.590 



BLEACHING. 



Method of 

 rendering 

 alkali caus- 

 tic. 



Bleaching, this is a mistake. When the alkaline ley is rendered 

 <r m r completely caustic, nothing more is necessary but to 

 reduce the quantity of the ley used. By this means 

 the linen cloth will not be too severely acted upon. 

 This process is now carried into effect by the more 

 intelligent bleachers ; and at least one-third of the 

 alkali they formerly used is thus saved. 



In order to render the alkali sufficiently caustic, 

 the following process may be followed : To two 

 parts of potash, dissolved in hot water, add one part 

 of fresh slaked lime, finely pulverised. After the 

 lime is added, make the mixture boil ; taking care, 

 that it is agitated by an iron rake, to keep it from 

 subsiding and fixing on the bottom of the boiler. 

 After it boils, the agitation will be sufficient to keep 

 the lime in suspension ; the ebullition may be conti- 

 nued for two hours, and the lime allowed to sub- 

 side : the clear liquor may then be run off for use, 

 and the precipitated lime well washed with water, 

 until it loses the alkaline taste. The washings may 

 be kept for making fresh alkaline solutions. 



On examining the quicklime which has been used, 

 it will now be found in the state of a carbonate ; 

 having, by its superior affinity for carbonic acid, 

 deprived the potash of this principle, which will con- 

 sequently be found nearly in the caustic state. 



From the experiments of Mr Kirwan, as narrated 

 above, it will be seen, that the power of caustic pot- 

 ash, in dissolving the colouring and resinous matter 

 contained in linen yarn, is at least double the power 

 which it possesses when in the mild or carbonated 

 state. This agrees also with the experience of every 

 well-informed bleacher. Hence at least one-half of 

 the alkali will be sufficient when used in the caustic 

 state, when put in opposition to the quantity which 

 will be required when in the mild state. 



As having the alkaline ley nearly of the same 

 specific gravity, is of considerable importance to the 

 bleacher, the hydrometer is generally used for as- 

 certaining its strength. Formerly this useful instru- 

 ment was constructed on no fixed principle, so that 

 when one of them was broken, another could not be 

 procured made to the same scale. This difficulty is 

 now overcome, and the instrument may be had from 

 Mr William Tweedale, of Glasgow, with invariably 

 the same scale. The principle on which he con- 

 structs these hydrometers, is, that the scale com- 

 mences at 1, and every degree indicates .005 of spe- 

 cific gravity. Hence, supposing the alkaline ley to 

 indicate '20 degrees on the scale of the hydrometer, 

 its specific gravity would be 1. + 20 X .005=1.100, 

 the specific gravity of water being unity. A com- 

 plete series of these, from No. 1 to 6, indicate the 

 specific gravity of fluids from distilled water as 0, 

 to sulphuric acid 2., the heaviest liquid known. 

 These instruments are now used in most parts of the 

 united kingdom. 



From the increase and variable price of potash, 

 and the dependance of Great Britain on foreign na- 

 tions for this necessary article, it is of importance, 

 that the expenditure of it be as much reduced as 

 possible. Accordingly, various attempts have been 

 made to recover the alkali from the strongest waste 

 ley which had been used in the boiling of linen 

 cloth. But the methods which have been followed 

 for this purpose, have, in general, been given up, 



Hydrome- 

 ter for al- 

 kali and 

 acids. 



on account of the great expenditure of fuel necessary Bleaching, 

 for evaporating the ley to a proper consistence for " ~~*" "' 

 procuring the alkali. We shali, therefore, take no 

 notice of the methods which have been unsuccessful; 

 but mention one, which to us appears practicable, 

 and which those who arc interested may use with 

 safety. It is scarcely necessary to observe, that the 

 alkaline ley must be supposed to be of such value, as 

 to render the recovery of the potash an object to the 

 bleacher. 



At some extensive chemical manufactories, where Recovery 

 it is necessary to evaporate very large quantities of "' waste al- 

 liquid to a given strength, at a small expence ; in * 

 place of evaporating these solutions in iron or leaden 

 boilers, it is found more economical to construct 

 what are called stone boilers for this purpose. These Plate 

 are nothing more than large oblong chambers, the ^. v '* 

 side walls of which are about two feet high, built S g ','* 

 into the ground to prevent them from giving way. ' ' * 

 The outside of the wall is well rammed with tem- 

 pered clay puddle, to prevent leakage. An arch of 

 brick is then thrown over between the walls, which 

 is covered with mortar to retain the heat. Proper 

 openings are, at the same time, left to examine the 

 state of the liquid : these are covered with a plate of 

 iron. At one end of the chamber, a furnace of a 

 sufficient capacity is built, having a breastwork in- 

 terposed between it and the liquid, over which the 

 flame plays. At the other end of the chamber, a 

 vent of sufficient height is built to carry off the 

 smoke. The fire being lighted, the flame plays 

 along the surface of the liquid, which by this means 

 is evaporated. Some of these stone boilers are so ca- 

 pacious as to contain 10,000 gallons. 



In evaporating waste ley for the recovery of the 

 alkali, all that is necessary, after it has been evapo- 

 rated to the consistence of tar, is, to carry it to 

 a reverberatory furnace, of a proper construction, 

 where, the mass being dried, it takes fire, and burns 

 with a vivid flame. So soon as the heat is suffi- 

 ciently strong, the alkali melts, and forms a liquid 

 mass, which is run out of the furnace, by a tap- 

 hole at the side of the furnace, into an old boiler 

 which lias been previously heated, to prevent the 

 inked mass from sparking up, and burning the 

 workman employed. 



On examining the alkali thus procured, it will be 

 found in a state of greater purity than when first 

 used ; because, in the incineration, every particle of 

 the resinous and colouring matter is completely con- 

 sumed, and the carbonaceous matter which it had 

 extracted out of the cloth, reduces any sulphate of 

 potash, which the purest imported alkali always con- 

 tains, to the state of a carbonate. Hence, when the 

 recovered alkali is dissolved and rendered caustic by 

 quicklime, its effects in bleaching will be found 

 equal, if not superior, to the first sort of pot- 

 ashes. 



Another method by which potash, when used in 

 boiling cotton goods, may be freed from a large pro- 

 portion of the impurity which it contains, is, by the 

 application of quicklime to the waste ley in the li- 

 quid state. 



It, to a solution of potash, saturated with the co- 

 loured extract from cotton cloth, a propi r quantity 

 of quicklime be added, and the mixture be well agi- 



