Blue Ochre
:
ON THE PRIMARY, BLUE.
which has been improperly called Native Prussian Blue, is a native
hydrated phosphate of iron of rare occurrence, found with iron pyrites
in Cornwall, and also in North America. What Indian red is to the colour
red, and Oxford ochre to yellow, this pigment is to the colour blue,
being sober and subdued rather than brilliant. It has the body of other
ochres, more transparency, and is of considerable depth. Both in water
and oil it works well, dries readily, and does not suffer in tint with
white lead, nor change when exposed to the action of strong light, damp,
or impure air. As far as its powers extend, therefore, it is an eligible
pigment, though not generally employed nor easily procured; it may,
however, be artificially prepared. Answering to similar acid tests as
ultramarine, it is distinguished therefrom by assuming an olive-brown
hue on exposure to a red heat.