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.
CHAPTER LXII.
THE COLOURING MATTER OF LOGWOOD.
The wood of Hæmatoxylon campechianum (logwood), a native of South America, comes into the market in small logs of a red colour; it contains a colouring matter whose properties are fairly well known. Before the discovery of the coal-tar dyes logwood was a most important colouring material; by means of it red, blue, violet and black colours can be obtained. At present it still plays an important part in dyeing.
Many varieties of logwood come into commerce. The most important are Campeachy wood, Honduras, Jamaica and St. Domingo logwood; the first named is the best, and the last the poorest quality. To facilitate the extraction of the colouring matter by water logwood is frequently sold in chips. Many dye-wood grinders moisten logwood with lime water; this gives the powder a better colour, but diminishes the yield of colouring matter. Since the only useful constituent of logwood can be extracted by treatment with water, and the wood is only a carrier of the colouring matter, which is only useful for fuel when exhausted, logwood extracts are largely used in place of the wood. The extracts are black resin-like masses, which easily dissolve in water; they are very hygroscopic, and should therefore be kept in closed vessels.
Logwood Extract.—Solid extracts can be obtained from the majority of dye-woods. The use of these is a great convenience to the dyer and the colour maker; the method by which extracts are made is therefore briefly described.
Dye-wood extracts can be made on the small scale, by washing out the finely divided wood until soluble matters are no longer taken up by the water, and cautiously evaporating the united extracts. When the extract becomes concentrated the greatest care must be taken to prevent burning on the bottom of the vessel; a burnt extract is always darkened by the presence of products of decomposition, and its solution has a brown colour. Apart from this, the complete extraction of the colouring matter requires much time, and such dilute solutions are produced that they cannot be evaporated, but at the best can be used in the place of water to extract fresh quantities of wood.
When steam is utilised to extract dye-woods these defects are removed. A burnt extract is not to be feared, and a small quantity of water is sufficient to extract the colouring matter completely. The apparatus illustrated in [Fig. 37] is very suitable for the extraction of dye-woods, and in general for obtaining vegetable extracts. The extraction vessel is pear-shaped, its two hollow axes move in hollow bearings. Thus it may be turned upside down, and steam and water can be introduced into the interior through the axes, which are connected with the pipes, E, W, and R. The opening at the pointed end of the pear is closed by the screw, S; through it the substances to be extracted are introduced. In the lower portion of the vessel is a sieve upon which the materials are spread out. The opening of the pipe connected with E and W is below the sieve.
