Page:The American Cyclopædia (1879) Volume II.djvu/724

 704 BLEACHING one third, by which their strength also is con- siderably impaired : the finer linens lose only from 12 to 25 per cent. In Silesia and Bohe- mia, where the chlorine process is not adopted, the linens are exposed to a fermenting process, then washed, and steeped in alkaline liquors, with alternate exposures upon grass, which processes are repeated a great number of times for 60 to 70 days ; but to render them properly white, they are afterward passed through a bath acidulated with sulphuric acid, then treated again with the potash lye several times and alternately exposed on the grass, and finally thoroughly cleansed by washing in a revolving cylinder called a dash-wheel. This machine is also employed in the English and Scotch processes for washing the goods with- out subjecting them to unnecessary wear. The frequent repetition of the different processes is rendered necessary by the complete diffusion of the coloring matters through the flax fibres, and their close union with them ; each opera- tion decomposing aud removing in succession small portions only. The discovery of chlorine gas by Scheele in 1774 led to the great im- provement in bleaching of applying this gas to the removal of the colors. The use of it was originally suggested by the French chemist Berthollet in 1785, and explained the next year by him to Watt of Glasgow, who was then in Paris. By Watt the process was soon intro- duced into Britain, the gas being used in solu- tion in water. Its preparation was found to be highly injurious to the health of the workmen, and the fibre of the cloth was weakened by the action of the chlorine. Berthollet improved the process by diluting the aqueous solution with water, and also by saturating with potash a por- tion of the acid. This was the first step toward the preparation of the chloride of lime, which was originally prepared after long continued ex- perimenting by Tennant of Glasgow in 1798. Its first employment was in the form of a satu- rated liquid solution ; but in 1799 he patented the use of the dry chloride of lime. (See BLEACHING POWDEK.) Bleaching by chlorine, as now practised, varies somewhat as applied to the ditferent i'abrics; but a succession of different processes is still adopted, as in the old methods. Thus, in bleaching cotton, there are the preparatory operations of singeing off the loose fibres by passing the cloth over heated cylinders ; then soaking some hours in water, followed by the dash-wheel ; then boiling in lime water, which acts upon the grease, and prepares it for easy removal by the next opera- tion of boiling in water. This is followed by the souring process, which dissolves out the adhering lime, and a succeeding washing pre- pares the cloth for bleaching. This consists in steeping the cloth in a dilute solution of the chloride of lime, which is called the chemicking process. The liquor consists, for every pound of cloth, of about half a pound of chloride of lime and three gallons of water. Souring and washing succeed this, and these processes are repeated, it may be, several times ; altogether they amount, including calendering, to about 25 in number. Though still very complicated, the time of the operation is greatly reduced from that of the old method. In two days is now accomplished what formerly required a whole summer, and the cost of the process amounts to only about 20 cents per piece of cot- ton cloth of 24 yards. Bleaching linens with chlorine, though somewhat more expeditious than the process already referred to in Bohemia and Silesia, is still a tedious operation, and prob- ably is susceptible of great improvements. It involves from 8 to 20 different processes of steep- ing, boiling, washing, souring, &c., with ex- posure upon the grass for from 30 to 60 days. Without this exposure a longer time is required for the bleaching action of the solution of chlo- ride of lime. Rags are bleached for the paper- makers, after being thoroughly washed in the engine and reduced to what is called half-stuff 1 , by soaking them from 6 to 12 hours in a solu- tion of chloride of lime; from 2 to 4 Ibs. of the dry chloride being used for every cwt. of rags. When the rags are strongly dyed, it is often necessary to add some sulphuric acid (half the weight of the bleaching powder), and cause the mixture, with the rags placed in it, to re- volve for some time in a tight cylindrical vessel, till the chlorine evolved has removed the colors. This process is followed by thorough washing. Wool requires a thorough preparation called scouring, to free it from the soapy and waxy matters exhaled from the skin of the sheep. Weak ammoniacal lye is found efficient for this purpose, and this is obtained by boiling putrefied urine with four to eight times its quantity of soft water. The wool is steeped and well washed in a warm bath of this liquor, until all the impurities are converted into soapy matters and removed by rinsing in clean water. Caustic soda is sometimes used instead of am- moniacal liquors. Chlorine cannot be employed to bleach animal fibre, because the nitrogen they contain causes them to become yellow, and sulphurous acid is the agent which is gen- erally used instead. Bleaching by sulphurous acid depends upon the production of colorless sulphites, the decomposition of which, how- ever, by alkalies or by prolonged exposure, will allow the color to reappear unless they are re- moved. This is accomplished by thoroughly washing the goods after the application of the acid. Woollen materials are generally bleached by hanging them in a moistened state in close chambers and passing the vapor of burning sul- phur over them; sometimes, however, a solu- tion of the acid in water is used. After sul- phuring they are washed and exposed to the air. The process may be briefly described as follows: 1. They are immersed three times in a bath composed of 24 Ibs. of carbonate of soda, 6 Ibs. of soap, and 130 gallons of water, at a temperature of 105 F. The bath is re- newed after each immersion by the addition of three fourths of a pound of soap. The goods