Ferrocyanide Of Iron &c Was Accidentally Discovered In 1710 By

: ON THE PRIMARY, BLUE.

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