BLEACHING is the art of whitening linen cloth, thread, cotton, &c. In the present advanced state of the linen and cotton manufactures of Great Britain and Ireland, the art of bleaching is one of the most interesting and important. Its object is to reduce flax, cotton, wool, or the threads or cloths manufactured from them, to a state of perfect whiteness. To attain this end, oils, metallic oxides, earthy impregrations, resins, and other animal, vegetable, or mineral particles, containing any colouring matter, must be discharged from the texture of the substances manufactured.
The process of bleaching is divided into five parts, viz. 1. Steeping and milling; 2. Bucking and boiling; 3. Alternate watering and drying; 4. Souring; and, 5. Rubbing with soap and warm water, starching and blueing. By the first of these methods, the cloth is in a great degree freed from its superficial foulness, and is rendered more pliant and soft. The second process is the most important of the whole. Its object is to loosen and carry off, by means of alkaline lies, that particular substance in cloth, which is the cause of its brown colour. The operation of alternate watering and drying is as follows: After the cloth has been bucked, it is carried out to the field, and frequently watered, during the first six hours. For, if in the course of that time it be allowed to dry, while strongly impregnated with salts, the latter, by approaching closer together, and being assisted by a degree of heat which increases in proportion to the dryness of the cloth, act with greater force, and destroy its texture. After this time, dry spots are suffered to appear before it receives any water.
By the continual evaporation which takes places on the surface of the cloth, it is evident that this operation is intended to carry off some impurities that remain after the former process of bucking. This is clearly proved from the fact, that the upper side of the cloth, where the evaporation is strongest, attains to a greater degree of whiteness than the reverse side; and the whole likewise turns much lighter on being exposed to the influence of the sun, air, and winds.
Souring. Every person, who possesses the smallest knowledge of chemistry, is aware that alkaline salts may, by various methods, be converted into absorbent earths. One of these is, frequent solution in water, and again evaporating it. A transmutation, therefore, of these salts must be continually going forwards in the cloth, during the alternate waterings and dryings of the former process. The souring process is sooner completed in cold, than in warm weather; and it is now experimentally ascertained, that vitriol is preferable to milk sours in bleaching.
The next is, hand rubbing with soap and warm water, rubbing-boards, starching, and blueing. After the cloth has been sufficiently soured, it is washed in the mill, to deprive it of the acrid particles which adhere to its surface. From the mill, it is taken to be washed by the hand, with soap and a water, to free it from the oily particles which could not be disengaged by the milling. Soft soap is preferred to hard, for this purpose, as the latter contains a considerable quantity of sea-salt, which is prejudicial to the cloth.
The management of coarse cloth in this operation is very different from that of fine; for the former, instead of being worked by the hands (a method which would be too expensive), is laid upon a table, rubbed over with soap, and then placed between what are called rubbing-boards, which have ridges and grooves from one side to the other, in the form of teeth.
The starching and blueing, which is the last operation, differs so little from the process employed by laundry-women, that it scarcely requires description. But it often happens, that the cloth, when exposed to dry in the open air, after being starched, is wetted by rain, which frustrates the effects intended by the operation: to remedy this inconvenience, many bleachers employ a dry-house, where the linen may be dried in all weathers.
As bleaching is a process connected with manufactures, and cannot be performed with advantage by private families, it is useless to enter into farther particulars. But the art is still susceptible of improvement; for scarcely a year elapses, which does not produce some new discovery in this useful branch of manufactures. We shall, therefore, content ourselves with communicating a few of such hints as may prove advantageous to the practical bleacher; and with which, we presume, there are many persons still unacquainted.
The new method of bleaching with the dephlogisticated or oxygenated muriatic acid, or spirit of salt combined with manganese, is founded upon the remarkable property which that acid possesses of destroying vegetable colours; and, though various attempts have been made to introduce it into this country, the difficulties or disadvantages attending it have prevented its general adoption. This acid was first applied to the purpose of bleaching, by M. Berthollet; and the particulars of the process are described at length in a treatise on bleaching, published a few years since at Edinburgh; and likewise in the Supplement to the Encyclopædia Britannica.
It is to be regretted, that no exact comparative statement of the difference of expence between the old and new methods of bleaching, has yet been laid before the public; but it is probable that the acid drawn from one pound of salt, will whiten four of linen cloth, without any addition. The expence in this case may appear trifling, but when we compute the vitriolic acid which is employed, and that the residuum is almost useless, it will soon be found to be very considerable; and upon the whole, the advantage may be only in the saving of time: but M. Berthollet asserts, that by this method the texture of the cloth is less injured than by that hitherto practised.
The oxy-muriatic acid is also very generally used for bleaching paper. According to M. Chaptal, blotting-paper, when put into it, is bleached without suffering any injury; and old books, and prints, when soiled in such a manner as to be scarcely distinguishable, have been completely restored to their original state. The simple immersion of a print in this acid, is sufficient to produce that desirable effect; but with books, some farther precaution is necessary: they should be unsewed, and the adhering leaves carefully separated, that the whole may be equally impregnated.
Mr. Higgins, chemist to the Irish Linen Board, has discovered that the oxy-muriate of lime is, in bleaching, not only cheaper, but in other respects preferable to that of pot-ash. The chemical attraction of the former is somewhat stronger than that of the latter; and, on account of this quality, it does less injury to the cloth. Alternate boilings in solutions of pot-ash, steepings in oxy-muriate of lime, exposure to the action of light, and evaporating water on the green, are found to complete within six weeks, at little more than half the expence, what otherwise cannot be performed in less than double the time.
Notwithstanding this great improvement, Mr. Higgins was anxious to diminish still farther the expence attending the process of bleaching. Convinced that the mixtures of sulphur with soda, are detergents, or cleansers of the most powerful kind, he was naturally led to conjecture, that lime, which, in other respects, possesses properties nearly similar to those of the fixed alkali, might also resemble them in the detergent effect of their combination with sulphur. He made trial: a sulphuret of lime, composed of four pounds of sulphur added to twenty pounds of lime, and diluted in sixteen gallons of water, formed a solution which answered cold, just as well for the bleaching of linen, as the boiling solution of pot-ash. In consequence of this experiment, he recommends, that linen, after being perfectly cleansed from the weaver's dressing, be immersed alternately in solutions of sulphuret of lime, and of oxy-muriate of lime, namely, six times in each. By this method, linen may be completely bleached, and with a considerable saving of expence. In Ireland, it is at present almost generally adopted.
The same process, with certain convenient modifications, yet always managed upon one common principle, is applicable to the bleaching of linen, cotton, silk, and wool. It has likewise been reduced to practice in France; but in a manner less simple and skilful than in Britain and Ireland.
We shall conclude this article by abstracting the patent lately granted to Mr. Turnbull, for an improvement in the common process of bleaching cotton, or linen pieces: Take any kind of earth which is easily mixable with water, such as clay, marl, or Fuller's earth, or if that cannot be had, any kind of soft mud or the like, which is put into a boiler to evaporate the moisture, dried, again mixed with water, and passed through fine sieves. This powder is then mixed with quick-lime, which is slacked in the earthy mass, and forms the materials for the several buckings which the cloth is to undergo. The pieces are to be worked in the bucking tubs for a number of times, alternating this operation with rinsing and souring, as is usual in the long established method, and afterwards exposing them to the air, on the bleaching ground. The only difference in the process here employed, is the admixture of earthy mud, or clay, to the lime, so that the corrosive power of the latter is diminished, and may consequently be used more freely. In the last buckings, pot-ash is also added to the earthy mixture. Hence the patentee's method unites that of fulling with soap, or washing with alkaline lye; and it is very probable, that by such a combination, not only time, but also expence may be saved, as alkali is the most valuable article used in the process.
The latest work published on this subject is, we believe, a treatise written by M.Pajot des Charmes, of which a translation was lately published in London, by Messrs. Robinson, in one vol. 8vo.