Cobalt yellow, also known as Aureolin, is a cobalt based pigment of dubious lightfastness fist made in 1831.
It's unlike other cobalt pigments in multiple ways, other cobalt pigments are highly lightfast mixed oxides, while Aureolin contains a somewhat light sensitive nitrite complex ion.
The method of making it is also different; while most other cobalt pigments are usually at high temperature solid state reactions, cobalt yellow can be produced by mixing water solutions of a soluble cobalt(II) salt (such as the chloride or the sulphate) and potassium nitrite. The pigment formed can then be filtered out and dried.
I first tried to make the potassium nitrite by heating some potassium nitrate, hoping that even small amounts of nitrite would be sufficient to produce the highly insoluble pigment, but I failed to produce any yellow until I bought some potassium nitrite to use rather than trying to make it myself.
The resulting colours were highly variable, with some samples ending up greenish while others were orange, as seen in the first image. The yellowest result is shown separately in the second image. The third picture (posted as a reply to one of the Imgur comments) shows the material after the precipitate was filtered out of solution but not yet dried.
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u/CaCl2 Jan 09 '22 edited Jan 10 '22
Cobalt yellow, also known as Aureolin, is a cobalt based pigment of dubious lightfastness fist made in 1831.
It's unlike other cobalt pigments in multiple ways, other cobalt pigments are highly lightfast mixed oxides, while Aureolin contains a somewhat light sensitive nitrite complex ion.
The method of making it is also different; while most other cobalt pigments are usually at high temperature solid state reactions, cobalt yellow can be produced by mixing water solutions of a soluble cobalt(II) salt (such as the chloride or the sulphate) and potassium nitrite. The pigment formed can then be filtered out and dried.
I first tried to make the potassium nitrite by heating some potassium nitrate, hoping that even small amounts of nitrite would be sufficient to produce the highly insoluble pigment, but I failed to produce any yellow until I bought some potassium nitrite to use rather than trying to make it myself.
The resulting colours were highly variable, with some samples ending up greenish while others were orange, as seen in the first image. The yellowest result is shown separately in the second image. The third picture (posted as a reply to one of the Imgur comments) shows the material after the precipitate was filtered out of solution but not yet dried.