Nickel Oxide is a powerful and refractory colorant that produces greys, blues, and yellows. It is more concentrated than nickel carbonate and as little as .02% gives a noticeable tint. Normal additions are 2-5%.
It can also soften and mute more intense colorants such as copper and cobalt.
It is refractory and can raise the melting temperature of glazes if used in large amounts.