Indigo carmine is used in the laundry, for blue inks and as an artists’ pigment; in chemical composition it is a mixture of the sodium salts of the two indigotin sulphonic acids described in the last chapter. In working on the large scale it is particularly important that the indigo should be finely powdered and completely free from moisture. It is a laborious process to powder indigo in mortars, and loss always occurs through the formation of dust. Thus when large quantities of indigo are to be powdered it is advisable to use rotating cylinders. These are strong casks bound with iron hoops and provided with a well-fitting slide, through which the indigo and a number of iron balls are introduced. When these casks are rotated for a sufficient length of time the indigo is converted into the finest powder without any loss.

Indigo Mills.—Special mills have been designed for powdering indigo. They may be advantageously employed for other materials also. The construction of a well-designed colour mill is represented in Fig. 36, in which indigo may be very finely powdered without loss. It consists of a cast-iron pan, which can be closed by a well-fitting cover. Through the cover passes the axis of the rotating part, which is moved by the bevel cog-wheels and the handle. Above the cog-wheel fastened to the axis is a cross piece carrying heavy balls at the ends, which serve to keep the motion regular. In the interior of the pan four vertical rods are attached to the axis; these are placed at such a distance that they drive before them the heavy balls in the circular depression of the pan. The action of this apparatus is very simple. After the introduction of the indigo through an opening in the cover the axis is put in rotation, the vertical rods attached to the axis roll the balls before them, and thus break up the lumps of indigo. The larger pieces of indigo oppose considerable resistance to the rotation, which should at first be slow. When the lumps have been broken down the speed of rotation can be considerably increased. After the operation has been continued for a sufficient length of time the indigo is converted into an impalpable powder.

To facilitate the removal of the ground indigo, an arm carrying a brush of fine hair should be attached to the horizontal arm of the axis. If this brush is placed so that it sweeps the bottom of the circular depression in which the balls move, all the indigo powder in this depression will be carried in one direction. In the depression is an opening closed by a slide during the grinding. The brush expels the ground indigo through this opening when the axis is slowly rotated; it is collected in a vessel below. The larger and heavier are the balls in this mill, the more rapidly will a given quantity of indigo be powdered, and the greater will be the power required to drive the machine. By means of a pulley on the axis of the cog-wheel the mill can be driven by mechanical power.

The powdered indigo is exposed for a long time to a temperature of 120° C., to completely dry it. For this purpose it is spread out on sheets of tin plate in a thin layer and heated so long as a test portion decreases in weight. The powder whilst still warm is at once mixed with the sulphuric acid; 4·5 kilogrammes of fuming sulphuric acid and 1 kilogramme of ordinary sulphuric acid (66° Bé) are used to 1 kilogramme of indigo. Heat is developed in the operation, and in order to prevent the indigo from being charred the vessel in which the mixture is made is placed in a larger vessel filled with cold water. The mixture is stirred with a glass spatula until it is quite uniform. The vessel is then maintained at a temperature of about 50° C. (not more than 60° C.) for 7 to 8 days. After the lapse of this time the indigo is completely dissolved, and the vessel is found to contain two distinct layers, the upper fluid and the lower a paste. The contents are mixed with cold water, about 10 kilogrammes of water to 1 kilogramme of indigo. A solution of 10 kilogrammes of common salt is then added in small quantities at a time. Indigo carmine is insoluble in strong salt solutions; it separates as a deep blue precipitate, which is allowed to settle and then filtered. If it were attempted to wash out the remainder of the salt solution retained by the carmine, a considerable quantity of the latter would dissolve as soon as the salt solution became diluted to a certain extent. To avoid such loss the precipitate is allowed to drain completely on the filter, and then spread out upon bricks, which absorb the water, so that the carmine forms a stiff paste.

Indigo carmine, from which the salt has not been completely washed out, dries completely when exposed to the air. This may be prevented by the addition of glycerine, which is a very hygroscopic substance, and so keeps the carmine always moist, thus preventing the crystallisation of the salt.

Indigo carmine easily dissolves in water to a deep blue liquid. It has tremendous colouring power, in which respect it is only equalled by genuine cochineal carmine. As mentioned above, indigo carmine is extensively employed in the manufacture of ink. The so-called alizarin inks show a very pale colour as they flow from the pen, and only become deep black after some time. To make these inks equally visible at the first indigo carmine is added, by which they acquire a handsome blue colour.

Indigo carmine may also be made by digesting 1 part of dry powdered indigo for 24 to 36 hours with 4 parts of strong sulphuric acid, diluting with water, quickly filtering through a linen cloth, and adding 4 parts of common salt. The precipitate is then collected and dried on gypsum or clay plates.

Blue Lake.—When alum is added to a solution of indigo sulphonic acid, and then soda solution, a blue precipitate is formed, which, when dry, resembles Chinese blue in appearance. It is used in painting. This indigo blue lake is superior to other blue lakes by reason of its fastness to light. It is rather dear, and is mixed with starch, so that it may be sold at low prices; the mixture is formed into slabs and sold as ”new blue,” ”indigo extract,” etc. It is chiefly used in the laundry.

Under various names a number of blue pigments are sold consisting of ordinary qualities of indigo mixed with Prussian blue, smalts, chalk, etc. These are used in the laundry, and also for distempering.