Linen bleaching process to which linen is subjected has for its object lbe destruction and elimination from the pure cellulose of the linen fibre all its impurities, the principal of which are insoluble pectic matters, occurring to the extent of 25 to 30 per cent.
Linen is bleached in the form of yarn, of thread, and of cloth. In the case of yarn, it is very usual to bleach it but partially, for reasona.to which reference will be made hereafter. Thread and cloth are generally bleached white. The operations of linen bleaching are, on the whole, very similar to those of cotton bleaching; but the details vary from the latter in some points, and the whole process is very much more difficult and tedious, owing to the larger percentage of natural impuritiea which adhere with great tenacity to the fibre. The process comprises—boiling with dilute caustic or carbonated alkalies, washing, chemicking, souriug, rubbing, and "grassing," or exposing.
Some of the machinery too, as used at the present time, differs from that employed in cotton bleaching. For yarn and thread, it is very usual to have the false bottom of the bleaching kier, or pot, movable and attached by ropes to a crane, so that after draining away the liquor, the whole potful of hanks can be lifted out at once. For washing, Gantert's machine is well adapted. For expressing the water after washing, either the hydro-extractor or the squeezers aro used, preferably the latter. These squeezers are made somewhat differently from those already described, and are shown in Fig. 380. They are provided with endless bands or aprons, on each side of the bowls, for entering and taking off the yarn, which is placed upon the entering band in a thick layer, the hanks overlapping each other. The levers, Src., are arranged so as to give the bowls considerable play up and down.
The chemicking machine, seen iu Fig. 381, consists of a large stone cistern, about 15 to 25 ft. long, 3 to 6 ft. wide, and 3 ft. deep, provided with a framework holding twenty or more square reels, on which the hanks of yarn or thread are hung, their lower ends dipping into the bleaching powder solution. The reels are driven by a series of bevel wheels placed along one side of the cistern, and having reversible gearing. Each reel rests loosely in its bearings, and can be readily detached while the machine is in motion. When necessary, e. g. at the end of the operation, the framework and all the reels can be raised out of the liquor at once, by means of chains attached to the four corners of the frame. With this machine, a whole kierful of yarn or thread is chemicked at once and with great equality ; it is generally run for from one to three hours, and, during the whole of that time, each portion of the hanks is successively exposed to the action of the liquor and of the air.
The machines for souring arc identical with those used for cotton yarn.
With regard to the machines in use for linen cloth bleaching, it rnay be remarked that the wash ing maehines, as described under " madder bleaching," have not been found at all adapted for linen, since the latter does not possess the elasticity of cotton (probably owing to the absence of tbe spiral character of the fibres), and the goods are liable to he "cracked " or torn, especially at the selvages, by a slight increase of tension. Many classes of goods, too, e. g. towelling, are of very uneven
thickness, owing to the divisions between each towel (i. e. the jeep forming the fringe), containing only warp threads, thus differing very much from the woven central portion. This inequality alone would ma,ke the cotton w.ashing machines quite inapplicable, since such goods would readily be torn. The machines in general use are the wash stocks, identical in construction with those described in the bleaching of cotton hank yarn. For many qualities of goods, it is more than likely. however, that success would attend the adoption of slack washing machines, similar to those in use with calico printers. In these machines, the washing trough is divided, by wooden spars or by perforated plates, into compartments, corresponding to the number of nips received by the cloth, so that each compsrtnaent can hold several yards of slack cloth before it is drawn again between the squeezing rollers.
The chemicking and souring of linen cloth consists, as a rule, in merely steeping the goods in stone cisterns containing the liquors, and then lifting them, after nine hours, on to drainers placed over a portion of the cistern. A preferable method, however, is to use pumps, and cisterns with false bottoms a,nd wells below, as mentioned in " market bleaching," so that the solutions can be circulated through the gooda, and a more even action of the reagents be obtained. A more recent and approved plan is to use a machine similar to the slack washing machines just referred to, with a continuous flow of liquor, as described in " madder bleaching." A special feature in linen cloth bleaching establishments is the " rubbing machine," a repre sentation of which is given in Fig. 382. It consists of a pair of heavy wooden boards A B, about 2 to 3 yds. long, 1 ft. broad, and 3 in. thick, resting on each other, and one of which is moved lengthwise to and fro, by means of a crank shaft D, while the pieces are led laterally between them. Where the pieces pass between the boards at c, are fixed pot-eyes, and the boards have their rubbing surfaces corrugated in order to increase the friction.
' By the term " grassing," or exposing, is understood tho spreading of the goods in a, field to subject them to the influences of air, light, and moisture. This is a characteristic of the present general method of linen bleaching ; practical experience has proved that it cannot conveniently be dispensed with.
The following is a