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.
Blue Ochre is a mineral colour of rare occurrence, found with iron pyrites in Cornwall, and also in North America, and is a sub-phospkate of iron. What Indian red is to the colour red, and Oxford ochre to yellow, this pigment is to the colour blue - they class in likeness of character: hence it is admirable rather for the modesty and solidity than for the brilliancy of its colour. It has the body of other ochres, more transparency, and is of considerable depth. It works well both in water and oil, dries readily, and does not suffer in tint with white lead, or change when exposed to the action of strong light, damp, or impure air: it is, therefore, as far as its powers extend, an eligible pigment, though it is not in general use, or easily procurable. It answers to the same acid tests as ultramarine, and is distinguishable from it by changing from a blue phosphate to an olive-brown ochrous oxide of iron when exposed to a red heat. It has been improperly called native Prussian blue.