Fig. 379 represents another kind of woollen cloth scouring machine, c,alled a " dolly," suitable only fur certain kinds of woollen goods, e.g. flannels, merinos, 80., the creasing of which during scour ing is of little consequence. It consists of two wooden bowls A and B, set over a wooden box C, 5 to 6 ft. deep, 4 ft. broad, and 5 ft. long, divided, by perforated wooden boards, into four compartments. Each division bolds eight pieces of 50 yds. each, and weighing 20 to 30 lb. ; ti3ey are all stitched together end to end, so as to form one endless band. This band, guided by the rollers D E, is drawn by the two squeezing bowls in the direction indicated by the arrows. When the scouring is finished, an end is loosened and led between the trac tion rollers F, in order to draw the pieces out of the machine. The box C is filled with a mixed solution of soap and soda, so a,s to form a good lather, say 2 lb. soda to 1 lb. soap.
The pieces are run from fifteen to twenty minutes, then taken out, and washed iu cold water in a similar machine; afterwards they are scoured again for fifteen to twenty minutes in a good solution of soap only, then washed, hydro-extracted or mangled, sulphured, and washed. The number of scourings and sulphurings depends greatly upon the quality of the material, and upon the degree of whiteness required.
Goods which have to remain white are blued, by being run through a padding machine containing 1 gall. boiling soap solution (1 lb. soap per gallon of water), 9 galls. water, to 2 pints indigo blue pulp (precipitated from a blue vat). The soap is required to keep the blue in suspension ; with less than the quantity given, the blueing might be uneven. The blueing process precedes the sul phuring. " All wool " goods require about half the given quantity of indigo blue pulp ; they are sometimes blued by using a weak aqueous solution of indigo extract (sulfindigotic acid) after scouring, and, when sufficiently blued, are rinsed in cold water, to wash off the blue adhering super ficially.
The method of bleaching by burning sulphur, as above described, is known as " gas bleaching," to distinguish it from " liquid bleaching," in which are employed a solution of sulphurous acid, or solutions of alkaline sulphites from which the sulphurous acid is expelled by a mineral acid.
Good results may be obtained by using a solution of bisulphite of soda, acidified with hydrochloric acid ; in this, the woollen goods are steeped for several hours and then thoroughly washed. Large
crystals of bisulphite are used, so that their solution and decomposition may take place gradually. A more rational method, however, is that in which the treatment with hydrochloric acid takes place in a separate bath, whereby the sulphurous acid is generated within the fibre, and, being in the nascent state, acts more powerfully upon the colouring matter of the wool. Liquid bleaching, when properly done, gives a more permanent white than gas bleaching, as may be inferred from the remarks, which will presently be made, on the theory of sulphurous acid bleaching.
After sulphuring with gas, the goods should be washed with a weak solution of carbonate of soda or ammonia ; after a final sulphuring, especially if .they contain cotton, with water only ; this is in order to remove the small quantity of sulphuric acid generated in the fibres, and which would otherwise be liable to render the goods tender. The treatment with soda after sulphuring has, also, the effect of restoring the suppleness of the wool, which the latter process has destroyed.
The sulphurous acid acts, by combining with the insoluble colouring matter fted on the wool, to form an e,asily soluble compound, which, as it happens, is in itself almost colourless, but which must nevertheless be washed away if the white is to be permanent ; hence repeated treatments with soda, soap, sulphurous acid, and water, suffice to eliminate every trace of this colouring and colour able matter. Since the last trace of any colouring matter remaining would necessarily be preci pitated by treatment with soda, and thus leave the wool with a yellowish hue, it will be readily understood that the washing after the final sulphuring should be done with water only, if a pure white is desired. Imperfectly bleached woollen goods, from which the colourless compound of colouring matter with sulphurous acid has not been entirely removed by washing, become slightly yellow after lengthened exposure to air.
With respect to the quality of the water suitable for scouring woollen, the freer it is from lime and magneaia salts the better, especially when soap is employed, since in this case insoluble soaps are formed, which cling with great tenacity to the fibre, and have an injurious effect on subse quent dyeing processes. When an alkaline carbonate, or urine is employed, the bad effects are not so great, as, frorn the powdery nature of the precipitates produced, they are more readily removed by washing.