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 This process, which is distinguished by the term of "Grass bleaching," has, however, been nearly superseded by another termed "Gas, or Chemical bleaching," founded upon one of those brilliant and useful discoveries, by which modern chemical science has so honourably distinguished itself, and rendered such service to the arts of life. Before proceeding to describe the new, (and now the ordinary process,) we shall give a brief description 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 occasions a kind of fermentation, 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 boiling in an alkaline lye, after which the skeins are exposed on the grass, for two or three weeks, which latter operation is called crofting. These alternate operations of bucking, washing, and crofting, are generally repeated four or five times, each time lessening the strength of the alkaline solution in which the bucking was performed. The next process is that of scouring, 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 technically 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 possesses, of rapidly destroying vegetable colours. By this system, (which furnishes one of the most beautiful illustrations of the immense benefits which science may render to the useful arts,) the practice of bleaching is conducted with a degree of precision before unknown, and with the most surprising 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 experi 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, 1786, 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 Berthollet (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 considerable 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 difficult 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 bleaching these, the oxymuriates of potash or soda must be employed. The peculiar advantages of combining chlorine with lime, or the alkalies, consists in the circumstance that the saline solution gives out the gas gradually to the goods which require bleaching, but does not part with it to the atmosphere with the same facility. In consequence of this, the operation of bleaching is now not injurious, 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 considerable 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.