Purple of the Ancients(CHAPTER XV. Indole Group.)

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.

Kappaleita lisätty luettavuuden helpottamiseksi. // Paragraphs added to help reading online.

The ancients derived their purple from certain molluscs or seasnails, the Purpura hoeinastoma, known to Pliny as Buccinum, and from the Murex brandaris, called by Pliny Purpura. At Athens and Pompeii, large quantities of the shells have been discovered lying in heaps close to ancient dyeworks. These molluscs are to be ound throughout the whole of the Mediterranean, and indeed, in the sea in numerous parts of the world varieties exist which may be employed for dyeing purposes. Two sorts of purple, known as Tyrian and Byzantium purple, were recognised by the ancients, the former possessing a redder tint than the latter.

From the observations of Cole (Phil. Trans., 1685), Reaumur (Mem. de 1'Acad. Royale des Sciences, 1711) and Bancroft ("Philosophy of Permanent Colours," i, 120, 1813), it appears that the colour-producing secretion, which resembles pus in appearance and consistence, is contained in a small whitish cyst or vein, placed transversely under, but in immediate contact with the shell, and near the head of the animal. This pus-like matter, either diluted with water or undiluted, on being applied to bits of white linen or calico, and exposed to sunlight, rapidly changes its colour, passing from yellow, through light green, deep green, and "watchet blue," to purplish-red or crimson.

To produce this change of colour the light of the sun is essential. It is effected more rapidly by the direct action of the sun's rays than by that of diffused light, but it does not take place in moonlight or in artificial light. If the linen or other fabric to which the secretion has been applied is kept in the dark, it remains unchanged, but when exposed to the sun it becomes purple, even after the lapse of years, though a little more slowly than at first. The metamorphosis which the change of colour indicates is not sensibly promoted by heat. It proceeds in a vacuum and in hydrogen or nitrogen gas as speedily as in air on exposure to light.

The colour produced is remarkably stable, resisting the action of soap, alkalis, and most acids, being destroyed only by nitric acid and chlorine (see also Bizis, Journ. de Ch. Med., 1835, 10, 99, and A. and G. de Negri, Gazz. chim. ital., 1875, 437). Schunck (Chem. Soc. Trans., 1879, 35, 591) who examined the Purpura capillus, which he procured from the rocks at Hastings, finds that the colouring matter (punicin) is quite insoluble in water, alcohol, or ether, sparingly soluble in boiling benzene or boiling glacial acetic acid, and readily soluble in boiling aniline, giving a solution which is at first green, but as it approaches saturation becomes purplish-blue. At this point it shows a broad, well-defined absorption band, beginning near C and extending beyond D; but as the solution cools, depositing the substance contained in it, the colour changes to green, and the band becomes gradually narrower, until it occupies the space midway between C and D, and then disappears.

The colouring matter as deposited from the solution in aniline is seen, under the microscope, to consist of star-shaped groups of irregular crystalline needles, which, when very thin, show by transmitted light a purple colour. Punicin is soluble in oil of vitriol, giving a dirty purple colour, and showing a broad ill-defined absorption-band between D and E, the green and blue of the spectrum being much darkened. On heating the solution slightly, or allowing it to stand for some time, the colour changes to a bright bluish-green and it now shows an absorption-band in the red. Punicin is also sparingly dissolved by a hot alkaline solution of stannous oxide, and the solution on exposure to air becomes covered with a blue pellicle. Punicin may be sublimed, giving crystals which show by reflected light a semi-metallic lustre, like that of sublimed indigo-blue (Schunck, loc. cit.). Witt (Technologic der Gespinnstfasern, 1888) expressed the opinion that the colouring matter yielded by these molluscs was an admixture of indigotin with a red colouring matter not so fast to light.

Friedlander (Annalen, 351, 390; Ber., 1906, 39, 1060) has examined the dye yielded by the Murex brandaris and Murex trunculus which he obtained from the zoological station at Trieste. Letellier (Comptes rend., 1891, 109, 82) had observed that, in addition to the colouring principle, organic sulphur compounds were also present in these glands, and it suggested itself, therefore, to Friedlander as possible that the "purple of the ancients" might in reality consist of the thioindigotin which he had lately discovered.

To isolate the colouring matter the glands of the molluscs were spread out as thinly as possible upon filter paper, which was then exposed to the sunlight for half an hour. The highly coloured product was now immersed in diluted hydrochloric acid (1:1), the mixture evaporated to dryness on the water-bath, the residue extracted with hot water, and washed with alcohol and ether. In this manner a product consisting only of cellulose and the pure colouring matter was obtained, and the latter could be readily removed by extraction with boiling anisole, from which it separates in the crystalline condition. Finally, it was recrystallised from nitrobenzene. It consists of dark violet crystals which possess a coppery lustre, dissolves in hot, high-boiling solvents with a blue-violet colour, yields a sublimate on heating, and in numerous respects resembles the colouring matters of the indigo group. Analysis indicated the absence of sulphur, although nitrogen was found to be present. The absorption spectrum is similar to that given by indigotin; but, on the other hand, it is distinguished from this latter colouring matter by its sparing solubility, and by giving with cold concentrated sulphuric acid a reddish-violet coloration. With fuming sulphuric acid it yields a soluble blue sulphonic acid, and when reduced in alkaline solution forms a pale yellow liquid, from which, when exposed to air, it separates as a reddish-violet precipitate. In a later paper by the same author (Ber., 1909, 42, 765) some slight modifications of the method for the isolation of the dye from the Murex brandaris are given, and it is shown that in this manner about 12,000 molluscs are required for the preparation of 1.4 grams of the substance. This colouring matter contains bromine, and by a comparison with the synthetic dye, there can be no doubt that it is in reality 6:6'-dibromoindigotin.

Further investigation has indicated that in addition to this dibromindigotin another dye of a bluer shade, also containing bromine, but possessing a greater solubility in organic solvents, is produced from the Murex brandaris. It contains more carbon and less bromine than dibromindigotin, but its constitution is as yet undetermined (Friedlander, Chem. Zeit., 1911, 640).