Bleaching Powder, a compound formed by the action of chlorine on dry slaked lime (see BLEACHING), was first manufactured on a large scale in Glasgow by Messrs Tennant & Co. The use of chlorine for bleaching was introduced into Glasgow in 1789 by Watt, the engineer. At first it was used in the form of solution; but soon the addition of an alkali to the liquid was found to be beneficial, and the 'Eau de Javelle,' as this compound was called, was generally used. The next improvement was the substitution of lime for the alkalies, when Mr Charles Tennant in 1798 patented his bleaching liquor. In 1799 his partner proposed the use of dry slaked lime, and from that time to this the bleaching powder has remained unaltered in its nature. The manufacture of bleaching powder is one of the leading chemical industries of Great Britain, and at the present time it is looked on as the prop which supports many other industries, more especially the Leblanc soda manufacture. At one time the large quantities of hydrochloric acid gas produced in the manufacture of washing soda were allowed to escape into the air, destroying all vegetation for miles around. Restrictive acts of parliament, limiting the amount of gas which might be allowed to escape, resulted in the production of liquid hydrochloric acid, which was allowed, in many cases, to run into the streams. The manufacture of chlorine afforded an outlet for this (see CHLORINE), and the introduction of bleaching powder following soon thereafter, the useless by-product became the nucleus of a new departure, equal in importance to the soda industry itself. On the introduction of the Brunner-Mond process for making soda, bleaching powder became the mainstay of old soda-works, only able to keep working by making the bleaching powder pay for the loss on the soda they manufacture. For the decomposition of salt into sodium and chlorine, see SODA; see also HYDROCHLORIC ACID. An electrolytic or electro-chemical method of producing caustic soda and chlorine—i.e. from the decomposition of salt in water by means of strong electric currents—was begun on a commercial scale in 1894. The average market price of 'bleach' has varied much: in 1805 it was £120 per ton; in 1815, £80; in 1825, £27; in 1850, £13; in 1868, £10; in 1883, £5; in 1892, £10; in 1894, £9. In the manufacture of bleaching powder, the chlorine is passed into large stone or leaden chambers, measuring about 60 feet by 30 feet, and 5 feet high. On the floor of the chambers a layer of dry slaked lime, about 6 inches deep, is placed, and this is allowed to remain in contact with the chlorine for some hours, during which time the temperature rises considerably. The greater part of the unabsorbed chlorine having been drawn into another chamber, the door is opened, and the lime stirred up with rakes to insure uniformity, the strongest bleaching powder being found on the surface. Care is taken to transfer it to casks without exposure to wet or sunshine, as either of these are sufficient to induce decomposition and render it worthless.
The chemical changes which take place in the manufacture of bleaching powder are still under discussion; but the resulting product consists of hypochlorite of calcium, , along with a variable proportion of chloride of calcium, , and uncombined lime. Its value is judged (see CHLORIMETRY) by the amount of chlorine which it can give up for bleaching purposes, the present market strength varying from 33 to 39 per cent. of chlorine, or 'available chlorine,' as it is called. Bleaching powder is a grayish-white powder, with a powerful odour resembling chlorine. It rapidly absorbs moisture in a damp atmosphere, at the same time losing its bleaching properties. Rubbed with water in small quantity, it forms a thin cream; but on the addition of more water, thickens to a paste, which again becomes thin when more water is added. It is principally used in the bleaching of paper and linen; but as a disinfectant it is extensively applied in the purification of sewage or other obnoxious matter, its virtues from this as from every other point of view depending on the freedom with which it liberates chlorine. In the manufacture of chloroform large quantities of bleaching powder are required, and no practical process for producing this substance has as yet been devised which can dispense with bleaching powder.