I a. In soaps made from fatty acids, the eoda is used in the form of a refined carbonated ash at 52° (that prepared by the Jarrow Co., Newc,astle-on-Tyne, ie recommended), every 100 lb. being dissolved in 160 lb. water in a lead-lined vat, and the solu tion allowed to settle previous to use. The etore-tanks of this, and of tho fatty acids employed, are connected with stnall gauge-tanks or measuring-tubes (Fig. 1238), for the purpose of obtaining uniformity in the smite by the use of exact quantities in every operation. For the delivery of the soda solutions into the soap-pan, a special feeder (Fig. 1239) is provided, closed with a movable tampon, by which the flow of liquid may be regulated at discretion ; a perforated rose-spout may be advantageously placed under the exit-pipe.
The soap-pan in which the operation is conducted, shown in Figs. 1240-2, is jacketed, the iulet-pipe being at Z, and the steam is either superheated, or used at a pres sure of 75-80 lb. Abovo the pan, is a movable curb 0, with elide at m, necessary to give room for the intumescence caused by the liberated carbonic acid ; a wheel arrangement W enables it to be readily drawn aside on a railway behind the columns N, whioh support the gearing M. This gearing moves the stirrer R at the rato of 40 rev. a minute ; the latter is made of wrought-irou, and moat efficient when the two sets of blades move in opposite directions; when thie is not the case, the pan itself should be provided with fixed traverses armed with vertical cross-teeth. In making soap with this apparatus, 1000 lb. oil are run into the pan with the curb 0 in its place, and 'Rated to I38°-160° (280°-320° F.), according to its quality. At this point, 190 lb. of soda ash for a neutral soap, or 210-225 lb. for a etrong soap, dissolved in the proper quantity of water, and at 100° (212° F.), is let into the pau at such a speed that it oocupies not less than 6 nor more than 12 minutes. The whole is well stirred meanwhile, and swells up enortnously ; but 5 minutes after the last por tions of alkali have beeu added, the mass subsides, and, in 15 minutes more, ohanges from a spongy to a clear, soft, brilliant, homogeneous paste. The curb is then removed, and, in about an hour, 100 lb. of hoiling water is lot in from the rose-spout of the soda-feeder, and the whole is twain well stirred ; if it be desired to mix silicate of soda or anything else with the soap, it is added at this stage, after which, the soap is transferred to the cooling-frames (pp. 1781-2), and a fresh batch is proceeded with. Soap thus made has the following composition :—Oleic acid, 65.00 ; soda, 6.7-7.50 ; water, 27.50. When rosin is used, it should be added to the oil while the latter is being heated ; or the rosin soap may be made in a separate pan, provided with a Morfit's ste,am-twirl (Fig. 1243), in which the tubular blades of the stirrer are perforated to emit steam while the whole is in motion ; 1200 lb.
rosin and 2200 lb. caustic leys at 11° B. are boiled together, and the thin jelly so produced is transferred in suitable quantities to other pans. It contains :—Roein, 54-5 ; soda, 7.8 ; water, 37.7. The apparatus described here is also suitable for several other kinds of soap, the eteam twirl, &c., being especially useful for making " hydrated " soaps (p. 1777).
I, b, a.—The so-called " cold process " consists in mixing given weights of fat, or a mixture of fats, previously melted at as low a temperature as possible, milli caustic soda solution of a given sp. gr., the quantities of each being so adjusted that only just enough soda shall be present to completely saponify the fat. After thorough incorporation, the mixture is covered up, and allowed to stand. In a few hours, the chemical reaction commences, accompanied by considerable evolution of heat, and the soap is formed. After the lapse of 2 or 3 days, it is usually hard enough for use. It is obvious that soaps made in this way retain all the glycerine originally combined with tho fatty acids, disseminated through the particles of soap. This, and the comparatively low temperature at which the soap is made, are the aief reasons why this process is much in vogue for the cheaper kinds of toilet soap, since the perfumes employed are not dissipated by heat. It is found, however, that soaps thus prepared are very apt to contain an excess of alkali, and hence they are unavailahle where perfectly neutral soaps are required.
Another objection is, that, as there is no opportunity of removing any extraneous matter, the materials employed must be of the purest,, and as tho soda leys are usually required in a con centrated (and therefore expensive) form, the process is not so advantageous as at first sight appears. It is chiefly applicable to soaps made on a small scale ; when larger quantities are operated on, a mechanical agitator, such as Hawes' boiler, represented in Fig. 1244, is necessary. This, for operating on 2/ tons of tallow, is a cylinder 6 ft. diam., 12 ft. long, with a central shaft provided with radiating arms, set in rotation by any convenient mechanism. Any saponifiahle fat or oil may be used, and for every 100 lb. of the pure fat, 50 lb. of caustic leys at 36° B. should be taken. When these are not very pure, i. e. if they contain much extraneous soda salts, especially sodium chloride, saponification will not take plac,e, unless some proportion (10 per cent. on the fat, at least) of coco-nut-oil be used. The following mixtures will be found useful for this process:— Noe. 1, 2, and 5 make good toilet soaps, which are improved if about of the soda used is replaced by an equivalent qnantity of potash. No. 4 with unbleached palm-oil gives a fine yellow soap, liable however to bleach in the light. No. 3 is given on the authority of Cristiaui, who recommends the use of 100 lb. leys at 25° B. to the 210 lh. mixed fat and rosin.