SOAP MANUFACTURE. Soap originally meant the compounds derived from the union between fatty bodies and the alkalies potash and soda. Although the name is still usually thus limited in its mean ing, it has nevertheless been extended to compounds of oleaginous bodies with some earthy and metallic bodies, having but few properties in common with soap properly so called.
Properties.—it has been found by Chevreuil that different varieties of fatty matter consist chiefly of two kinds : one hard, to which he gave the name of stearin ; and the other soft, which he termed olein. He also discovered that stearin is composed of stearie acid and a pecu liar principle which, on account of its sweet taste, he named glycerin ; and it was further proved by his experiments that olein consists of oleic acid and glycerin. Stearin is therefore a stearate of glycerin, and olein an oleate of the same substance. 'When, in the manufacture of soap, an alkali (soda, for example) is heated with tallow, the soda gradually dislodges the glycerin from combination with the stearie and oleic acids, and by combining with them, forms soap, or, in other words, a compound of atearate and oleate of soda, and the glycerin remains in solution.
In vegetable fat oils—olive-oil, for example—the glycerin is combined with margarie and oleic acids, forming margarate and oleate of glycerin; and, consequently, soap made with this oil is a margamte and oleate of soda, instead of a stearate and oleate of this base.
The soaps which have the alkalies for their bases are soluble in water, though the solution is in general milky; they are also soluble in alcohol, and the solution Is used frequently as a test of what is called the hardness of water. [Soar-Tesr.] Acids also decompose soaps, and though the effect is apparently similar, yet it is in reality different : thus when sulphuric acid is added to soap, a white precipitate is formed, but this is merely the fatty acid which the soap contained, and shows the change which the fat employed has undergone; it is either stearie, oleic, or inargarie acid, ke., or a mixture of two or more of them. Sulphate of soda remains in solution when a soda soap has been thus decomposed. There are certain preparations used in medicine under the names of emulsions and liniments, which are obtained by merely agitating either potash, soda, or lime-water, with oil. The first of these is an aininoniacal soap; the second and third are imperfect alkaline soaps; and the fourth is an earthy soap to which barytes and atrontia-water form compounds nearly analogous; these earthy soaps are Insoluble In water, or nearly so. Metallic soaps are formed by heating
certain metallic oxides, as those of lead, mercury, and bismuth, with fatty matter; glycerin is separated, as has already been mentioned, and the metallic soaps formed are insoluble in water. The only soap of this kind extensively employed is that of oxide of lead, which is largely used under the name of diachylon, or lead plaster.
Manufacture.—Some of the soap-factories of the present day carry on operations with the aid of considerable chemical and mechanical skill. There is, however, not much that requires description here.
Mottled soap, a kind much used in England, Is made of tallow, kitchen stuff, soda, water, and a little salt. The tallow principally employed is brought from Russia, and arrives In a solid state in barrels; it is tolerably pure, and is ready for use at once. The impure grease known as kitchen-stuff requires much heating, straining, and purifying before it is fitted for use ; and even then it is not employed for the better soaps. The alkali formerly used, as has already been stated, was obtained from kelp and barilla; but the carbonate of soda obtained from common salt is now almost exclusively used. The soda being required almost in a caustic state, the carbonic mid is ()rivet' off for tho soap-maker's -purposes. The caustic soda is dissolved in water to form a ley or lye. The ley is pumped into boilers, and mixed with the fatty substances. Steam-heat is applied, and the mixture is boiled until the fat has combined with n11 the alkali of the ley. The spent ley is pumped up, fresh ley is introduced, and the boiling proceeds. This is repeated several times, a stronger Icy being used each time than before. When the soap is nearly finished, the " mottling " is given by sprinkling a small quantity of very dense ley; this percolates slowly through the mass, leaving dark-coloured veins in its track. When the soap is finished, it is laded into buckets, and thence transferred to frames. These frames are upright oblong boxes, made either of wood or iron, and easily taken to pieces. The soap, when the frames are full, is allowed to cool and solidify. Each frame being taken to pieces, the mass of soap, sometimes weighing as much as three or four thousand pounds, is exposed to view. It is cut up into slabs or layers by n wire being drawn through it, following the marks of certain gauge-lines. The application of a similar wire in a different way after wards separates the slabs into bars about 15 inches long, 3 inches wide, and 3 inches deep.