A name given to those bodies which are compounds of the alkalies with fat and the fixed oils. The earths, and the other metallic oxides also, combine with fat and oils, forming neutral compounds. The former have been called earthy, and the latter metallic soaps. The soaps formed by the alkalies have the distinguishing character of being soluble in water and alcohol. The earthy soaps are perfectly insoluble: and since any of the earths have a stronger attraction for oil than the alkalies, the alkaline soaps are always decomposed by the earths. This occasions the curdy appearance when soap is used with water containing any earthy or metallic salt: it is from this quality that waters are ?aid to be hard.
The soaps used in the manufactures and domestic economy, are made with the fixed alkalies, combined with different kinds of fat and oil. These, in the manufacture of soap, are divided into two principal varieties, viz. hard and soft. The alkali employed for hard soap is soda, generally obtained from the different sea vegetables, and called by different names, according to the name of the plant, in different countries. Most of the algae, but particularly the fucus and salsola, afford soda by burning. The vegetables are first dried, and then burnt in pits formed with loose stones. The earthy matter, and the soda, with some neutral salts, fuse into a crude mass, in which state it is sold. This substance is furnished in great abundance from the Highlands of Scotland, under the name of kelp, and from Alicant, in Spain, under the name of barilla. In France it is known by the name of varec; this being the name of the plant from which it is generally produced there. It is commonly, however, in this state that it comes to the soap-maker, varying frequently in its value, and often occasioning much uncertainty in its employment. It should be the first business, therefore, of the manufacturer to assay the substance from which he gets his alkali, even before he purchases it.
When the exact value of the alkali is known, it is then to be treated as follows, to prepare it for mixing with the fat. The kelp, or barilla, is first to be pounded, and then mixed with one-fifth its weight of quick lime, in a large vat. These vats are generally three or four in number to each boiler. Besides these vats for the infusion of crude alkali, each of them has a cavity made under it. The bottom of each vat is even with the ground, the under cavity being sunk below, and is intended to receive the liquor which runs from a plug-hole in the upper vat, when the infusion has gone on to a certain extent. One of these vats, with its under reservoir, is sufficient for one boiling, but they are generally all at work, in order to give time for the solution of the alkali from the crude mass. In charging a vat, the barilla, kelp, or potash, and sometimes mixtures of these, are first coarsely powdered and mixed with quicklime, also coarsely powdered; some water is then thrown upon these, to slake the lime. In the side of the vat some straw is first placed about the plug-hole, to prevent bits from passing through.
The vat is now charged, and water poured upon the materials till it stands considerably above the solid mass; after standing several hours the plug is withdrawn, to let out the solution into the lower reservoir. The plug is now returned, and fresh water poured upon the materials. Some, or all of the first ley is now removed into one of the other lower reservoirs before the second infusion is drawn off. This is done that the soapboiler may always have at command two leys of different degrees of strength, as, in the course of every boiling, he finds it necessary to use sometimes the weak, and, at other times, the strong. The number of waters to be added to the materials, depends upon the judgment of the workman, who, by his taste, can tell when the water has dissolved the whole of the alkali. The ley being ready to lade out of the reservoir, which is near to the boiler, the tallow or oil, first weighed, is put in. When it is sufficiently melted, the workman begins by adding the ley and stirring the mixture. The alkali and the oil soon begin to unite, forming a milky fluid. As more ley is added, and the stirring continued, the liquid thickens.
This is continued generally for thirty hours, and frequently more, till small portions of the soap, taken out from time to time, assume a proper consistence, which the workman, by constant experience, understands. He now adds a quantity of common salt, which has the effect of separating the watery part from the soap, which contains a portion of neutral salts, that existed in the crude alkali, especially when more than enough has been added. The fire has now to be withdrawn, and the mass left to cool. The watery part will be found at the bottom, and requires to be drawn out by a pump, which is a fixture on the side of the boiler. When this has been removed the fire is rekindled, and if the mass does not melt freely, a little water is added. As soon as the whole becomes liquid, and is made uniform by agitation with wooden poles, the fire is again withdrawn, and the mass allowed to assume a proper consistence for lading. It is laded into square moulds; these are composed ofa number of strata lying one upon another, so that when the soap has become solid, each layer of frame-work can be removed, beginning at the top, and the soap is cut into cakes with a small piece of brass wire at every interval; these cakes are afterwards cut into square prismatic pieces, in which state they are sold.
Yellow hard soap is formed of similar proportions of soda and tallow with the last; but it also contains resin, and sometimes palm oil. In boiling the yellow soap, the resin, oil, and tallow, are put into the boiler first. The ley is prepared in a similar vat, and managed, in other respects, like the white soap.
Soft soap differs in its composition from hard, in containing no alkali, but potash. Scft soap made with colourless fat, such as tallow, is a white unctuous substance, about the consistency of lard. Ifthefatbe coloured, the soap partakes of the same. In France, and other parts of the continent, it is generally coloured, sometimes with metallic oxides. Those made with yellow oil are sometimes coloured with indigo, which gives them a green colour. The oils employed are seldom olive oil, but the cheaper oils, such as rape-oil, the oil of hempseed, linseed, and others. In Holland it was made with whale-oil. This oil was forbidden on some parts of the continent, on account of its disagreeable smell. In this country, however, all the soft soaps are made with whale-oil, which gives a transparent mass of a yellow colour. In commerce, however, we do not find it uniform in its colour; besides the yellow part, it appears interspersed with white spots, giving the whole a strong resemblance to the inside of a dried fig.