In each case about 50-60 gallons of water are required.
To determine the amount of barytes present in a sample, boil about 50 gr. with caustic soda, filter, wash the residue free from soda, treat with sulphuric acid, well wash the insoluble residue, dry, and weigh.
Chinese Blue.—This well-known and favourite form of Prussian blue is prepared with great care, and is usually sold in fine powder or little cubes. Its composition is virtually identical with that of ordinary Prussian blue, but it is more free from impurities, and shows a fine bronze bloom or lustre on newly fractured surfaces. Being pure, it is entirely dissolved by oxalic acid; and its composition is about 52 per cent. oxide of iron, 43½ cyanogen, and 4½ water. In dyeing and calico-printing it is extensively employed. Its tint varies from greenish to violet, according to modifications in the method of manufacture, the chief difference being that yellow prussiate gives a greenish tone and red prussiate a violet.
The process of preparation is mainly as follows. In about 40 gallons of cold water dissolve 1 cwt. of green copperas selecting it carefully for freedom from insoluble oxide; add about 5 pints of sulphuric acid. This liquor very rapidly undergoes oxidation, by which oxide of iron is thrown down, and the solution is rendered unfit for making the best quality pigment. Therefore it should be prepared only immediately before it is used. In another vessel containing about 40 gallons of cold water, dissolve 1 cwt. of yellow prussiate (if a green shade is desired), or of red prussiate (if a violet tint is wished for). Even larger quantities of water may be used for the solutions, as the more dilute they are the finer is the colour precipitated and the greater the lustre on the surface of the finished pigment.
When the two solutions of yellow or red prussiate and acidified green copperas are brought together, a bluish-white precipitate is thrown down. This is allowed to completely separate itself, and then the clear supernatant liquid is drawn off.
The next step is to thoroughly oxidise the precipitate. This cannot be satisfactorily accomplished by utilising the oxygen of the atmosphere, as is done in other cases, because that method entails the production of a certain amount of oxide of iron, which prejudicially affects the purity of colour of the finished article. Of the chemical oxidising agents which are available, the most satisfactory in point of cost and efficiency is chloride of lime (bleaching powder). For each cwt. of green copperas, mix about 20 lb. of bleaching powder into a thin cream with water, and add it, in small quantities at a time, to the precipitate, constantly stirring so as to ensure the absorption of the whole of the chlorine by the blue. Without the addition be made gradually and under agitation, the chlorine will be generated more quickly than it can be absorbed, entailing a waste of gas and a noxious vapour to be breathed by the workmen. Sometimes the bleaching powder is added at an earlier stage, viz. to the green copperas solution, and in that case the blue assumes a violet tone.
After the addition of the bleaching powder solution to the bluish-white precipitate, it is acidified with hydrochloric acid, which develops the blue. When the whole has settled, the supernatant liquor is drawn off, and the blue powder is well washed and strained on a filter, then placed in pans and dried very gradually indeed in the dark, at a temperature never exceeding 130° F. The slower the drying the better is the gloss of the pigment. It is most essential that iron be excluded during the final grinding operation, or it may cause ignition of the mass, and its conversion into oxide of iron would speedily follow.
It has been proposed to treat the white precipitate (obtained in the usual manner from green copperas and yellow prussiate) by the chlorine contained in aqua regia (nitro-hydrochloric acid). The copperas, however, must be as free as possible from basic sulphate (oxide), which is ensured by keeping a little metallic iron in the acid solution of copperas. It is also desirable to effect the precipitation with crude prussiate, so as to avoid absorption of oxygen and premature development of the blue colour. Habich considers that the mistake is generally made of using too little copperas, and he has found that when 90 lb. of copperas have been added to 100 lb. of yellow prussiate, a drop of iron solution in the filtered liquor produces no precipitate, while the white precipitate has carried with it a certain proportion of prussiate, which can be washed out. He therefore proposes to avoid this waste by pouring the copperas solution into the prussiate solution, with constant agitation, till no further precipitate goes down, then adding one volume of the copperas solution equal to one-ninth of that already used. After fifteen minutes stirring, it is certain that all the prussiate carried down is decomposed.
The drained precipitate is blued (peroxidised) by adding aqua regia prepared several days previously, and in proportions depending on the strengths of the two acids. Generally, the aqua regia mixture will be 100 lb. of commercial nitric acid at 30° B. (containing 35·4 lb. of anhydrous acid) and 62·2 lb. commercial hydrochloric acid at 23° B. (containing 23·9 lb. of the anhydrous acid); and 40 lb. of this mixture will suffice for bluing the precipitate resulting from 100 lb. yellow prussiate. The addition of the aqua regia should take place in a wooden vessel with constant agitation.
According to another modification, the white precipitate obtained in the usual way is blued by adding a solution of perchloride of iron, which may be made from a hematite ore free from clay and carbonate of lime, or from rouge. The iron oxide, from whatever source, is ground to a very fine state, and treated with crude hydrochloric acid in a lead-lined tank, where the mixture remains for several days, and is constantly stirred. When saturated with iron the clear liquid is withdrawn for use. To receive it, the white precipitate is rapidly heated to boiling in a copper vessel, and is then transferred to a wooden vat, and the iron perchloride solution is stirred in till the desired tint is produced. The pigment is washed and dried in the ordinary way, while the supernatant liquor (essentially protochloride of iron) is poured over old scrap iron and used instead of copperas for a fresh batch of yellow prussiate.