INDIGO.

453

the following plants, which are used for blue dyeing, contain-likewise indigotin,viz., the woad, or pastel, Isatis tinctoria ; Polygonum tinctoriam, largely cul-tivated in Russia, and there applied for the manufacture of indigo; Ncriumtinctoriam, Asclepias tingcns, Enpatorium tinctoriam, Galega tinctoria.

According to Dr. Schunck, indigo-blue is obtained from the leaves ofBlesia Tankervillic? , a fact established also by the late Dr. Calvert. Theycontain also indican, which the former chemist considers to be in every casethe parent substance of indigotin. Dr. Schunck did not succeed in obtainingindigo-blue from the Galega tinctoria and Spilanthes tinctorial grown in theopen air in this country, but considers it possible that the colour might bedeveloped in a warmer climate.

Woad, or Pastel ( Isatis tinctoria), is a'biennial plant, belonging to theCruciferce. Its leaves are gathered in the month of June of the second year,and are sometimes simply cleaned, and serve, after having been dried, for thepurpose of dyeing; more frequently, however, the leaves are converted by theaid of water into a paste, which is heaped together to a height of fromgo centimetres to i metre, and left fermenting for about a fortnight, andthen made up into the shape of small balls, similar in size to those sold aswashing blue. In some few instances the fermented mass is submitted toanother fermentation, which is accompanied by a disengagement of ammonia.The material thus obtained is known in the trade (at least on the Continent,since woad and its preparations are quite out of use in England)* as pastel;the best kind thereof is made in Provence and Languedoc; next in value isthe Normandy; while the growths of Thuringia, Bohemia, and Hungary areof equal quality. Pastel is very frequently used for dyeing along with indigo,and some dyers pretend that they cannot get good shades without it. Pastelballs of good quality should be light, exhale a sweetish odour, have a green orgreenish yellow colour, and should when cut exhibit a greasy lustre, and leavewhen rubbed on paper a greenish streak. They improve in tindlorial powerby being kept, so that when six or eight years old their value is nearlydoubled. It does not appear that indigo ever has been industrially preparedeither from pastel or the fresh plant, but there is no doubt that it can beobtained from these substances; and before the re-introdudtion of indigo inEurope in the 17th century woad was generally applied for dyeing a fast blue.

From the dried leaves of the same plant Dr. Schunck-obtained indican bythe following process :—The leaves are treated with cold alcohol; to theliquid thus obtained water is first added, and it is next concentrated byevaporation at the ordinary temperature, aided by a current of dry air. Theconcentrated liquid is filtered and shaken up with freshly precipitated hydratedoxide of copper. The liquid is again filtered, and the filtrate treated withsulphuretted hydrogen to eliminate the oxide of copper which had beendissolved. It is again filtered, and the filtrate evaporated by the aid of acurrent of air at the ordinary temperature until there remains a syrupy mass,from which indican is extradted by cold alcohol. This solution deposits abrown material, and on the addition of ether other foreign matters are throwndown. The liquid having been filtered is evaporated, leaving indican as a lightbrown, syrupy, bitter mass, which has an acid readlion with test-paper ; it issoluble in water, alcohol, and ether, and susceptible of yielding, under theinfluence of acids or of a ferment in the plant, indigo-blue and sugar.

* Woad, though no longer used as a dye, aids the fermentation of the indigo»vats.