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.
Water.
Pure water in large quantity is necessary for the manufacture of natural indigo (Crookes, "Handbook of Dyeing and Calico Printing," 1874; "Indigo Manufacture," Bridges-Lee, 1892; Rawson, Report on the Cultivation and Manufacture of Indigo, 1902). When such is not available, Bridges-Lee recommends its purification, if hard, by treatment with lime water, and should much organic matter be present by the use of permanganate. Rawson also lays stress on this point, and recommends a similar method of procedure. In special circumstances the employment of aluminoferric is advisable (Bergtheil, 1909). As the duration of the fermentation varies with the temperature of the water, Rawson recommends, in case this should be lower than 90° F., a preliminary heating in the reservoir.
Fermentation.
The duration of this process is given by Crookes (loc. cit.) as nine to fourteen hours, according to the prevailing temperature of the water, whereas Georgievics (Der Indigo, 1892) suggests eighteen hours when the external temperature is 35,6° C. In very hot weather the fermentation is completed in six hours. According to Rawson (loc. cit.) when the temperature of the fermentation vat is from 90-92° F. a twelve hours' steeping gives the best result in the case of the I. sumatrana; whereas Bergtheil (Indigo Research Station, Sirsiah, 1906) is of opinion that a ten hours' fermentation is sufficient. With the I. arrecta the steeping should vary from thirteen to fifteen hours at 90, according to the indican content of the plant. In other respects, according to Rawson and Bergtheil, there is practically no improvement necessary in the steeping operation as carried out in well-managed factories. The addition of such chemicals as mercuric chloride, sodium and potassium carbonates, lime, carbolic acid, formaldehyde, and sugar are not of advantage, although sodium nitrate, which has been employed by planters for many years past, may facilitate the deposition of the indigo in the oxidising vat. On the other hand, the work of Thomas, Bloxam, and Perkin indicates as beneficial the curtailment, as far as possible, of the steeping operation, and the addition of sulphuric or oxalic acid in small quantity to the vat as advantageous.
Hot-water Extraction.
The extraction of the indigo plant with hot water has been employed for many years, and in Bancroft's "Philosophy of Permanent Colours" an account is given by Dr. Roxburgh, dated 1797, of such a method:
" The hot-water process begins to be used over these provinces ... with it they can make indigo when the weather is too cold for the usual process of fermentation, and it gives a more beautiful and lighter indigo.... A more complete and certain extraction of the basis of indigo is effected by subjecting the plant to the action of water heated to about 150-160°F." Bridges-Lee (loc. cit.) claims an advantage by the employment of hot water, and heats the contents of the steeping vat gradually, either by direct fire or steam pipes. It is also well known that the Java planters who have employed the I. arrecta for several years past have favoured a hot-water process, and although the exact details of their methods have not been disclosed, it is certain that sulphuric acid is also employed in the manufacture. Perkin (Chem. Soc. Trans., 1907, 91, 435) refers to samples of Java indigo prepared by three distinct methods, viz. "the new process with hot water," "the new process with cold water," and "the old process in which no chemicals are used ". There can be no doubt that by these hot-water processes the indican is very rapidly hydrolysed by the ferment, and that the indigo eventually produced is of a superior quality. During this hot extraction it appears preferable, as far as possible, to exclude air from the vat by means of a cover, and the necessity in this case is easy to understand because the evolution of carbon dioxide and other gases which act as a protection to the indoxyl during the ordinary process of fermentation, is greatly decreased when operating in this manner (Roxburgh, loc. cit.). Rawson (loc. cit.) who refers to a patent No. 157, 1892, granted to A. Schulte in Hofe, for manufacturing indigo pn these lines, and also to Henly's "heating process of 1888," carried out numerous experiments on this subject with the I. sumatrana. In order to economise fuel the indigo plant was, in the first instance, extracted by the accumulative method; but, contrary to expectation, this did not give such good results as a simple extraction in ordinary vats fitted with perforated steam pipes. He finally concludes, however, that except in wet or cold weather the hot-water system offers no advantage over ordinary steeping carried out under favourable conditions. The indigo made by this method was, however, of better quality (75-77 per cent.) than that made in the ordinary way (50-55 per cent.). In regard to extraction of the plant by means of boiling water, or extraction by steaming, Rawson considers that the cost would be prohibitive.
When the fermented liquid is run into the oxidising vat, the residual plant still contains a small quantity of indoxyl. The question of a second steeping in order to recover this is referred to by Roxburgh as early as about 1797; he considers that a considerable economy would probably be effected thereby; but Rawson's (loc. cit.) experiments in this direction gave an unsuccessful result. Thomas, Perkin, and Bloxam (loc. cit.) suggest that the employment of a slightly acid water for this purpose should be advantageous, and that the amount of indoxyl retained by the plant residue is probably greater than the 5 per cent, (on the total colouring matter) believed by Rawson to be present. The extracted plant, known as "seet," is a valuable manure.
The Oxidation Vat.
Although the oxidation of the fermented liquid was until very recently carried out to some extent by "hand beating," a method practised over a century ago, according to Bancroft (loc. cit.), this operation is commonly effected by machinery. The apparatus is identical with, or very similar to, the "beating wheel," a rimless wheel, the spokes of which are paddles, and which is now very generally employed in India. Geneste in 1888 patented the pumping in of air, and Bridges-Lee (loc. cit.) in 1891 a showerbath arrangement, as improvements in the method of oxidation. Rawson, again (1902, Eng. Pat., 173), proposed to treat the liquid with acid and an alkaline persulphate; but although excellent results were obtained in the laboratory, these were not satisfactory on the manufacturing scale. As the outcome of an elaborate investigation, Rawson considers that the oxidation of the fermented liquid by blowers and compressors is superior to wheel beating, the yield being thereby increased about 20 per cent.
It has long been the custom to facilitate the deposition of the indigo by what were termed "precipitants," and experiments are recorded by Roxburgh, who employed for this purpose ammonia, stale urine, caustic lye, lime water, and potassium ferrocyanide. That such chemicals must be considered to have assisted in the moje rapid oxidation of the indoxyl is certain, and their effect is not tp be confused with the mere settlement of the indigo by the use of slaked lime, as adopted by the Chinese.
In 1894 Coventry patented a process which was based on the employment of lime under certain conditions. The invention consisted in the employment of a special vat intermediate between the steeping and oxidising vats, in which the fermented liquid was treated with lime. A copious precipitate of calcium and magnesium carbonates was thus produced, which on settling carried down various impurities. The supernatant liquid was then oxidised in the usual manner. The indigo thus produced is somewhat contaminated with lime, and the removal of this is subsequently effected by the addition of a certain amount of acid to the "mal" in the boiler. Indigo prepared in this manner is of superior quality, and although not equal to the Java product resembles the latter in containing some quantity of indirubin. According to Rawson a substantially increased yield of colouring matter is given by this process.
Caustic soda added to indigo liquor before oxidising behaves very similarly to lime, and on the large scale gave an increase of 43 per cent, of dry indigo as weighed. Sodium peroxide also gave an average increase of 33 per cent., but on the whole was not so serviceable as caustic soda (Rawson).
The oxidation of the fermented plant extract in the presence of ammonia, first mentioned by Roxburgh (loc. cit.), was patented by Michea in 1876, whereas Geneste in 1889 suggested the use of caustic soda and ammonium sulphate instead of liquid ammonia itself. The use of ammonia is mentioned as beneficial by Georgievics (loc. cit.), and there appears to be no doubt that it is superior to the other reagents which have been employed for this purpose. In its presence the indoxyl is rapidly oxidised to indigotin, and the precipitated colouring matter settles well. The more general employment of ammonia in India has resulted from the work of Rawson, and its use in conjunction with the steam injector blower constitutes the most important improvement which he has recommended to the notice of the indigo planters. The procedure adopted by Rawson consists briefly in connecting the outlet of an ammonia still (containing lime and ammonium sulphate) loosely with the steam blower, so that when in action, ammonia, air, and steam are injected into the vat by means of perforated pipes laid at the bottom of the receptacle. During the operation the temperature rises 10-15°F., and the oxidation is rapidly completed. The employment of ammonia gas and steam in connection with the beating wheel gives also satisfactory results. By these methods Rawson describes increases in the yields of dry indigo, varying from 37-63,8 per cent., and considers that the average increase of colouring matter is about 34 per cent, as compared with that given by the ordinary oxidising process.
On the other hand, Bergtheil (Report of the Indigo Research Station, Sirsiah, 1906, 6) states that the ammonia process effects very little, if any, improvement over ordinary oxidising when this is carried out under optimum conditions of speed, weather, etc.
After the indigo has settled in the vat, the supernatant liquid, or "seeth" water, is run off as completely as possible. This seeth water, as a rule, contains more or less colouring matter in suspension, and it is during this operation that a considerable loss of indigo occurs, which may reach as much as 20 per cent. (Rawson). This, as a rule, is much reduced by using an alkali in oxidising, on account of the readier settlement of the precipitate. Rawson found that filter pressing cannot be employed for recovering the indigo, but suggests treating the "seeth" water with an alkali which causes the suspended indigo to more readily subside. On the other hand, Bergtheil (1909) recommends the employment of alumino- ferric as an aid to the deposition of the indigo precipitate in the oxidation vat.
Final Treatment of Indigo.
According to Bancroft (loc. cit.) it was the practice of some manufacturers in the East Indies to purify their indigo by boiling it with water and fossil alkali (soda), whereas Roxburgh, as well as de Cosigny, recommended also the action of a diluted sulphuric acid. The mere general procedure, until very recently, in India has consisted in merely boiling the semi-fluid indigo paste in a large cauldron, but the addition of dilute sulphuric acid appears now to be generally adopted. According to Rawson the quality of the indigo may be in this way improved 5-10 per cent. At the close of the operation the indigo is allowed to settle, the acid liquid run off, and the precipitate treated with fresh water and again boiled.
The subsequent filtering, pressing, and drying operations call for no special comment. The slow drying of the product appears to be most advantageous, and in this way an indigo of slightly higher percentage than when the mass is dried artificially is obtained. This is accounted for by the fact that certain impurities of the indigo in the presence of moisture undergo gradual decomposition with evolution of ammonia and other gases.
Briggs (Pat. Spec. 292, 1906) devised an apparatus for drying the indigo paste, and simultaneously converting it into powder. An illustration of this machine, essentially a revolving drum, appears in Bergtheil's Report, 1906, 12. Attempts, moreover, are being made to place natural indigo on the market in the paste form (ibid., 1910).
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