Bleaching

BLEACHING. The art of freeing cloths and various other substances from their natural brown or dusky tinge, and rendering them perfectly white. The most ancient, and at one time the only known, method of bleaching linen, or cotton cloths, consists in frequently wetting them, and exposing them upon the grass to the rays of the sun • the powerful action of which in the destruction of colours is well known. This process, which is distinguished by the term of " Grass bleaching," has, however, been nearly superseded by another termed " Gas, or Chemical bleaching," founded upon one of those brilliant and useful discoveries, by which modern chemical science has so shed itself, and rendered such service to the arts of life. Before • to describe the new, (and now the ordinary process,) we shall give a brief descrip tion of the method of grass bleaching, as, although not generally practised, it is still in use in some parts. The details of the process vary of course with the nature of the goods; but the following is the process for bleaching flax-yarn, which constitutes an important branch of business. The first operation, called steeping, consists in immersing the brown yarn in hot water, or in allowing it to macerate in cold water, or in alkaline ley. This occasion. a kind of fermen tation, which loosens the saliva employed in spinning the yarn, and so far separates the other impurities attached to it, that the whole may be easily removed by washing in river or spring water. The next operation is that of bucking, or bWing in an alkaline lye, Am. which the skeins are exposed on the grass, for two or three weeks, which latter operation is called erofting. These alternate operations of bucking, washing, and are generally repeated four or five times, each time lessening the strength of the alkaline solution in which the bucking was performed. The next is that of aeowwg, which, as more anciently practised, consisted in soaking the yarn in milk, which had become acidulous by age, which was usually employed, for the first time, immediately after the fourth or fifth bucking. In this liquor, which was tech nically called the first sour, the goods generally lay for three weeks, or until such time as the scum began to crack and subside, when they were usually taken out and submitted to a repetition of the processes already described. Thus, whenever the goods had been once soured, the operations of bucking, washing, scouring, and crofting, were repeated in regular rotation, until the yarn came to a good colour, and was esteemed perfectly clear. These tedious operations have been much shortened by substituting very dilute sulphuric acid for the sour milk. This improvement (suggested by Dr. Home,) so much accelerated the process, that one souring by sulphuric acid may be performed in from 12 to 24 hours ; whilst every souring by the milk process required from two to six weeks; and the whole process may by this means be completed in four months, which before required seven or eight months. We shall now proceed to give a slight history and description of the new system of bleaching, founded upon the property which chlorine pos sesses, of rapidly destroying vegetable colours. By this system, (which fur Dishes one of the most beautiful illustrations of the immense benefits which science may render to the useful arts,) the practice of bleaching is con ducted with a degree of precision before unknown, and with the most surpri sing expedition. For the discovery of chlorine, we are indebted to Scheele, who, in the year 1774, first formed it by art, and afterwards ascertained its powers in destroying vegetable colours. But the first person who made ments upon this gas, with a view to its application in the arts, was Mr. Ber thollet, who, in the Journal de Physique, for June, 1785, and again in the number for August, 1788, explained the nature of its action on vegetable colours, and suggested how it might be applied with advantage to the process of bleaching. The subject soon attracted the attention of various scientific persons and enterprising manufacturers, and numerous establishments were formed, in which the process of Berth°Het (modified by subsequent discoveries,) was adopted. Amongst the first to introduce and perfect the new process in this country, were Professor Copland, the celebrated Watt, and Mr. Henry, of Manchester. Mr. Watt, so early as 1787, had introduced it in the bleaching field of Mr. Macgregor, at Glasgow ; and in his first attempt, he bleached 500 pieces of cloth ; and Mr. Henry, in the year 1788, published an account of the process, as practised by himself, which account comprehends every thing at this time known respecting the use of chlorine gas in bleaching, excepting the condensation of the gas, by means of lime. At the first introduction of the new process, the chlorine was employed in the state of gas, but this method was found to be attended with many inconveniences ; the fumes occasioning consi derable annoyance to the workmen, and the texture of the cloth being frequently injured by the too great energy of the gas. It was also found extremely diffi cult to expose all the surfaces equally to its action, without which no perfect bleaching can ever be effected. The first remedy for these inconveniences consisted in condensing the gas in water, and subsequently in a solution of potash, which imbibed the gas more readily than water alone, and formed a more concentrated liquor. This latter process was invented by some manufacturers at Javelle, whence the liquid was named—" Liquer de Javelle." In the year 1798, Mr. Tennant, of Glasgow, took out a patent for a new bleaching liquor, which consisted of a solution of chlorine of lime, instead of oxymuriate of potash, which, besides being equally efficacious with the former for general purposes, has the advantage of being much cheaper. It is not, however, applicable where cottons are subsequently to be dyed with madder : for bleach ing these, the oxymuriates of potash or soda must be employed. The peculiar advantages of combining chlorine with lime, or the alkalies, consists in the cir cumstance that the saline solution gives out the gas gradually to thewhich require bleaching, but does not part with it to the atmosphere witr the e same facility. In consequence of this, the operation of bleaching is now not inju rious, nor even very disagreeable, to the workmen; whereas, in the former process, when the gas was merely received into water, it was so freely given out again that no man could long endure to work in it, or even, for any consider able time, to superintend the operations. This advantage of the new process more than compensates for the diminution of the bleaching power of chlorine, which results from the aforesaid combinations. Mr. Tennant's patent for the liquid chloride of lime was afterwards set aside ; but he subsequently obtained a patent for combining chlorine with lime in the dry state. This is a most valuable improvement, the dry chloride being less liable to decomposition than the liquid, and being so much more portable, the smaller manufacturers find it more advantageous to purchase the article of those whose business it is to prepare it, than to establish works for the preparation of the article themselves. The fol lowing is the process as practised in bleaching linen, or cotton cloth, or yarn. The same methods are followed as far as the fourth or fifth bucking, as described in the process of grass bleaching, only good washing is substituted for crofting. The goods are then immersed in a solution of chloride of lime, or of the chlorides of potash or soda, and are then well washed, by machinery, in pure water. They are then taken to the souring vessels, containing a portion of very dilute sulphuric acid, and when taken out of these vessels, are again well washed in water ; and, lastly, they are submitted once more to the alkaline process already described. Linen goods require, at least, three immersions in the solution of chloride of lime, followed by an equal number of alternate immersions in the sours and in the alkaline solutions, carefully and thoroughly washing them in pure water between each of these processes. however, require fewer immersions in the bleaching liquor, which may likewise be more diluted with water for cottons than for linens. By this method of bleaching, linen goods constantly acquire a yellowish tinge ; this, however, is so superficial, that mere exposure to the air for a few days generally removes it. The goods are then finished by boiling them for a short time in a diluted solu tion of pearlash and white soap, which removes the disagreeable odour which would otherwise be attached to articles bleached by this process. Cotton goods do not require crofting, as the yellow tinge, just mentioned, does not appear in them when finished, being removed by the sulphuric acid, although this acid will not remove it from linen goods. But the routine just described, Mr. Parkes (from whose essay on bleaching much of the preceding account is taken,) observes, is not sufficient for bleaching calicoes intended for the best madder work. The following outline of a process adopted by a scientific printer in Scotland, for bleaching calicoes for madder work, or resist work, or for pale blue dipping, was communicated by him to Mr. Parkes, with an assur ance that it may be relied on. The goods, after being singed, steeped, and squeezed, by passing between rollers, are boiled four times, ten or twelve hours each time, in a solution of caustic potash of a spec. gray. from 1.0127 to 2.0156, washing them carefully and thoroughly between each boiling. They are then immersed in a solution of the chloride of potash, originally of the strength of 1.00625, and afterwards reduced, with twenty-four times its weight of water but as the specific gravity alone is not a perfect guide, the bleaching power of the liquid is tested by a solution of indigo of a known strength. In the above preparation, the goods are allowed to remain 12 hours, and are then, by some printers, laid, whilst wet, on the grass, and exposed to the sun and weather for two or three days. From thence they are removed to the sours, made of the

spec. gray. of about 1.0254, at the temperature of 110° Fahr. ; and after lying five or six hours, are taken to the wheel, and washed. The four boiling, in caustic potash, with the washings between each, are then repeated, and the goods, after being again immersed in the diluted chloride of potash, are well washed in pure water, and then rinced in common sours for half an hour. The last process is that of careful washing in clean water, after which they are immediately hung up in the airing sheds to dry gradually. Various articles, besides cloth, such as wax, paper which has become mildewed, &c. are now bleached by means of chlorine, either in the state of gas, or combined with alkalies or alkaline earths. The annexed engraving represents the apparatus employed for the preparation of chloride of potash, or soda, for the use of calico printers. a is the furnace for heating the materials which furnish the chlorine; b a cast iron vessel containing water, and forming a water bath for the recep tion of the still ; c the body of the still made of lead, and the upper part sur rounded by a deep cup d d formed in one piece with it, and containmg a portion of water into which the head of the still descends to the depth of about six inches, thus forming what is termed a water-joint, which prevents the escape of the gas ; e is the still head descending, as we have just stated, into the water; in the cup f is a bent funnel, through which the acid is poured into the still ; g an agitator for stirring the materials in the still, working through an air-tight aperture ; and h the eduction pipe, by which the gas passes into i, an inter mediate vessel, partly filled with water, and designed to arrest any uncombined muriatic acid which may occasionally rise from the still during the process ; k a safety tube ; and 1 the pipe which conveys the gas into m, the large receiver made of lead, and charged with the alkaline solution ; n the agitator for con stantly stirring the alkaline solution ; this is necessary to promote the absorption of the gas ; and, in large works, the agitator is moved by power from a steam engine; o an opening for filling the receiver, occasionally cleaning it out, &c.; p discharge cock for drawing off the saturated solution. The junction of all the various pipes and openings are rendered airtight by water-joints. At the first introduction of the new process, the chlorine was obtained by distilling muriatic acid upon the black oxide of manganese ; but it is now procured in a simpler and more economical manner, by mixing together black oxide of manganese, common salt, and diluted sulphuric acid ; various proportions are used by different manufacturers : Mr. Tennant recommends equal weights of salt, oxide, and acid, and a quantity of water equal to the measure of acid. Silk and woollen goods being animal productions, different processes are employed in bleaching them. The colouring principle of silk being resinous, M. 13aume has proposed a process for extracting it by digesting the silk in alcohol acidulated by muriatic acid, but the ordinary method of bleaching silk is the following. The silk being still raw, is inclosed in a linen bag, and boiled in a solution of soap for two or three hours, the bag being frequently turned. It is then taken out and beaten, and next washed in cold water ; and, after being slightly wrung, it is a second time put into the boiler filled with cold water, mixed with soap and a little indigo, which gives it that bluish cast commonly observed in white silk. When the silk is taken out of this second water, they wring it hard with a wooden peg to press out all the soap and water ; after which they shake it to untwist it, and separate the threads. It is then suspended in a kind of stove constructed for that purpose, in which sulphur is burning, the vapour of which gives the last degree of whiteness to the silk. Woollen cloths are sometimes bleached by simply scouring them with soap and water after the operation of the fulling mills, and sometimes by sulphuric acid es, which is effected as follows : The stuffs are first well washed and cleansed in river water, and then put upon poles to dry. When half dry, they are exposed to the vapour of burning sul phur, or sulphurous acid gas, in a very close stove ; the gas gradually adhering to the surface of the stuff, renders it beautifully white. An improvement upon this method is to condense the sulphurous acid in water, and immerse the stuffs therein; by which means the acid acts more equally over the whole surface, than when in the state of gas. The gas may also be obtained by digesting sul phuric acid upon chopped straw, saw-dust, or other carbonaceous matter, in a retort, and the gas may be condensed by an apparatus similar to that used for condensing the chlorine gas. High pressure steam has been lately employed instead of chlorine for bleaching cloths. It is said that this method of bleaching has long been practised in the east, but Chaptal is the first writer who recom mended it to the European bleacher; and Mr. S. Wright has taken out a patent in this country for an apparatus for washing and bleaching upon this principle, which apparatus is represented in the annexed engraving. The goods to be bleached are first packed closely into a conical vessel, through which the steam is caused to pass for a while ; the steam is then made to force an alkaline solu tion through the goods, to remove the impurities and colouring matter (which operation is repeated as often as may be necessary) ; hot water is next im pelled through the goods to remove all the alkaline matter ; and, lastly, steam of a high pressure is forced through to expel the water, by which the goods are left in nearly a dry state, and perfectly clean. a is a copper vessel, formed as ft ustrum of a cone, at the lower part of which is a perforated false bottom, or grating, and below this the real bottom, from which a pipe descends. The articles to be operated upon having been previously laid in water and rubbed with soap, are to be closely packed in this vessel, the lid of which is then to be screwed down and rendered steam-tight at the junction. In the diagram this vessel is represented enclosed in a jacket, to prevent the radiation of heat ; B is a vessel (also of copper, as well as the other vessels and tubes repre sented), containing soap and water, or the usual alkaline solutions of pearl-ash, soda, 8rc.; C is a pipe leading from a steam boiler, through which is introduced steam at a pressure of 50 lbs. on the inch, which is first to be gradually admitted into the apparatus, by partially opening the stop-cock a, when it passes into the vessel A, where it is allowed to act upon the goods therein deposited, for half an hour ; after which the cock a may be completely opened, and the full force of the steam allowed to operate, first opening the cocks b, c, d, e, when the steam will pass up the pipe D into the vessel B containing the alkaline solution. The pressure of the steam upon the surface of the liquid in this vessel will now cause it to descend through the pipe E into the vessel A, and herein the steam continuing to press, will force the alkaline liquid through the goods, satu rating part, and carrying the dirt and other impurities to the bottom, the liquid passing off through the pipe F into the receiver G underneath. The pressure of steam is next employed to refill vessel B with the discharged alkaline liquor; for this purpose the cocks b, c, d, e, are to be closed, and the cocks f and g, to be opened ; the steam will now pass down the pipe H, and operate with its full force urn G, thereby forcing the liquid up the pipe I I again into B, from whence it is again forced through the goods in the vessel A, repeating the operation as often as may be necessary, in order perfectly to cleanse them. The dirt, and other impurities, being removed, the next process is that of rinsing, which is effected by closing the cocks b, c, d, e, f, g, those at h, i,k, when the steam from C passes up the pipe K into the vessel L, which is filled with clean hot water; the full pressure of the steam being now transferred to the surface of the hot water, forces it through the pipe M and through the goods in the conical vessel A, carrying away all the alkaline and other impurities through the pipe N into the vessel O. The hot liquor in 0 is now to be returned into L, by closing the cocks i, k, h, and opening those at 1, m, when the steam passes down the pipe P, and forces the liquor contained in 0, up the pipe Q Q, again into L, for the renewal of the operation; this part of the process being also repeated as many times as ntay be deemed desirable, which will depend upon the condition of the goods. The next stage of the process is drying, which is effected , by closing all the cocks, except those at d, e, and allowing steam, at a reduced pressure, to pass direct from C into the vessel A again, by which all the water is driven out from the goods, leaving them nearly all in a dry state, the steam passing off tit/Ugh the pipe F, and escaping at R. In this part of the process it is necessary to observe that steam should not be employed at a greater pressure than 20 lbs. on the inch, and that its action should not be pro longed beyond the time necessary to drive off the water. For the bleaching of piece goods, in lieu of the circular-sided vessel A, the patentee recommends one with straight sides, diminishing downwards ; in this vessel the goods having been carefully folded are to be closely packed, and, in addition to the steaming and washing, by means of alkaline solutions, currents of cold air, produced by a blowing machine, are to be admitted through the pipe S, which, it is said, greatly assists in whitening the fabric.