Nickel is the least predictable of the common colourants. Depending on the glaze base it can give grey, brown, green, or a distinctive muted blue-grey, and it is used at very low percentages. It is a material for potters who like testing rather than for anyone wanting a colour on a first attempt.
Why the colour is base dependent
Nickel responds strongly to what else is in the glaze, particularly to the balance of fluxes. A high-magnesium base can push it towards a soft blue-grey, a high-zinc base towards yellow-brown, and an ordinary base towards a muted grey-green. That variability is why published recipes containing nickel usually specify the base very precisely.
How much to use
A fraction of a percent up to about one percent. Above that nickel tends to matt and deaden a glaze rather than colour it further, and it can push a stable glaze towards dryness. Because the quantities are so small and the results so base sensitive, this is a material to test in a line blend rather than to add by eye.
With other colourants
Nickel is most often used alongside cobalt or iron to knock the brightness off them, which is where its muting quality becomes an advantage rather than an inconvenience. A small nickel addition to a hard cobalt blue moves it towards the softer slate blues that are difficult to achieve any other way.
Handling
Nickel compounds are among the materials that warrant more than routine care: a fitted respirator when weighing, good ventilation, and wet cleaning of the bench afterwards. Buy the small quantity you will actually use, since the percentages involved mean a modest jar lasts for years.
Questions people ask about nickel oxide
What colour does nickel oxide give?
It depends heavily on the glaze base: grey, brown, green or a muted blue-grey. It is the most base-sensitive of the common colourants.
How much nickel should I use?
A fraction of a percent up to about one percent. More tends to matt and deaden the glaze rather than deepen the colour.
Why would I use nickel at all?
Mostly to mute other colourants. A small addition softens a hard cobalt blue towards slate in a way little else achieves.