This section is from the book "Chromatography; Or, A Treatise On Colours And Pigments, And Of Their Powers In Painting", by George Field. Also available from Amazon: Chromatography, or A Treatise on Colours and Pigments, and of Their Powers in Painting.
Burnt Verdigris is what its name expresses, and is an olive-coloured oxide of copper deprived of acid. It dries remarkably well in oil, and is more durable; and, in other respects, an improved and more eligible pigment than the original verdigris. Scheele's green affords by burning also a series of similar olive colours, which are as durable as their original pigment, and most of the copper greens may be subjected to the same process with the same results: indeed we have remarked in many instances that the action of fire anticipates the effects of long-continued time, and that many of the primary and secondary colours may, by different degrees of burning, be converted into their analogous secondary and tertiary, or semi-neutral colours.