All new soaps contain a considerable portion of adhering water, a great part of I which they lose when kept in a dry place; hence the economy and excellence of otc/ soap ; and hence the dealers in soap ge nerally keep it in a damp cellar, that it may not lose weight by evaporation • or, as it is said, sometimes immerse it in brine, which does not dissolve it, but keeps it in its utmost state of humidity. Soap may be considered as a necessary of life ; in all civilized countries its con sumption is immense. According to Pliny, the invention of soap must be ascribed to the Gauls, by whom, he says, it was composed of tallow and ashes, though the German soap was considered the best.
Lichig has forcibly observed that we may estimate the conditions of comfort and civilization of a nation by the glum titv of soap which it consumes.
Of the manufacture of hard soap.—The fat of this soap is usually tallow or coarse olive oil. Different species of grease are saponified by sods, with different degrees of facility; among oils, the olive, sweet almond, rapeseed, and castor oil ; and among solid flits, tallow, bone grease, and butter, are most easily saponified. From 12 to 13 ewts. of tallow are estimated to produce one ton of good soap. Some years ago, in many manufactories the tallow used to be saponified with potash leys, and the resulting soft soap was con verted, in the course of the process, into hard soap, by the introduction of mu•iate of soda, or weak kelp leys, in sufficient quantity to furnish the proper quantity of soda by the reaction of the potash up on the neutral salts. But of potash, and the diminished price as well as improved quality of the crude sodas, have led to their general adoption in soap-works. The soda-ash used by the soap-boiler, contains in general about 86 per cent. of real soda, in the state of dry carbonate, mixed with muriate of soda, and more or less nndecomposed sulphate. The barillas from Spain and Teneriffe contain from 18 to 24 per cent. of real soda.
The crude soda being ground, is to be stratified with lime in cylindrical cast iron vats, from 6 to 7 feet wide, and from I to 5 feet deep ; the lowest layer con sisting, of course, of unslaked or shell quicklime. The vats have a false bottom, perforated with holes, and a lateral tubu lure under it, closed commonly with a wooden plug, similar to the (pine of the French soap pans, by which the leys trickle off clear and caustic, after lnfiltra tion through the beds of lime. The quantity of lime must be proportional to the carbonic acid in the soda.
Upon 1 ton of tallow put into the soap pan, about 200 gallons of soda ley, of specific gravity 1.040, being poured, heat is applied, and after a very gentle ebulli tion of about 4 hours, the fat will he found to be completely saponified, by the test of the spatula, trowel, or pallet knife; for the fluid ley will be seen to separate at once upon the steel blade, from the soapy paste. Such hays, if composed of pure caustic soda, would contain 4 per cent. of alkali ; but from the presence of neutro-saline matter, they seldom contain so much as 2 per cent.; in fact, a gallon may he estimated to contain not more than 2 ounces ; so that 200 gallons con tain 25 pounds of real soda. The fire be ing withdrawn from the soap-pan, the mass is allowed to cool during one hour, or a little more, after which the spent leys, which are not at all alkaline, are run off by a spigot below, or pumped off i above, by a pump set into the pan. A second similar charge of ley is now intro duced into the pan, and a similar boiling process is renewed. Three such boils
may be given in the course of one day's work, by an active soap-maker. Next day the same routine is resumed with somewhat stronger leys, and so progres sively, till, towards the sixth day, the ley may have the density of 1.160, and will be found to contain 6 per cent. of real soda. Were the ley a solution of pure caustic soda, it would contain at this density no less than 14: per cent. of al kali. The neutro-saline matter present in the spent ley is essential to the proper granulation and separation of the sapo naccous compound ; for otherwise the watery menstruum would dilute and even liquefy the soap. Supposing 121 owls. of tallow to yield upon an average 20 cwts. of hard soap, then 20 cwts. of tallow will produce 82 cwts.
Of yellow or rosin soap.—}Zosin, al though very soluble in alkaline menstrua, is not however susceptible, like fats, of being transformed into an acid, and will not of course saponify, or form a proper soap by itself. The more caustic the al kali, the less consistence has the resin ous compound which is made with it. Hence fat of some kind, in considerable proportion, must be used along with the rosin, the minimum being equal parts ; and then the soap is far from being good. As alkaline matter cannot be neutralized by rosin, it preserves its peculiar acri• mony in a soap poor in fat, and is ready to act too powerfully upon woollen and all other animal fibres to which it is ap plied. It is said that rancid tallow serves to mask the strong odor of rosin in soap, more than any oil or other species of fat. From what we have just said, it is ob viously needless to make the rosin used for yellow soaps pass through all the stages of the saponifying process ; nor would this indeed be proper, as a portion of the rosin would be carried away, and wasted with the spent leys. The best mode of proceeding, therefore, is first of all to make the hard soap in the usual manner, and at the last service or charge of ley, namely, when this ceases to be absorbed, and preserves in the boiling pan its entire causticity, to add the pro portion of rosin intended for the soap. In order to facilitate the solution of the rosin in the soap, it should be reduced to coarse powder, and well incorporated by stirring with the rake. The propor tion of rosin is usually from one-third to one-fourth the weight of the tallow. The boil must be kept up for some time with an excess of caustic ley ; and when the paste is found, on cooling a sample of it, to acquire a solid consistence, and when diffused in a little water, not to leave a resinous varnish on the skin, we may consider the soap to be finished. We next proceed to draw off the superfluous leys, and to purify the paste. For this purpose, a quantity of leys at B. be ing poured in, the mass is heated, work ed well with a rake, then allowed to set tle, and drained of its leys. A second service of at 4° B., is now intro duced, and finally one at 2° j after each of which, there is the usual agitation and period of repose. The pan being now skimmed, and the scum removed for another operation, the soap is laded off by hand-pails into its frame-moulds. A little palm oil is usually employed in the manufacture of yellow soap, in order to correct the flavor of the rosin, and brighten the color. This soap, when well made, ought to be of a fine wax-yel low hue, be transparent upon the edges of the bars, dissolve readily in water, and afford, even with hard pump-water, an excellent lather.