The Natural Organic Colouring Matters
By
Arthur George Perkin, F.R.S., F.R.S.E., F.I.C., professor of colour chemistry and dyeing in the University of Leeds
and
Arthur Ernest Everest, D.Sc., Ph.D., F.I.C., of the Wilton Research Laboratories; Late head of the Department of Coal-tar Colour Chemistry; Technical College, Huddersfield
Longmans, Green and Co.
39 Paternoster Row, London
Fourth Avenue & 30th Street, New York
Bombay, Calcutta, and Madras
1918
Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.
INDIGO has been known in Asia from a remote period of antiquity, and there exist very ancient records in Sanskrit describing its methods of preparation. The Romans appear to have recognised it only as a pigment (indicum), but evidence as to its use as a dye by the ancient Egyptians has been abundantly proved from the examination of mummy cloths. Its employment in Europe was very limited until in 1516 when it began to be imported from India by way of the Cape of Good Hope, but its introduction in large quantity did not occur until about 1602. Owing chiefly to the opposition of the growers of woad, its European rival, as a dyeware it met with much opposition, and various laws were enacted both on the Continent and in England prohibiting its use. It was called a "devilish drug," and was said to be injurious to fabrics. In 1737 its employment was legally permitted in France, and from this period its valuable properties appear to have become gradually recognised throughout Europe.
The most important plants which yield indigo are those of the genus Indigofera belonging to the natural order of the Leguminosa; these have been cultivated in India, China, Egypt, the Philippines, Caracas, and Brazil.
For the purpose of indigo manufacture the Indigofera tinctoria (Linn.), I. sumatrana (Garrtn.) (the Indian plant), I. disperma (Linn.), I. argentea (Linn.), and I. arrecta (Höchst.) (the Natal plant), the I. paucifolia (Delile) (Madagascar plant), and I. secundiflora (Poir.) (Guatemala plant), have been mainly used, though certain less valuable varieties, viz. the I. pseudotinctoria (R. Br.), I. angustifolia (Linn.), I. arcuata (Willd.), I. caroliniana (Walt.), I. cinerea (Willd), I. longeracemosa (Boiv.), I. cœrulea (Roxb.), I. endecaphylla (Jacq.), I. glabra (Linn.), I. hirsuta (Linn.), I. indica (Lam.), I. mexicana (Benth.), I. leptostachya (DC.), have been employed. In Japan, China, and Russia the plant usually cultivated has been the Polygonum tinctorium (Ait.), but the Isatis tinctoria (Linn.), or woad plant, at one time very largely grown in Europe, is now only used in very limited quantity as an adjunct in the dyeing of indigo (woad vat). The native source of indigo in Western Africa appears to consist almost entirely of the Lonchocarpus cyanescens (Benth.) (Perkin, J. Soc. Chem. Ind., 1907, 26).
Other indigo-yielding plants are the Nerium tinctorium, Gymnema tingens (Spreng.), Eupatorium laeve (DC.), Tephrosia tinctoria (Pers.), Marsdenia tinctoria (R. Br.), and certain species of orchids such as the Phaius grandiflorus (Reich.), and Calanthe veratrifolia (R. Br.).
In addition to these, various plants, of which the Mercurialis perennis (Linn.), Fagopyrum esculentum (Moench.), Fraxinus excelsior (Linn.), Baptisia tinctoria (R. Br.), and Rhamnus alaternus (Linn.) (Georgievics, Der Indigo, 1892) may be given as examples, are stated to yield indigo, or a very similar colouring matter, but this requires confirmation.
The production of indigo from the indigo plant is of a simple character and consists mainly of two processes, viz. a steeping of the plant with water (fermentation), followed by the oxidation of the solution with air in a separate vessel. Until very recently but little modification appears to have been introduced into this ancient process, and there is also but little variation to be found in the main features as described by Bancroft ("Philosophy of Permanent Colours," 1813), Crookes ("Manual of Dyeing and Calico Printing," 1874), Bridges-Lee ("Indigo Manufacture," 1892), Georgievics (loc. cit., 1892), and Rawson ("The Cultivation and Manufacture of Indigo," J. Soc. Dyers, 1899).
* It has long been known that the percentage of indican rapidly disappears from the leaf in the freshly plucked moist condition, and on this account it is advisable that the plant when cut down should be dealt with at once. On the other hand, it has been the practice to a small extent in certain districts to airdry the leaf before proceeding with the manufacture of indigo. In order to determine if when air-dried under ideal conditions, the leaf in these circumstances loses a material amount of indican, experiments were made by Watson (Jour. Chem. Soc. Ind.,1918, 37, 81) with indigo plant specially grown for the purpose. The leaf when gathered was divided into two portions, one of which was analysed at once, and the second dried by spreading in a thin layer upon filter paper in a north verandah for three or four days until constant in weight.
The analyses were carried out by the isatin method (loc. cit.) with 10 grammes of leaf weighed in each case in the fresh condition, and the results expressed as indirubin indicate that a serious loss of colouring principle does occur in these circumstances:
(a) Fresh Leaf. 0,1008 / Air-dried Leaf. 0,0568
(b) Fresh Leaf. 0,0825 / Air-dried Leaf. 0,0610
(c) Fresh Leaf. 0,1057 / Air-dried Leaf. 0,0763 Directly the plants are cut down they are tied in bundles and brought to the factory without delay, because it is necessary that the material should be operated on at once.* The tanks for the extraction (steeping vats) and precipitation of the indigo by oxidation (beating vats) are sometimes of stone, but more usually of brickwork lined inside with cement, and are respectively ranged in two rows one above the other, so that, the former can be drained into the latter. The steeping vats may have a capacity of about 1000 cubic feet, and are usually of much smaller dimensions than the beating vats, of which less are consequently required. According to Rawson (loc. cit.), who describes a small indigo factory, each range of beating vat runs the whole length of six steeping vats, and has a width of 13 feet 6 inches.
Into each of the upper tanks the bundles of the plant are tightly packed (preferably in a horizontal position, Bridges-Lee, loc. cit.) on the top of this is laid a horizontal trellis of bamboo, and the whole is wedged down into the tanks by means of timber, so that the material is unable to float during the fermentation process. Water is then run in, in such quantity that the bundles are entirely submerged. After about two hours an active fermentation is observed, and the surface of the liquid becomes covered with froth owing to the evolution of a mixture of carbon dioxide, oxygen, and nitrogen (Georgievics, loc. cit.); in the later stages (Rawson, loc. cit.) either marsh gas or hydrogen or a mixture of the two is freely produced. After ten to fifteen hours, according to the prevailing temperature of the water, the straw-yellow, orange, or olive-green coloured liquid is drawn off into the tanks below, and submitted to oxidation with air.
This may be accomplished by "hand beating," by machinery (the beating wheel), by blowing air through the liquid, or by the showerbath method. During this operation the colour of the liquid gradually changes, becoming first dark green and then blue, and considerable frothing is produced. When it is observed that the indigo precipitate or "fecula" readily settles, the beating is discontinued and the mixture allowed to rest for some two hours. The supernatant liquid, or "seeth water," having been drained off as completely as possible, the indigo sludge or "mal" is led into a reservoir inside the factory, from which it is subsequently elevated by means of a hand pump or steam injector into a large cauldron known as the "mal boiler". It is here heated by direct fire or by the admission of steam, and this has for its object the prevention of a further fermentation, the solution of certain brown impurities, and a more complete granulation of the "mal".
The product is then run on to a filter known as a "table," consisting of stout cotton or linen cloth stretched over a shallow rectangular basin of stone or cement, with a drainage opening at one corner, and allowed to remain until it has the consistency of a stiff paste. In order to remove excess of moisture the indigo is transferred to perforated wooden boxes lined with sail cloth and cautiously pressed. Finally, the resulting slab is cut into cakes by means of a guillotine or metal wires and allowed to dry at the ordinary temperature on trellis-work shelves in a specially constructed drying house.
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