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Although the chemical constitution of this pigment can hardly be said to have been absolutely ascertained, yet it is generally believed that there are at least three different though closely allied chemical compounds included under the above names, not to mention those varieties of this pigment which contain added or extraneous substances, such as alumina, plaster-of- Paris, or zinc-white. The three typical and distinct compounds are:
I. Soluble Prussian Blue
This is made by pouring a solution of ferric chloride or ferric nitrate into an excess of potassium ferrocyanide solution (yellow prussiate of potash), or by pouring ferrous sulphate solution into excess of potassium ferricyanide solution. The blue precipitate formed is washed with distilled water until the wash-water begins to acquire a blue tint. The composition of the pigment thus prepared is, when dry, represented by the formula K2Fe2(CN)12Fe2. It contains potassium, and is, in reality, a double ferrocyanide - a 'potassio-ferric ferrocyanide.' It is less stable than either of the other kinds of Prussian blue, while its solubility in water causes it to stain the paper on which it is spread in water-colour painting. It should invariably be rejected by artists, although it must be owned that it works very smoothly both in water and in oil. It may always be distinguished from the superior kinds of Prussian blue by very simple tests. One of these consists in roasting a small portion of the dry powdered pigment in a porcelain basin or iron tray, allowing the brown residue to cool, and then throwing it into a little pure water.
Then place the mixture on a wetted filter contained in a funnel, and see whether the clear filtrate is alkaline by dipping a piece of yellow turmeric paper into it; if the yellow tint of this paper is reddened, then the Prussian blue belongs to this section. Another test is applied by simply washing some of the powdered blue with warm distilled water on a Swedish filter - the filtrate becomes blue.
II. Insoluble Prussian Blue
II. Insoluble Prussian Blue may be prepared by boiling No. I. (the soluble kind) with a solution of ferric chloride, by mixing solutions of ferrocyanic acid and ferric chloride, by pouring potassium ferrocyanide solution into an excess of a solution of ferric chloride, or of ferric nitrate, and heating the mixture for some time, or by precipitating a watery solution of Blue No. I. with an excess of either of the above-named iron salts. It may also be obtained by oxidizing Turnbull's blue (No. III.) with chlorine water or nitric acid. The chemical composition of this pigment is very complex, the simplest empirical formula for it being Fe7(CN)18: it will be seen that it contains no potassium. It always contains some combined water, which cannot be driven off by heat without decomposition of the salt. This blue is more permanent than No. I.
III. Turnbull's Blue
The chief constituent of the original Turnbull's blue (more properly, Gmelin's blue) closely resembles ordinary soluble Prussian blue, and, like it, contains potassium. But the potassium may be removed from it by stannous chloride solution, a substance being produced having the empirical formula Fe5(CN)12, but containing some water. Or the same body may be made by precipitating a solution of ferri-cyanic acid with a solution of ferrous sulphate or ferrous chloride. This blue is of good colour, but is more difficult to obtain pure than No. II., the other insoluble Prussian blue. Exposed to light, all the forms of Turn-bull's blue, pure and impure, have a more decided tendency to become greenish or to fade than No. II.
The ordinary commercial Prussian blue is a mixture, in varying proportions, of the three blues above described. It is made by adding green vitriol (ferrous sulphate) solution to a solution of yellow prussiate of potash (potassium ferrocyanide). The precipitate formed (which varies in colour from a light to a deep blue, according to the amount of ferric salt present in the green vitriol) is then oxidized by means of dilute nitric acid or of a solution of bleaching powder. After having been washed, the substance is treated with hydrochloric acid, and is then again washed with water.
All the above blues are of a very deep blue colour in powder or in the lump, but when pressed or rubbed they all show a coppery lustre. The only one fit for artists' use is the insoluble variety (No. II.), the others being less stable or having other defects. The insoluble form is, moreover, the only one which yields, when roasted, a perfectly satisfactory 'Prussian brown.'
Prussian blue is a transparent colour of great force and richness, and works well in oil as well as in water. In thin washes or layers it has a slightly greenish hue. Its colour is changed by lime and by the weakest alkalies, so that it cannot be employed in fresco or on newly-plastered walls. Long-continued exposure to strong light weakens and alters the colour of Prussian blue, but the insoluble varieties are less affected than the soluble. When this fading of the pigment in water-colour washes has taken place, a brief sojourn in darkness generally suffices to restore the hue almost to its original depth and quality. This strange phenomenon, which awaits explanation, has been long familiar to artists' colourmen. The influence of moisture in determining the fading of Prussian blue under solar exposure is seen in three comparative trials with water-colour washes on paper. In a sealed tube with ordinary air the intensity was reduced from 10° to 1°, and the colour became sea-green in thirteen months; after four years a part of the same wash retained its full depth when the slip was exposed in air kept dry; another portion was reduced to 8.5° by four years' exposure in an ordinary frame.
A sample of Prussian blue (as ordinarily made) in oil, after five years' exposure, had become somewhat greenish, with a loss of about one-tenth of its depth. These changes were more obvious in the pure transparent pigment than in its tint with flake-white. A second specimen, from another maker, similarly exposed, was rather less affected, both as to loss of colour and as to change of hue.
Prussian blue was discovered in 1704 by a colour-maker of Berlin, Diesbach by name. When, as is sometimes the case, this pigment is found in water-colour paintings of the seventeenth century, it is scarcely necessary to state that its presence betrays the brush of the restorer or the forger.
Antwerp blue is a sort of Prussian blue lake, the pigment consisting of a colourless base dyed with Prussian blue. According to one method of preparation, a solution of 1 part of green vitriol and 2 parts of zinc sulphate in 40 parts of water is precipitated by adding to it a solution of 4 parts of potassium ferrocyanide in 40 parts of water. The blue colour of the precipitate deepens as it is washed. Alumina is sometimes used as the base upon which the blue pigment is thrown. Antwerp blue is less transparent and less intense in colour than Prussian blue; it has about the same degree of stability.
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