PRUSSIAN BLUE 667 



potash in a strongly-ignited iron pot, a dark grey mass is obtained, that affords to 

 water the liquor originally called lixivium sanguinis, or blood-lye. This solution 

 yields crystals, known in commerce as the yellow prussiate of potash. If to this salt 

 solutions of any per-salt of iron be added, Prussian blue is formed. If the iron in the 

 salt employed be present as protoxide, it will afford a precipitate, at first pale blue, 

 which turns dark blue in the air. If, however, the salt employed contains peroxide 

 of iron (ferric salts) the precipitate is at once a dark blue. The white cyanide of iron 

 (the prussiate of the pure protoxide) when exposed to the air in a moist condition, 

 becomes, as above stated, dark blue ; yet the new combination formed in this case 

 through absorption of oxygen, is essentially different from that resulting from the 

 precipitation by the peroxide of iron, since it contains an excess of the peroxide in 

 addition to the usual two cyanides of iron. It has been therefore called basic Prussian 

 blue, and, from its dissolving in pure water, soluble Prussian blue. 



Both kinds of Prussian blue agree in being void of taste and smell, in attracting 

 humidity from the air when they are artificially dried, and being decomposed at a 

 heat above 348 Fahr. The neutral or insoluble Prussian blue is not affected by alcohol; 

 the basic, when dissolved in water, is not precipitated by that liquid. Neither is 

 it acted upon by dilute acids ; but they form with concentrated sulphuric acid a white 

 pasty mass, from which they are again reproduced by the action of cold water. They 

 are decomposed by strong sulphuric acid at a boiling-heat, and by strong nitric acid 

 at common temperatures; but they are hardly affected by the muriatic. They 

 become green with chlorine, but resume their blue colour when treated with de- 

 oxidising reagents. When Prussian blue is digested in warm water along with potash, 

 soda, or lime, peroxide of iron is separated, and a ferroprussiate of potash, soda, or 

 lime remains in solution. 



The precipitation of Prussian blue. Green sulphate of iron is commonly employed by 

 the manufacturer, on account of its cheapness, for mixing with solution of the ferro- 

 prussiate, in forming Prussian blue, though the persulphate, nitrate, or muriate of 

 iron would afford a much richer blue pigment. Whatever salt of iron be preferred, 

 it should be carefully freed from any cupreous impregnation, 'as this would give the 

 pure blue a dirty brownish cast. The green sulphate of iron is the most advantageous 

 precipitant, on account of its affording protoxide, to convert into ferrocyanide any 

 cyanide of potassium that may happen to be present in the uncrystallised lixivium. 

 The carbonate of potash in that lixivium might be saturated with sulphuric acid 

 before adding the solution of sulphate of iron ; but it is more commonly done by 

 adding a certain portion of alum, in which case alumina falls along with the Prus- 

 sian blue ; and though it renders it somewhat paler, yet it proportionally increases its 

 weight ; whilst the acid of the alum saturates the carbonate of potash, and prevents 

 its throwing down iron-oxide, to degrade by its brown-red tint the tone of the blue. 

 1'or every pound of pearlash used in the calcination, from two to three pounds of 

 alum are employed in the precipitation. When a rich blue is wished for, the free 

 alkali in the Prussian lye may be partly saturated with sulphuric acid, before adding 

 the micgled solutions of copperas and alum. One part of the sulphate of iron is 

 generally allowed for 15 or 20 parts of dried blood, and 2 or 3 of horn-shavings or 

 hoofs. But the proportion will depend very much upon the manipulations ; which, 

 if skilfully conducted, will produce more of the cyanides of iron, and require more 

 copperas to neutralise them. The mixed solutions of alum and copperas should be 

 progressively added to the lye as long as they produce any precipitate. This is not 

 at first a fine blue, but a greenish grey, in consequence of the admixture of some 

 white cyanide of iron ; it becomes gradually blue by the absorption of oxygen from 

 the air, which is favoured by agitation of the liquor. Whenever the colour seems 

 to be as beautiful as it is likely to become, the liquor is to be run off by a spigot or 

 cock from the bottom of the precipitation vats, into flat cisterns, to settle. The clear 

 supernatant fluid, which is chiefly a solution of sulphate of potash, is then drawn off 

 by a syphon ; more water is run on with agitation to wash it, which after settling is 

 again drawn off; and whenever the washings become tasteless, the sediment is thrown 

 upon filter sieves, and exposed to dry, first in the air of a stove, but finally upon 

 slabs of chalk or Paris-plaster. But for several purposes, Prussian blue may be best 

 employed in the fresh pasty state, as it then spreads more evenly over paper and other 

 surfaces. 



A good article is known by the following tests : It feels light in the hand, adheres 

 to the tongue, has a dark lively blue colour, and gives a smooth deep trace ; it should 

 not effervesce with acids, as when adulterated with chalk ; nor become pasty with boil- 

 ing water, as when adulterated with starch. The Paris blue, prepared without alum, 

 with a peroxide salt of iron, displays, when rubbed, a copper-red lustre, like indigo. 

 Prussian blue, degraded in its colour by an admixture of free oxide of iron, may be 

 improved by digestion in dilute sulphuric or muriatic acid, washing, and drying. 



