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.
Logwood or Campeachy wood is one of the most important dyestuffs, and at the present time is very largely employed. Although for a long time it has successfully competed with the artificial colouring matters, its supremacy as a cotton dye has been affected to a considerable extent by the introduction of the sulphide blacks.
Logwood appears to have been first imported into Europe by the Spaniards shortly after the discovery of America, and has been cultivated in Jamaica from the year 1715. In the time of Elizabeth its employment as a dye was prohibited by Act of Parliament, and large quantities of the wood were burned, because it was said to produce fugitive colours.
It is derived from the Haematoxylon campechianum (Linn.), a tree which belongs to the family Cæsalpiniacæe, and is a native of the warmer part of South America and some of the West India Islands. It also grows in the island of Mauritius, and trials indicate that this wood is of good quality (private communication).
Logwood usually comes into the market in large blocks weighing about 400 lbs. These externally are of a deep brown colour, but internally have a much lighter tint. The best qualities come from Jamaica, Honduras, and St. Domingo, for the true Campeachy wood, at one time esteemed to be the best, is now practically exhausted.
The colouring principle of logwood, hæmatoxylin, was first isolated in a crystalline condition by Chevreul (Ann. Chim. Phys., (ii.), 82, 53, 126), who obtained it by extracting the wood with ether, evaporating the extract, and digesting the residue with alcohol. After distilling off the alcohol the residue was allowed to stand in contact with water, when the haematoxylin separated in crystals. It may also be prepared by similarly treating commercial logwood extract which has been incorporated with a large quantity of sand (O. L. Erdmann, Annalen, 44, 292; J. pr. Chem., 26, 193; 36, 205; 75, 318). For this purpose ether containing water is preferable (Hesse, Annalen, 109, 332). It is, however, more easily obtained from the dark-coloured crusts which slowly separate when concentrated logwood liquor stands for some time in a cool place. The crude mass is ground to a fine powder, extracted repeatedly with ether, the ethereal solution evaporated, and the residue left in contact with water, when dark-coloured crystals separate, which by recrystallisation from water containing a small quantity of sodium bisulphite, may be obtained colourless (W. H. Perkin and Yates, Chem. Soc. Trans., 1902, 81, 236).
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Hummel and A. G. Perkin (Chem. Soc. Trans., 1882, 41, 373) exposed an ammoniacal solution of logwood extract to the air for two or three days. The precipitated ammonia compound of haematein was collected, dissolved in water, dilute acetic acid added, and the mixture digested on the water-bath to dissolve as much as possible of the suspended amorphous hsematein. The clear liquid after partial evaporation deposited crystals of the colouring matter, possessing a yellowish-green iridescence, and having the composition C16H12O6.
According to Mayer (Chem. Zentr., 1904, i, 228), haematein may also be prepared by oxidising haematoxylin in aqueous solution with sodium iodate.
Engels, W. H. Perkin and Robinson (Chem. Soc. Trans., 1908, 93, 1140) passed air for six hours through a solution of 15 grams of haematoxylin, dissolved in a solution of 15 c.c. of concentrated ammonia in 150 c.c. of water. The product of the oxidation was added in a thin stream to dilute acetic acid (250 c.c. of 10 per cent.) heated on the water-bath, when haematein separated in crystals.
Haematein is very sparingly soluble in water and the usual solvents. Alkalis dissolve it readily; ammonia dissolves it with a brown-violet colour, whilst its strong alkaline solution has a rich purplish-blue colour. On exposure to air the colour of these alkaline solutions gradually becomes red and finally brown, the colouring matter being destroyed. By addition of potassium acetate to the boiling alcoholic solution of haematein, the monopotassium salt C16H11O6K is deposited (A. G. Perkin, Chem. Soc. Trans., 1899, 75, 443).
Sulphurous acid or sodium bisulphite solution converts haematein into a colourless addition product, readily soluble in water, but no reduction hereby appears to occur, as on boiling the solution or by addition of acid haematein is precipitated. With zinc and hydrochloric acid or with stannous chloride and caustic soda, a solution of haematein is decolorised, but on standing the liquid regains its former tint.
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Before being used by the dyer, logwood is reduced to chips or powder, and these products are known as "chipped," "rasped," or "ground" wood. When first cut down the wood has a pale yellow colour and contains only hsematoxylin, but during transport or storage this colour gradually deepens owing to a surface oxidation of the haematoxylin into haematein. Formerly, and even to some extent at the present time, it was the practice to submit the chipped or rasped logwood to what is known as the "ageing" process, in order to facilitate the haematein formation. This consists in wetting the wood thoroughly with water, then forming it into heaps, 4 to 5 feet high, in large airy chambers. Very soon the mass begins to ferment, which is indicated by a rise of its internal temperature, the wood gradually darkens in colour, and during the operation the mixture is repeatedly turned over and remixed in order to avoid too vigorous a fermentation and to obtain an even result. Constant attention is necessary during this "ageing" process, for if the reaction is allowed to proceed too far, a portion of the colouring matter is destroyed, and "burnt" or "over-aged" wood is produced. In order to expedite this operation, the use of various oxidising agents has been suggested; a sprinkling of the wood with ammonia has been said to exercise a beneficial result; but, on the other hand, such processes are best avoided, for it is difficult under these circumstances to control the reaction and prevent an over-oxidation.
Latterly this ageing operation has been less in vogue, not only on account of the increased employment of logwood extract, but because it is now recognised that where the mordant employed for dyeing purposes can exert an oxidising action, this preliminary treatment is unnecessary. Formerly, logwood blacks on wool were produced by means of an iron mordant, but this is now largely replaced by the use of potassium dichromate, which provides a mordant possessing the power of converting at least a portion of the hsematoxylin into haematein.
Logwood extract is now prepared in enormous quantity not only in this country but also in Jamaica in the neighbourhood of the logwood plantations, and in the latter case an economy is naturally effected in the carriage of some varieties of the extract rather than that of the more bulky wood to this and other countries. Logwood extract is almost invariably prepared from the "unaged" wood, because whereas haematoxylin is readily soluble in water, haematein is not. On this account, an aged wood is difficult to exhaust. For the manufacture on the large scale, two processes are in vogue known respectively as the "American" and "French" methods. These differ merely in the manner in which the wood is lixiviated, for this is carried out by the French process in open pans with warm or boiling water, whereas in the American process, closed vessels are used in which steam at from 15 to 30 lb. pressure is employed. In both cases the liquid is afterwards concentrated, preferably in vacuum pans, and the product is sold at about 15 Tw. as logwood liquor, at 51 Tw. as logwood extract, or in the solid condition as solid logwood extract. As a rule, it is considered that the extract made at the lower temperature gives the brighter shade.
Certain processes are in use for the "ageing" of "logwood liquor," that is the conversion of the dissolved hsematoxylin into hsematein. For this purpose the following methods have been suggested : (a) treat the logwood liquor with bleaching powder solution; (b) half neutralise with alkali or ammonia and blow air or oxygen through the liquid; (c) employ air or oxygen for this purpose without the addition of alkali; (d) boil with manganese dioxide (Weldon mud) and filter. Interesting also in this respect is the patented process of Haak (Chem. Zentr., 1905, ii., 867), who treats the extract with sodium nitrite.
A very important product more recently introduced is the socalled crystalline hamatein paste, which is a treacly extract containing in suspension minute crystals of haematein. The details of the preparation of this material, which appears to have been first introduced from Jamaica, although now also manufactured in this country, are kept secret. For experimental purposes, this paste may be employed as the source of a comparatively pure haematein, which can be isolated from it by dilution with glacial acetic acid and subsequent filtration (private communication).
Logwood and its extracts are enormously employed for the dyeing of blacks on silk, wool, and to a less extent with cotton, chiefly in conjunction with iron and chromium mordants. It also enters into the composition of numerous compound shades.
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