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Ferrocyanide Of Iron &c Was Accidentally Discovered In 1710 By

Diesbach, a colour-maker at Berlin. It is a compound of iron and
cyanogen, of varying composition, formed by adding yellow prussiate of
potash to a persalt of iron, or by oxidizing the precipitate obtained
from the prussiate and a protosalt. The finest blue is furnished by
sesquinitrate of iron, but the salt almost exclusively employed is the
protosulphate, the freedom of which from copper is essential to the
colour of the blue. As is the case with other pigments, Prussian blue
differs considerably in colour, in depth, and in permanence, according
to the purity of the materials, the mode of manufacture, and the absence
of adulterants. Like smalt, it is known in the washtub as well as in the
studio; and in the cheaper varieties, alumina, starch, chalk, oxide of
iron, &c., are often largely present. A good unsophisticated sample in
the dry state is intense blue, almost black, hard and brittle, much
resembling in appearance the best indigo, and having a similar
copper-red fracture. It does not effervesce with acids, as when
adulterated with chalk; nor become pasty with boiling water, as when
sophisticated with starch. Further, it feels light in the hand, adheres
to the tongue, is inodorous, tasteless, not poisonous, and is insoluble
in water. Forming a bulky mass while moist, Prussian blue shrinks to a
comparatively small compass when well washed and dried by gentle heat;
and, when once dried, being difficult to reduce again to the state of
extreme division which it possessed while wet, it is frequently sold and
used in paste for common purposes. We have said that a good sample of
Prussian blue is insoluble in water, and for artistic use it should
certainly be so, as otherwise it has a tendency to stain the fabric on
which it is employed, a defect formerly very prevalent. All Prussian
blues, however, are not insoluble, and these are not only liable to the
drawback named, but are less to be depended on for permanence. Improper
proportions, for instance, of sesquichloride of iron and
potash-ferrocyanide will yield a blue which, when washed even with cold
water, continually imparts to it a yellow or green colour, through the
partial solution of the prussiate. All commercial Prussian blue, and
indeed that which is prepared by careful chemical processes, give up the
ferrocyanide to boiling water, thereby colouring it greenish yellow; but

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