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.
This commercial product is a dark clay-like preparation made from the leaves of the woad-plant, Isatis tinctoria (Linn.), an erect, herbaceous, biennial plant, belonging to the Cruciferæ, bearing yellow flowers, small flat elliptical pods, and large smooth lanceolate or spathulate leaves.
The term "woad" is derived from the Saxon "wad," which it has been suggested is derived from Woden, the Saxon God of War. It is synonymous with the Gallic glastum, with which, according to Pliny, the ancient Britons dyed their skin blue, in time of war and in connection with certain religious observances.
The plant is a native of Southern Europe, and from very early times has been employed in dyeing blue, for which purpose, previous to the introduction of indigo from India, it was largely cultivated in various parts of Europe - e.g. Thuringia, Languedoc, Piedmont, etc. Its cultivation has now declined almost to the vanishing-point.
In this country woad is now only grown, to a very small extent, in the fen lands of Lincolnshire and Huntingdon. The seed is sown in the early spring, March or April, and the young plants having been duly thinned and weeded, the leaves are ready for the first plucking in June, which, at intervals of five or six weeks, is repeated once or twice, or as often as fresh leaves shoot up.
The newly-gathered leaves are at once crushed or ground in edge-runner mills to a pulp, which is then placed in small heaps to drain, till sufficiently dry to cohere and be submitted to the "balling" process. This consists in working the pasty mass by hand into balls, 4-6 ins. in diameter. These are at once spread out on wicker-work trays Or "fleaks," and thoroughly dried in well-ventilated sheds. The balls are stored in a dry airy place till the whole crop has been gathered, and are then submitted to the so-called "couching" - i.e. a fermentation - process. For this purpose the balls are ground to a coarse powder, which is spread on the floor of the couching- house to a depth of 2 or 3 feet, and there reduced again to the consistency of a paste by frequent sprinkling with water and turning over with shovels. During this process, which lasts from twenty to forty days, the mass becomes heated and abundant offensive odours are given off. The operation needs to be conducted with some care and skill, so that the fermentation is neither so slow that a "heavy" product is obtained, nor so rapid as to give one which is "foxy". When the fermentation has subsided, and the stiff, pasty mass is sufficiently cooled, it is packed in casks ready for the market.
It has been calculated that 9 parts by weight of woad leaves yield 1 part of the prepared product.
Although woad was formerly used for the indigo contained in it, it is at present only employed for the purpose of exciting fermentation in the indigo-vat ordinarily used by the woollen dyer, which is therefore termed the "woad-vat".
According to Wendelstadt and Binz (Ber., 1906, 39, 1627) woad contains two distinct micro-organisms, one of which under suitable conditions appears to be able to reduce indigo.
Spurious woad was sometimes prepared from the leaves of the rhubarb, cabbage, etc., but these products were very inferior to the true woad.
The colouring principle of woad leaves, considered by Schunck to be identical with that present in the Indigoferæ, is now known to be a distinct substance. This has not been isolated in a pure condition, but in its general reactions resembles indoxylic acid (see INDIGO, NATURAL).
Other Literature. - Chevreul, J. Pharm. Chim., 1808, 66, 369; 1817, 350; Ann. Chim. Phys., 68, 284; Gilbert, Annalen, 41, 245; 42, 315; Trommsdorff, J. Pharm. China., 19, 93; Paris, Mus. Hist. Nat. Ann., 18, 251.
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