THE INDIGENOUS DYES OF BENGAL. 33 



This list includes direct cotton dyes, basic dyes, acid dyes and developed dyes, 

 and the fastness varies from the lowest to the highest grade. 



The dyed samples were prepared according to the best European methods as 

 described in modern text-books. 



Determination of Fastness of dyed samples. 



The dyed samples were examined with regard to the following points : — 

 (i) Fastness to light, 

 (ii) ,, to washing with soap, 

 (iii) ,, to alkali, 

 (iv) ,, to acid. 



(7.) Fastness to Light. 



In order to determine the fastness to light the samples were exposed to the full 

 sunlight from sunrise to sunset for many days. Strips of the samples (10 cm. x 1*5 

 cm.) were arranged on white writing paper fastened to plates of glass. The glasses 

 were laid on stools on the roof of a building, so that no shadows should fall on them. 

 The strips were so arranged that half of each could be covered by an opaque screen. 

 The screen could be removed to compare the exposed and unexposed portions and 

 replaced exactly in its original position. All the samples were exposed together, both 

 those dyed with indigenous and those with synthetic materials. Since the time required 

 to produce an appreciable fading depends to some extent on the depth or intensity 

 of the dyeing, in many cases dyeings of different depths were made and exposed ; and 

 in all cases a dyeing with an indigenous material was matched by an equally deep 

 dyeing with some standard synthetic dye, which was exposed under the same condi- 

 tions. The samples were exposed to light at the Civil Engineering Col 1 ege, Sibpur, 

 during the latter half of November 1906, December 1906, and January 1907, viz., on 

 the following days : Nov. 25, 26, 27, 29, 30 ; Dec. 1, 2, 3, 4, 5, 6, 7, 8, 30, 31 ; Jan. 

 1 , 2, 3, 4, 5, 6, 7, n, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21. The samples were examin- 

 ed each day and thus it was determined how many days were required (1) to pro- 

 duce the first sign of fading; (2) to reduce the shade to one-quarter of the original 

 intensity ; (3) to completely bleach the fabric. 



Although the majority of the days could be considered as typical, sunshiny, cold 

 weather days, there were a few days on which the sun was for a time obscured by 

 clouds. These days would have less effect than cloudless days, and their relative values 

 were obtained in the following manner : The Sunshine Recorder at the Alipore Meteoro- 

 logical Observatory (which is only about two miles distant from Sibpur) registered 

 the number of hours of sunshine per diem. The maximum number during this period 

 was 8-i hours and on many days the number registered was a close approximat'on to 

 this. The day on which 8*i hours of sunshine were recorded was taken as the unit 

 '* 1 day of bright sunshine." It was assumed that one hour during which the sun 

 was obscured was only equal to half an hour of bright sunshine, and on this assump- 

 tion the value of each day was determined. These values are given in Table O, 

 overleaf. 



