NA TURE 



[November i, 1900 



brief survey of the processes employed for producing the 

 natural and artificial indigo may be of interest. 

 ~ Indigo is one of the oldest dye-stuffs known, having 

 been used in India and Egypt before the Christian era. 

 Egyptian mummies are sometimes found with wrappings 

 which have been dyed with indigo. The ancient Romans 

 and Greeks were also familiar with this dye-stuff. Pliny 

 the Younger mentions indigo in his writings, and in this 

 connection it is interesting to note that adulteration of 

 commercial articles was even practised in his days, for 

 he states that indigo was at times adulterated with the 

 excrement of pigeons and with chalk coloured with 

 woad, but he says. the pure article may be known by its 

 burning with a purple flame when heated. Indigo was 

 not introduced into Europe until the sixteenth century, 

 and even then, owing to the strong opposition of the woad 

 cultivators,^ it was a long time before it came into general 

 use. Indeed, so strong was the opposition and so great 

 was the influence of the woad cultivators,'that the em- 

 ployment of indigo was prohibited in England, France 

 and Germany, its use in France being in the time of 

 Henry IV. punished by death, it being called " Devil's 

 Food." However, notwithstanding this powerful opposi- 

 tion, the employment of indigo as a dye gradually gained 

 ground until to-day Woad is scarcely cultivated and is no 

 longer employed as a colouring matter, but is used in a 

 certain process of indigo dyeing to cause fermentation 

 and reduce the insoluble indigo blue into soluble indigo 

 white. 



The indigo plant {Indigofera tinctorla) belongs to the 

 natural order Leguminosse. It is obtained chiefly from 

 India, especially from the provinces of Bengal, Madras 

 and Oude. But it is also grown in some parts of Africa, 

 in Java, China, Japan, Central America and Brazil. The 

 land is ploughed in October or November, and the seed 

 sown at the end of March or the beginning of April. The 

 growth is very rapid, and the plant attains a height of 

 about three feet. It is cut for the first time between the 

 middle of June and the beginning of July. Two months 

 later a second crop is taken, but the yield is smaller than 

 that of the first crop. The land on which the indigo is 

 cultivated is frequently very poor, and contains very 

 little nitrogen, yet indigo is grown on the same land from 

 year to year with only very occasional change of cropping, 

 and this in spite of the fact that practically the only 

 manure employed is seet^ i.e. the indigo refuse, leaves, 

 stalks, &c., which have been taken from the steeping 

 vats. Notwithstanding, the crops obtained from year to 

 year do not show much deterioration either in yield or 

 quality. Dr. D. A. Voelcker, in his report on Indian 

 agriculture, suggests that since indigo belongs to the 

 order Leguminosae, and it has been shown that certain 

 legumes are able to absorb atmospheric nitrogen through 

 the medium of nodules which form on the rootlets, that 

 perhaps the indigo plant obtains the nitrogen it requires 

 in this manner. The writer of this article is, however, 

 not aware whether the subject has been investigated. 



The dye indigo does not occur ready formed in the 

 plant, but exists in the form of a glucoside called indi- 

 can. This glucoside was isolated by Schunk. It is a 

 brown, transparent, uncrystallisable syrup, which, by the 

 action of dilute acids, is split up into indigotin (the 

 colouring matter of indigo) and a sugar called indiglucin. 

 A reaction similar to this is supposed to take place during 

 the fermentation process in the production of natural 

 indigo. 



Manufacture. 

 The cut plant is tied into bundles, which are then 

 packed into the fermenting vats and covered with clear 

 fresh water. The vats, which are usually made of brick 

 lined with cement, have an area of about 400 square feet 

 and are 3 feet deep, are arranged in two rows, the tops 

 of the bottom or " beating vats " being generally on a 



1 The colouring matter of woad, Isatis iincioria, is indigo. 



NO. 16 18, VOL. 63] 



lever with the bottoms of the fermenting vats. The 

 indigo plant is allowed to steep till the rapid fermenta- 

 tion, which quickly sets in, has almost ceased, the time 

 required being from 10-15 hours. The liquor,, which 

 varies from a pale straw colour to a golden-yellow, is 

 then run into the beaters, where it is agitated either by 

 men entering the vats and beating with oars, or by 

 machinery. The colour of the liquid becomes greeny 

 then blue, and, finally, the indigo separates out as flakes,, 

 and is precipitated to the bottom of the vats. The indigo- 

 is allowed to thoroughly settle, when the supernatant 

 liquid is drawn off. The pulpy mass of indiigo is then 

 boiled with water for some hours to remove impurities, 

 filtered through thick woollen or coarse canvas bags, 

 then pressed to remove as much of the moisture as- 

 possible, after which it is cut into cubes and finally air- 

 dried. 



Another method is to treat the plant with dilute am- 

 monia or alkalis. This method is said to more com- 

 pletely decompose the indican, and thus to give a larger 

 yield of indigo. 



The value of indigo as a dye-stuff depends upon, the 

 quantity of indigotin which it contains. The percentage 

 of indigotin in the natural indigo varies from 20-90 per- 

 cent. Beside indigotin, natural indigo also contains small 

 and varying quantities of indigo red, indigO' brown andi 

 indigo gluten. The following is an analysis of a good, 

 sample of Bengal indigo :^ 



Indigo blue 

 Indigo red 

 Indigo brown 

 Indigo gluten 

 Mineral matter 

 Water 



61 '4, per cent. 



4-6 ■ „ 



1-5. ,. 



I9'6 „ 



5-7 „ 



Artificial Indigo. 

 After many years of careful and laborious scientific "^ox^i.. 

 artificial indigo is beginning to compete with natural in- 

 digo, and there seems to be but little doubt that, unless 

 the producers of the natural article are able to improve 

 the process of manufacture, in the near future the arti- 

 ficial product will, in all probability, get the upper hand 

 in the struggle. Engler and Emmerling appear to have 

 been the first chemists to obtain artificial indigo. 

 They obtained it by the action^ of zinc dust, 

 and soda lime upon ortho-nitroacetophenon, but 

 the quantity obtained was very minute, and, as the- 

 mechanism of the reaction was not at that time under- 

 stood, it did not much help in paving the way for further 

 research work. For most of our present knowledge of 

 indigo we have to thank von Baeyer, whose work on indigo- 

 may be looked upon as one of the chemical triumphs of 

 the century. So far back as 1868, von; Baeyer obtained 

 indol directly from indigo, and, in the following, year, in 

 conjunction with Emmerling, he prepared this substance 

 by fusing crude nitrocinnamic acid with caustic potash 

 and iron filings ; shortly afterwards they discovered that 

 by the action of phosphorus trichloride, phosphorus and 

 acetylchloride on isatin, a product was obtained, which, 

 when exposed in aqueous solution to the action of the 

 air, gradually deposited indigo ; this method was subse- 

 quently improved. In 1875 Nencki obtained indigo by 

 the oxidation of indol with ozone. But it was not till the 

 year 1880 that any great progress was made in the syn- 

 thesis of indigo. In this year von Baeyer published a 

 series of brilliant researches showing how indigo could 

 be obtained from orlho-nitrocinnamic acid. He showed 

 that when ortho-nitrocinnamic acid is subjected to the 

 action of bromine, ortho-nitrodibromcinnamic acid is 

 obtained, which when treated with alkalis in the cold is 

 converted into ortho-nitrophenylpropiolic acid, and this 

 substance,on being warmed with a dilute solution of caustic 

 soda and grape sugar, or some other alkaline reducing 

 agent, is converted into indigo, the yield compared with 



