Indigo is judged commercially by its lightness, by a copper gloss on the surface, and by exhibiting no foreign ingredients when broken. There are several ways of testing it chemically, to ascertain the exact proportion of indigotine present; one method is as follows:—Finely pulverised indigo, 1 part; green copperas, 2 parts; and water containing 10 per cent. of caustic soda, 200 parts; are well boiled in a flask, and left to cool. The clear liquor is exposed in shallow vessels to the air, when the soluble indigo is oxidised, and precipitated as pure indigotine. The residue in the flask is thus treated three times; the whole indigotine is then collected on a filter, dried, and weighed. The consumption of indigo is still very large. Artificial indigo has not, as yet, been manufactured on a commercial scale, nor at a commercial price; but it has been produced, in the laboratory, from coal-tar derivatives, and further experiment may reveal a process for preparing the article at a sufficiently low price to compete with the natural colour.

Several preparations of indigo are in use:—(1) Sulphopurpuric acid, phenicine, or indigo-purple, is made by mixing 1 part of indigo with 4 parts of sulphuric acid (sp. gr. 1·845), and heating for ½-1 hour; the acid mass is thrown into 40-50 parts of water, when the purple falls down; it is collected on a filter, and washed with dilute hydrochloric acid. (2) Sulphindigotic acid is prepared by mixing indigotine, 1 part, with sulphuric acid (sp. gr. 1·845) 6 parts; the operation must be performed in a leaden vessel, cooled outside, and the indigo must be added by degrees, to avoid heating; the mixture is then left for 8 days, when the conversion will be complete. Fuming or anhydrous acid may be used, in less proportion, but the reaction is more difficult to manage. Weaker acid will require a longer period, say a month for “brown acid” (145° Tw.). (3) Sulphindigotic acids are transformed into neutral paste, or “carmine,” by neutralising with carbonate of soda, and washing the paste, on a woollen filter, with a solution of chloride of sodium (common salt).

Manganese Blue.—(1) Kuhlmann found a blue mass of manganate of lime in furnaces used for making calcium chloride by calcining a mixture of chalk and residues from chlorine making. The formation of this beautiful coloured manganate he attributes to the decomposition of the calcium chloride by steam, and to a certain solubility of the lime in undecomposed calcium chloride. Unsuccessful attempts to reproduce this result were apparently due to the lime not being under such favourable conditions for acting upon the manganese oxide as when it is in solution in the calcium chloride. As accidentally produced in reverberatory furnaces, the manganate of lime is of an ultramarine tint, and is insoluble in water though not permanent under its influence; it is acted upon by the weakest acids.

(2) Bong has proposed several formulæ for making manganese blues, the ingredients in each case being heated to redness in an oxidising atmosphere, taking special care to avoid iron. The following are his recipes:—(a) 6 parts soda ash, 5 of calcium carbonate, 3 of silica, and 3 of manganese oxide; (b) 8 of barium nitrate, 2 of kaolin, and 3 of manganese oxide; (c) 8 of barium nitrate, 3 of silica, and 3 of manganese oxide. The tint can be varied from violet to green by altering the proportions.

Prussian Blue.—This blue owes its colour to a combination of iron and ferrocyanogen. The commercial products vary very much in tint, depth of colour, covering power, and solubility. They are used for a variety of purposes, nearly all of which require the blue to have some property different from what it should have for other uses. For some purposes a green shade blue is wanted, for others a violet shade blue; some users want the blue to be soluble in water, others for it to be soluble in oxalic acid, others require it to be insoluble. Ordinary Prussian blue is insoluble in water, acids, and alkaline salts; bleaching powder has no action on it, and therefore it is largely used for tinting paper. It is capable of resisting acids; but alkalies, such as caustic soda, caustic potash, the carbonates of the same metals, lime and ammonia, decompose it, oxide of iron and a solution of a ferrocyanide of the alkali being formed, the decomposition being shown by a change of colour from blue to a reddish brown. On this account Prussian blues cannot be used for colouring soaps and alkaline products, or used as a pigment in distemper painting along with lime. The change of colour, from blue to brown, by the action of alkalies, distinguishes this blue from other blues.

Prussian blues require to be tested for their solubility in oxalic acid, by taking about 20 gr. of the blue, mixing with 1 oz. of water and 20 gr. of oxalic acid, in which a good blue ought to dissolve completely. Some brands are soluble in strong hydrochloric acid while others are not. It is decomposed by boiling sulphuric acid, and turned green by boiling nitric acid. For all the ordinary uses of a pigment Prussian blue is quite durable, and possesses a depth of colour and a definite tint which is proof against the destructive agencies of light and air; and though its covering powers are not great, it is one of the most important blue pigments in use.

When a salt of the higher oxide of iron is added to a solution of yellow prussiate of potash (or ferrocyanide of potassium) a blue compound is formed which is called Prussian blue. But this is not the method adopted for its commercial manufacture. In that case a ferrous salt, the proto-sulphate of iron, is added to a potassium ferrocyanide solution, the result of which is that a dirty bluish white precipitate is thrown down. On adding to this a little solution of bichromate of potash and sulphuric acid, the full deep blue is obtained. This is the industrial method of manufacturing Prussian blue.

Yellow Prussiate.—The first step is the preparation of the yellow prussiate of potash. The manufacture of this substance, although an industry of considerable importance, is comparatively little understood, either from a scientific or a practical point of view. At all events, many prussiate makers seem completely at sea with regard to the most favourable conditions for carrying on the manufacture, and there can be no doubt that in many cases great waste occurs, through ignorance of the various reactions which take place during the process. The raw materials usually consist of carbonate of potash, iron filings or turnings, and organic matters containing carbon and nitrogen—such as dried blood, woollen rags, horn, hair, leather scraps, &c. The most suitable substances for use are, of course, those containing the largest proportion of nitrogen. The following are the percentages of nitrogen in various kinds of animal matter:—

Horn15 to 17
Dried blood15 to 17
Woollen rags10 to 16
Sheep shearings16 to 17
Calves’ hair15 to 17
Bristles9 to 10
Feathers16 to 17
Hide clippings4 to 5
Old shoes6 to 7
Horn charcoal2 to 7
Rag charcoal2 to 12

Animal matters always contain more carbon than is necessary for the formation of cyanogen by combining with the nitrogen also present. Consequently, when such substances are heated with pearlash, the excess of carbon reduces a portion of the carbonate to the metallic state, and this potassium combines with the cyanogen to produce potassium cyanide. The manufacture of yellow prussiate of potash may be conveniently divided into three stages: (1) The production of the molten mass technically known as “metal”; (2) the lixiviation; and (3) the crystallisation.