LokaoCHAPTER XVIII. Colouring Matters of Unknown Constitution. (Osa artikkelista)

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.

[Rhamnus davurica = Dahurian buckthorn]

Lokao, a green dyestuff of Chinese origin, is, or rather was, since it has been supplanted by artificial colouring matters, met with in commerce in the form of thin laminae of a dark bluish or bluish - green colour. Though formerly employed to some extent in Europe, it does not appear, owing to its extremely expensive nature, to have been extensively used. According to Crookes ("Dyeing and Calico Printing," 429), lokao contains from 21,5 to 33 per cent, of ash, 9,3 per cent, of water, and 61,7 per cent, of colouring matter. It is insoluble in water and the usual solvents, but dissolves in sulphuric acid with a red-brown coloration, and also in alkalis, the solution thus obtained gradually acquiring a brown tint.

According to Hellot (Crookes, loc. cit.) and others, lokao requires for its preparation two distinct plants, the Rhamnus dahurica (Pall.) (hong, pi, lo, chou) and the R. tinctoria (VValldst. and Kit.) (pé, pi, lo, chou), the first of which is said to yield a deep and fast colour, whilst from the latter a weak but brilliant colour can be obtained. According to Rüpe ("Die Chemie der natürlichen Farbstoffe," 1900), the plant bark is extracted with hot water, the extract allowed to stand over-night, filtered, and then treated with potassium carbonate or milk of lime. Cotton yarn is immersed in this mixture, subsequently withdrawn and spread out in meadows over-night and a portion of the daytime, and the operation repeated ten or twenty times. The colouring matter is removed by rubbing the yarn with the hands under cold water, and the precipitate, which settles to the bottom of the receptacle, is washed by decantation, spread on paper and dried in the shade (cf. also Crookes, loc. cit.}.

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According to Persoz, cotton can be dyed in a weak alkaline solution of lokao, and it behaves also as a vat dyestuff, for which purpose it may be reduced by faintly acid or alkaline stannous chloride. The colour thus obtained is blue, and can be converted into green by the subsequent employment of a yellow dyestuff.

In order to dye cotton a green with lokao, Persoz recommends a bath prepared by adding the pigment to a solution of soap, whereas for silk Michel obtained excellent results by employing lokao and alum solution in the presence of lime salts. For the Chinese and other methods of employing this dyestuff, see Crookes (loc. cit.) and Rûpe (loc. cit.).