RESEARCH AND THE COAL-TAR DYE INDUSTRY 413 



The Fourth Period : 1897 AND ON - The Technical Syn- 

 thesis of Indigo, New "Vat-dyes"; Sulphur Colours 



From 1880 onwards there was one question overshadowing 

 all others in importance, namely the possibility of synthesising 

 indigo from coal-tar products. 



It will be recalled that in 1826 Unverdorben had prepared 

 aniline for the first time by distilling indigo, but all attempts 

 to reverse the process, if any had been made, were fruitless. 



Adolph von Baeyer, whilst working at Berlin University 

 from 1 866-1 870, had proved the constitution of indigo and 

 had indeed synthesised the dye by the reduction of isatin, which 

 was however of no practical importance, since the latter was 

 only obtainable from indigo itself. 



Nevertheless the fact that the annual production of indigo 

 in those days was some 4,000 to 5,000 tons a year, worth 

 £4,000,000 to £5,000,000, convinced chemists that the problem 

 was one well worth attempting to solve. 



The final impulse undoubtedly came from the synthetic 

 production of alizarine during the years 1 867-1 870, where for 

 the first time a naturally occurring dye was produced in the 

 chemical works from coal-tar, and thus encouraged chemists 

 to attempt the yet more difficult problem of indigo. 



From 1870 to 1880 renewed attempts were made to work 

 out a commercially practicable method for synthesising indigo, 

 but without success. 



In 1880 a step was taken which could only have happened 

 in Germany and where the boards of directors were composed 

 largely of able and far-seeing chemists : the two great firms 

 of the Badische Company at Ludwigshaven, and Meister, Lucius 

 and Briining at Hoechst joined forces in order to attack the 

 problem systematically, and entered into an agreement to 

 carry on researches conjointly on the commercial synthesis of 

 indigo, sharing expenses and results. 



They acquired the patent from von Baeyer for his synthesis 

 of indigo from toluene via ortho-nitro-benzaldehyde, but the 

 method was found to be impracticable on a commercial scale 

 although a small amount appears to have been made this way. 



After a decade of unsatisfactory results, which would have 

 discouraged any firms not possessed of a great degree of 

 optimism founded on the highest chemical perception, they 



