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A Sample Which Parts With Its Prussiate To Cold Water Is Quite








unfitted for the palette, for which the most perfect specimen is none
too stable.

In spite of the learned researches of Professor Williamson, whose name
is as closely connected with the pigment as are the names of Schunck and
De La Rue with madder and cochineal, Prussian blue is not yet entirely
understood. Complex and uncertain in composition, uncertain too in its
habitudes, our best course perhaps will be not to attempt a complete
survey, but to state briefly those facts which bear on the artist's
craft.

Prussian blue is a colour of vast body and wonderful transparency, with
a soft velvety richness, and of such intense depth as to appear black in
its deepest washes. Notwithstanding it lasts a long time under
favourable circumstances, its tints fade by the action of strong light;
becoming white, according to Chevreul, in the direct rays of the sun,
but regaining its blue colour in the dark; hence that subdued light
which is favourable to all colours is particularly so to this blue. Its
colour has the singular property of fluctuating, or of coming and going,
under certain conditions; and which it owes to the action and reaction
by which it acquires or relinquishes oxygen alternately. It also becomes
greenish sometimes by a development of the oxide of iron; and is
purpled, darkened, or otherwise discoloured by damp or impure air. Time
has a neutralizing tendency upon its colour, which forms tints of much
beauty with white lead, though they are not equal either in purity,
brilliancy, or permanence to those of cobalt and ultramarine. When
carefully heated, Prussian blue gives off water and assumes a pale green
hue; its colour, therefore, depending on the presence of water, must not
be exposed to a high temperature. And as it is likewise injured or
destroyed by alkalis, which decompose it into oxide of iron and a
soluble prussiate, the blue should be avoided in fresco, on account of
the lime; neither should it be employed with pigments of an alkaline
nature, nor with hard water containing bicarbonate of lime in solution,
but with clean rain or distilled water, either of which is preferable
for colours generally.

Prussian blue dries and glazes well in oil, but its great and principal
use is in painting deep blues, in which its body helps to secure its
permanence, and its transparency gives force to its depth. It is also
valuable in compounding deep purples with lake, and is a powerful
neutralizer and component of black, to the intensity of which it adds
considerably. Prussian blue borders slightly on green, a quality which
militates against its use in skies and distances. In spite, however, of
its want of, or deficiency in, durability, the old water-colour painters
so employed it, neutralized by the addition of a little crimson lake. It
is serviceable in mixed tints of greens, affording with light red a
sea-green neutral. Dissolved in oxalic acid, the blue is available as an
ink, or for tinting maps.





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