The average composition of Prussian-blue is 3 equivalents iron protocyanide, 2 sesquicyanide, and 9 water. The proportions of the cyanides are liable to differ, and this fact, combined with varying quality of the raw materials and mode of preparation, renders the colouring intensity subject to fluctuation. According to the most general mode of preparation, a solution of alum and iron sulphate in water is mixed with one of yellow prussiate of potash (potassium ferrocyanide). Prussiate of potash is made by heating together crude potash carbonate and refuse animal matters, such as " flocks" (the waste or refuse woollen dust from wool-mills), clippings of leather, horn, hoof, etc. The process is conducted in a closed building with louvre openings in or along the roof. Iron .pots 2 to 3 ft. wide and deep are used, arranged in a bank, and heated by fires underneath. There are always more pots than are in use at any one time, since they wear out after a few, weeks, the material of the pot furnishing the iron necessary for the formation of the salt.

Each pot is furnished with a flat-iron cover, capable of partial removal for introduction of material; also with a stirrer inside, the vertical shaft of which passes through the cover, and is set in motion by a shaft passing along above the series of pots. The potash being put in, the animal material is added in shovelfuls from time to time, the removable portion of the cover being raised for the purpose; the heating operation is continued for 3 or 4 hours, and then the contents of the pot are taken out and thrown into a tank of cold water, which dissolves out the prussiate. A carbonaceous matter remains, and is usually sent away to sewage or manure works to be used as a deodorizer. The further processes consist in recrystallization of the salt upon strings suspended in the crystallizing tank.

During the heating process, flame and offensive smoke issue from the gaps about the covers of the pots, and directly from the pots themselves when the cover is raised to put in fresh material. This smoke is often a source of nuisance. The best mode of preventing it is in operation at Sir . Buckley's works, Edge Lane, Clayton, near Manchester. The arrangements which have been most effectual in obviating nuisance were devised by Prof. Roscoe, and are represented in Fig. 15. From the back part of the lid of each pot A, a pipe c passes, first upwards, then horizontally, and downwards to the back part of the flue d, which surrounds and heats the pot. The pipe is hinged to allow of the lid a, with the part of the pipe connected with it, being raised. The smoke and fumes are drawn down to the flue, where they meet the flame of the fire and are consumed, the products of combustion being carried off by a tall chimney. The building, in which 21 pots are arranged along one side, is 50 yd. long by 11 yd. wide and high.

It is lit all along the top of the roof by skylights, below which are louvres, and is ventilated by openings in the side and end walls: b is the stirrer, which makes 24 revolutions per minute. (Dr. Ballard.)

For making Prussian-blue, the solution of (yellow) prussiate of potash is poured by degrees into a hot solution of pure iron sulphate and alum. The proportions are varied according to the quality desired. The common relative quantities are 1 part alum' to 7 or 8 of iron sulphate; for inferior grades, these are reduced to 1 alum and 2 or 3 iron sulphate, or even to equal parts of each.

Each addition of cyanide lye to the iron solution produces abundant hydro-sulphuric and carbonic acids, whose escape is aided by stirring the liquor with a wooden rod. The precipitate is brownish-green, and is washed with pure water until it turns entirely blue. After settling and decanting the liquors, the blue precipitate is placed upon a cloth filter, and washed with water holding a little sulphuric acid. The drained blue is pressed in boxes, to remove the greater part of its water, and the thick resulting paste is divided into rectangular blocks, which are dried in the dark, or in a stove-room not above 77° to 86° F. (25° to 30° C).

Prussian-blue is next in purity of tone after ultramarine and cobalt, and though inferior in durability, contains more colouring power - 10 or 11 times; but all alkalies alter it. Large quantities are used by house-painters and decorators, and by manufacturers of paper-hangings. Of all blues it is the most intense. Mixed with white lead, the hue is slightly greenish. A mixture of 1 gr. Prussian-blue and 90 white produces sky-blue; 200 white and 1 blue give azure-white. To judge of the beauty of a Prussian-blue, it should be incorporated with 50 to 100 times its weight of fine white-lead. Mixed with 15 to 20 times its weight of chvome-yellnw, it produces handsome greens, not very lasting, however. It is employed with glue, size, or oil; in the latter case, it should not be kept long, or it becomes thick, and does not flow well under the brush. The pure blue ground in oil produces velvety blacks. Old damp walls destroy the colour of Prussian blue by the nitrate of lime they contain, producing, by double decomposition, a ferrocyanide of calcium and a nitrate of iron, (Rimiult.)

Prussian Blue Paints 20017

Fig. 15.

This colour may be made in small quantities in the following manners: - (a) Dissolve I oz. iron sulphate and 8 oi. alum in 1 gal. water; add separate solutions of prussiate (yellow) of potash and peariash until the precipitation ceases; collect the precipitate alter soma time, wash thoroughly, and dry. (b) Mil a solution of iron protosulphate with one of red pmulate of potash (ferricyanide); wash, and dry.