The apparatus consists of a number of 10 to 20 extractors connected together, also with a water tank above, and with a boiler. A single extractor is represented in section in [Fig. 39], together with the necessary pipes and valves by which it is connected with the neighbouring extractors, the water tank and the boiler. [Fig. 40] is a plan of three extractors connected together.
The extractors consist of wooden or copper vessels, slightly conical in shape, and of sufficient strength to resist the pressure of one atmosphere. On the top is a copper dome closed by a lid, through which the raw material is introduced, and also steam and water. Immediately above the bottom is an opening, also closed by a screw, which serves to remove the exhausted materials. An inclined sieve is placed at some distance above the bottom; upon this the material rests. The extract collects below the sieve, and may be run off by the pipe or brought into another extractor. All the metallic portions of the apparatus which come in contact with the liquid must be made of a metal such as copper, which does not act upon tannic acid. Iron cannot be used; it forms deep bluish or greenish black compounds with tannic acid, which would cause the extract, instead of being pale and clear, to resemble ordinary writing ink.
In a range of 10 extractors the process is carried out in the following manner: The extractors numbered 1 to 10 are filled with bark or dye-wood and closed; 1 is then filled with water from the tank, and heated by steam to 50° to 70° C. After some time the contents of 1 are forced into 2, and 1 is again filled with water, so that the material in 2 is in contact with the solution from 1, whilst the material in 1 is warmed with a fresh quantity of water under pressure; the extract in 2 is transferred to 3, that in 1 to 2, and 1 is again filled with water, and so on. Finally from 10 a very strong extract of tannin or colouring matter is obtained. The quantity of water required to fill one extractor has come ten times in contact with fresh bark or dye-wood. The material in 1 has been treated with ten times the quantity of water; it is now exhausted, and is replaced by fresh material. The sequence of the vessels is now changed. The original extractor 2 is to be regarded as 1, and 1 as 10. After ten repetitions the original order of the extractors re-obtains. Ninety-nine per cent. of the tannic acid of bark is extracted in this way.
The concentrated extracts obtained in this apparatus should be mixed with a little carbolic acid to prevent decomposition; they may then be filled into barrels. The extracts may also be so far concentrated by evaporation that they become syrupy. The tannins are readily decomposed; they would be considerably altered if their solutions were evaporated in open vessels. The extracts are therefore evaporated at a very low temperature under diminished pressure in vacuum pans, which are now much used for the concentration of solutions of substances, such as sugar, which would be injured by heating above a certain temperature. Essentially, a vacuum pan is a thick-walled copper vessel, in which the liquid is warmed by a steam coil. It is connected with an air pump, which exhausts the air at the commencement of an operation, and afterwards steam. The liquid is thus constantly evaporated under a low pressure. Extracts of tan-bark and dye-woods boil briskly under these conditions at temperatures below 60° C., at which no decomposition of the tannin or the colouring matter is to be feared. When the solutions have been evaporated to the proper strength they are run off directly into the packages in which they are to be despatched, and in which they become syrupy or even solid masses, according to the extent to which the evaporation has been driven.
The packages should be at once closed; the thick extract is thereby most simply and safely prevented from decomposing, to which risk it would be exposed by the access of mould spores. If these were already present in the barrels or were communicated to the extract by the air, they would either be killed by the hot liquid or would be prevented from developing for a long time. In the closed vessels the extracts remain completely unaltered.
The concentrated solution of colouring matter obtained by extracting dye-woods in the above apparatus can be at once used, after dilution, in the preparation of lakes and in dyeing, but it is not an extract in the ordinary commercial sense of the term, i.e., it does not solidify on cooling. To obtain solid extracts the concentrated solutions must be evaporated; concentration with fire heat would be attended with danger to the quality of the extract, steam heat is therefore used. The liquid to be evaporated is brought into shallow steam-jacketed pans, in which the operation is continued until the liquid solidifies into a resin-like mass when dropped on cold stone. When sufficiently evaporated the extract is allowed to solidify, broken into lumps, and these packed whilst still warm into barrels lined with paper. The lining is necessary on account of the hygroscopic nature of the extracts; when they are exposed to the air they absorb water and form a viscous fluid which soon becomes mouldy.
A properly prepared dye-wood extract should dissolve in water without residue, and the solution when largely diluted should show the characteristic colour of the wood with no brownish shade, and when the colouring matter is precipitated from such a solution by a metallic salt the residual solution should be almost colourless. If the extract dissolves incompletely in water and the solution is brown after precipitation of the colouring matter, the extract has been burnt in the evaporation.
Logwood and logwood extract contain two substances of importance in dyeing and colour making. These are hæmatoxylin and hæmateïn. Hæmatoxylin is found in logwood in greatest amount shortly before the wood is cut. When pure it forms colourless crystals of a peculiar sweet taste, which are soluble in cold water with difficulty, more easily in hot, and readily soluble in alcohol or ether. The composition of hæmatoxylin is expressed by the formula C₁₆H₁₄O₆.
Hæmatoxylin is not a colouring matter. It is important because from it is obtained the essential colouring matter of logwood—hæmateïn. When a trace of ammonia is added to the colourless solution of hæmatoxylin, the liquid at once becomes dark red owing to the formation of hæmateïn. When a larger quantity of ammonia is added the liquid acquires a deep red colour, and then contains only hæmateïn (its ammonia compound), which is formed according to the following equation:—