BLEACHING 303 



washed by rinsing three to four times in cold water. A little indigo blue is used to 

 give it a pearly appearance. The use of archil, which hns been mentioned, depends 

 upon the shade of white, so to speak, which is wanted. 7- The silk is now dried by 

 the hydro-extractor first, and then by exposing to a temperature of 85-90. If 

 heavily laden with gums, silk must be dried at a still cooler temperature. In this 

 operation of bleaching, 1 Ib. of good silk loses 4 oz. ; but as it seldom arrives very 

 pure, the usual loss to the pound of silk is 5 oz. 



The first, or simmering operation, mentioned here, is not necessary for the white 

 silk of China. 



The silks intended for the manufacture of blondes and gauzes are not subjected to 

 the ordinary scouring process, because it is essential in these cases for them to 

 preserve their natural stiffness. We must therefore select the raw silk of China, or 

 the whitest raw silks of other countries ; steep them, rinse them in a bath of pure 

 water, or in one containing a little soap ; wring them, expose them to the vapour of 

 sulphur, and then pass them through the azure -water. Sometimes this process is 

 repeated. 



Before the memoir of M. Hoard appeared, extremely vague ideas were entertained 

 about the composition of the native varnish of silk. He has shown that this sub- 

 stance, so far from being of a gummy nature, as had been believed, may bo rather 

 compared to bees'-wax, with a species of oil and a colouring-matter which exist only 

 in raw silks. It is contained in them to the amount of from 23 to 24 per cent., and 

 forms the portion of weight which is lost in the ungumming. It possesses, however, 

 some of the properties of vegetable gums, though it differs essentially as to others. 

 In a dry mass, it is friable and has a vitreous fracture; it is soluble in water, and 

 affords a solution which lathers like soap ; but when thrown upon burning coals, it 

 does not soften like gum, but burns with the exhalation of a fetid odour. Its solu- 

 tion, when left exposed to the open air, is at first of a golden yellow, becomes soon 

 greenish, and ere long putrefies, as a solution of animal matter would do in similar 

 circumstances. M. Board assures us that the city of Lyons alone could furnish 

 several thousand quintals of this substance per annum, were it applicable to any use- 

 ful purpose. 



The yellow varnish is of a resinous nature, altogether insoluble in water, very 

 soluble in alcohol, and contains a little volatile oil, which gives it a rank smell. The 

 colour of this resin is easily dissipated, either by exposure to the sun or by the action 

 of chlorine : it forms about one fifty-fifth of its weight. 



Bees'-wax exists also in all the sorts of silk, even in that of China ; but the whiter 

 the filaments, the less wax do they contain. 



M. Eoard has observed that, if the silk be exposed to the soap-baths for some time 

 after it has been stripped of its foreign matters, it begins to lose body, and has its 

 valuable qualities impaired. It becomes dull, stiff, and coloured in consequence of 

 the solution, more or less considerable, of its substance ; a solution which takes place 

 in all liquids, and even in boiling water. It is for this reason that silks cannot be 

 alumed with heat ; and that they lose some of their lustre in being dyed brown, <i 

 colour which requires a boiling hot bath. The best mode, therefore, of avoiding 

 these inconveniences, is to boil the silks in the soap-bath no longer than is absolutely 

 necessary for the scouring process, and to expose them in the various dyeing operations 

 to a temperature as moderate as may be sufficient to communicate the colour. When 

 silks are to be dyed, much less soap should be used in the cleansing, and very little 

 for the dark colours. According to M. Eoard, raw silks, white or yellow, may be 

 completely scoured in one hour, with 15 Ibs. of water for one of silk, and a suitable 

 proportion of soap. The soap and the silk should bo put into the bath half an hour 

 before its ebullition, and the silks should be turned about frequently. The dull silks, 

 in which the varnish has already undergone some alteration, never acquire a fine 

 white until they are exposed to sxilphurous acid gas. Exposure to light has also a 

 very good effect in whitening silks, and is had recourse to, it is said, with advantage, 

 by the Chinese. 



Baume contrived a process which does not appear to have received the sanction of 

 experience, but which may bo a guide in the right way. He macerates the yellow raw 

 silk in a mixture of alcohol at 36 (sp. gr. '0837) and one thirty-second part of pure 

 muriatic acid. At the end of forty-eight hours, it is as white as possible, and the 

 more so, the better the quality of silk. The loss which it suffers in the menstruum, 

 is only one-fortieth ; showing that nothing but the colouring-matter is abstracted. 

 The expense of this menstruum is the groat obstacle to Baume's process. The alcohol, 

 however, might be in a very great measure recovered, by saturating the acid with 

 chalk, and redistilling. 



