and the reason blue and white pottery exists across so many traditions. A fraction of a percent gives a definite blue and two percent gives something close to black-blue, so it is weighed carefully rather than scooped.
Oxide and carbonate, and why the choice matters here
Cobalt oxide is much stronger by weight than cobalt carbonate and considerably coarser. Because so little is used, a coarse particle that does not disperse shows as a dark speck in the fired glaze. Carbonate is finer and gives smoother, more even blue, which is why most recipes aiming at a clean colour name it. Roughly two and a half parts carbonate replace one part oxide.
How little is enough
A quarter to half a percent gives a soft blue; one percent a strong blue; two percent and above a very dark, almost black blue that also starts to affect the melt. Because the quantities are so small, accurate scales matter more with cobalt than with anything else, and a badly mixed batch shows as blotches rather than as a slightly wrong shade.
Blending it away from primary blue
Cobalt on its own reads as a hard, bright blue that many potters find difficult to use. Combining it with iron, manganese or rutile softens it towards grey-blue, slate and the muted blues of traditional ware. Those blends are where most of the pleasing blues in ceramics come from, rather than from cobalt alone.
Cost and handling
Cobalt is one of the more expensive materials on a supplier's list per kilogram and one of the cheapest in use, because a small jar colours a great deal of glaze. It is a fine powder with the usual weighing precautions, and it stains skin and surfaces strongly, so gloves and wet cleaning save trouble.
Questions people ask about cobalt oxide
How much cobalt gives a blue glaze?
A quarter to half a percent for a soft blue, around one percent for a strong one. Two percent and above reads almost black.
Why does my cobalt glaze have dark specks?
Usually cobalt oxide rather than carbonate, with coarse particles that did not disperse. Carbonate and thorough sieving give an even colour.
Is cobalt expensive?
Per kilogram, yes, and in use it is cheap because so little is needed. A small jar colours a large quantity of glaze.