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.
These are morbid excrescences produced by the puncture of an insect called Cynips gallæ-tinctoria upon the leaves and young twigs of certain kinds of oak trees, more especially those of Quercus infectoria (Oliver), Q. lusitanica (Lam.), growing in the East Indies, Persia, the Levant. If the fully developed nut be broken open it will be seen to contain a central cavity, in which the larva of the insect will be found. As a rule the galls are collected before the larvae are fully developed, and therefore before they have perforated the galls and escaped as mature insects. In this condition they contain the most tannic acid and are known as blue, black, or true nuts. The less valuable or perforated variety are larger and paler in colour and are known as white or false nuts.
Aleppo galls are one of the best varieties on the market, and should contain from 50-60 per cent, of tannic acid. This same oak, the Quercus infectoria, also bears a large gall known as the Apple of Sodom, due to a different insect, which contains from 24-34 per cent, of tannin and has been used for tanning purposes.
Other varieties are Smyrna galls, Austrian, and Hungarian galls, and of these the former are considered best. English oaks yield several species of galls and oak apples, which, however, are not of much value.
Chinese galls are produced by the action of an insect termed the Aphis chinensis on a species of sumach, Rhus semilata. These are hollow and possess very thin walls, but are much larger and more irregular in shape than the ordinary Aleppo variety, moreover, when freshly gathered, they are covered with a very fine down. They are much esteemed owing to their richness in tannin matter, of which frequently as much as 70 per cent, is present. On this account they are largely employed for the manufacture of tannic acid.
Gall-nut extract is employed for mordanting purposes when very delicate shades are required. In addition to tannic acid, all varieties of gall-nuts appear to contain minute traces of ellagitannic acid.
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