Soap is the term applied to that class of compounds formed when alkalies act upon fats or fatty acids. Chemically it includes also the insoluble compounds formed from metallic oxides, as lime, litharge, &c.; but industrially it is confined to the compounds of soda and potash. Soap is first mentioned by Pliny, who states that it was prepared from goat's tallow and beech-tree ashes, and was employed for giving brilliance to the hair. The excavations at Pompeii brought to light a complete soap-making establishment, containing some well-preserved soap. The industry, however, advanced very slowly, and it is only within comparatively recent years that it has really flourished. The most important discoveries which have assisted its development and led to its present enormous proportions are Chevreul's researches on the constitution of fats and Leblanc's process for the manufacture of soda on a large scale. It has also been stimulated in Britain by the repeal in 1853 of the duty (amounting to from 1d. to 3d. per lb.), first imposed in 1711. Chevreul showed that the neutral fats are mixtures of 'glycerides,' consisting of glycerine combined with the elements of certain fatty acids. In the process of saponification the fat is decomposed, the fatty acids combining with the alkali to form soap, while the glycerine is set free.
Soaps are distinguished as 'hard' (soda-soaps) and 'soft' (potash-soaps). They have a characteristic taste, and are soluble in water and in alcohol. When agitated with water a 'lather' is produced, and when their hot aqueous solutions are allowed to cool jellies are formed. Salt precipitates them from their solutions, but in the case of a potash-soap the bulk is converted into a soda-soap by double decomposition. The cleansing power of soap has not been satisfactorily explained; it is generally supposed to be due to the 'hydrolysis,' or partial decomposition into free alkali and insoluble acid-soap which takes place when the soap is dissolved in water. Dittmar considers that the cleansing power of soap may be attributed to 'the inherent property of its solution to emulsionise fats,' which seems a more plausible theory.
The principal fats and oils employed in the manufacture of soap are tallow, greases, palm-oil, lard, cocoa-nut oil, and olive-oil for 'hard' soaps; distilled fatty acids are also used, and rosin in conjunction with tallow, &c. For 'soft' soaps linseed-oil, castor-oil, and fish-oils are used, as well as some of those already mentioned. The caustic alkalies, potash and soda, were formerly prepared from the carbonates by the soap-maker himself, but are now almost entirely procured from the soda-manufacturer.
The following processes, arranged in order of simplicity, are those used in the manufacture.
(1) The direct union of free fatty or resinous acids and alkalies.—This process is seldom used. The fatty acid (e.g. oleic) is run into a steam-jacketed pan, provided with a mechanical agitator, and raised to about 300° F. by super-heated steam. A strong solution of the necessary quantity of alkali is then added, and the whole well stirred. The mixture is allowed to stand for some time, some water added, again stirred, and removed to cooling-frames.
(2) Treatment of fats with definite quantities of alkali, the glycerine remaining with the soap; known as the 'cold process.'—Given weights of fat are melted at the lowest possible temperature, and mechanically mixed in a pan with a definite quantity of caustic lye of known strength, just sufficient to effect complete saponification. After standing for from one to four days, according to the quantity, the soap is hard enough for use. This process possesses the advantages of economy and retention of the glycerine. Soaps made in this way, however, are liable to contain free alkali, and of necessity contain also the impurities of the soda.
(3) Treatment of fats by boiling with indefinite quantities of alkaline lye.—This process is the most important of the three, and will now be described as it is carried out for 'hard soaps.'
Hard Soaps constitute the great bulk of the soaps used, and may be divided into three varieties—curd, mottled, and yellow. The general method is the same for the three. The vessel used (called the 'copper') is made of wrought-iron plates riveted together, and is provided with coils for supplying open and close steam. These 'coppers' are generally of circular form, and capable of turning out from 20 to 30 tons of soap at one operation, although some of them are much larger. In the first stage (called 'killing the goods') a quantity of the fat is melted in the 'copper,' weak caustic lye added, and the steam turned on. The mass becomes pasty after some time, and the boiling is continued, and fat and lye added, until a sample appears somewhat firm, and has very little or no caustic taste. In the second stage ('cutting the soap' or 'salting') the water is separated from the soap (the boiling being continued) by the cautious addition of common salt, or strong brine, until clear liquor runs from a small sample taken out. After standing for a few hours the 'spent lye' (containing the bulk of the glycerine of the fat, common salt, and the impurities of the caustic soda) is removed. In the third operation ('clear boiling') the granulated soap is boiled for two to three hours with fresh lye, in order to cause more complete saponification, and to remove the brine retained by the soap. After settling, the lye is removed, and may be used for the treatment of more fresh fat. The contents of the 'copper' are again boiled with open steam and lye for several hours, to complete the saponification of the last traces of fat, and 'make' the soap.
The method of finishing the soap varies with the kind of soap required, and will now be explained under the names of the three varieties.
Curd Soaps.—The finest quality is made from tallow, but other fats may be used. When the soap has been 'made,' as just explained, the lye is concentrated by means of close steam, until a sample of the soap appears hard. The boiling is then stopped, and the soap removed, after settling.
Mottled Soaps, as far as composition is concerned, are practically 'curd' soaps; darker fats, however, are used in their manufacture, and after the soaps have been 'made' the lye is not concentrated so far as with 'curd' soaps. Many precautions require to be observed in order to obtain the natural 'mottling' which characterises them. It is due to the presence of small quantities of lime, magnesia, &c., from the materials used, and to oxide of iron, from the 'copper.' These oxides form insoluble soaps, and when the soap, retaining a little lye, is transferred to the cooling frames, they collect together, producing the well-known appearance.
Natural 'mottling' may be accepted as a guarantee of the absence of an undue amount of water in a soap. Artificial 'mottling' of inferior qualities of soap with ultramarine, oxide of iron, &c. is largely practised, sometimes with fraudulent intentions. 'Mottled' soaps are much used for laundry work and such purposes.
Yellow Soaps contain rosin as an essential constituent. The finest qualities are made from the best tallow and light-coloured rosin; inferior qualities from darker tallow, bleached palm-oil, &c., and darker rosin. The rosin is usually introduced after the second stage of the general process. The finishing operation, or 'fitting' of yellow soaps, requires much experience. After the soap has been 'made' the 'copper' is allowed to stand for about twelve hours, the lye removed, and the soap well boiled until homogeneous, with some fresh lye if necessary. When a sample has been found to be in the proper state, the 'copper' is covered up, and allowed to stand for some days, when a separation into three layers takes place—the scum or 'fob' on the top, the 'nigre' (or dark alkaline soap-lye) underneath, and the finished or 'neat' soap in the centre, which is then removed to frames. The 'nigres' are utilised in the making of darker soaps. The principal soap-manufacturers now recover the glycerine from their 'spent lyes.'
Cocoa-nut or Marine Soap.—Cocoa-nut oil is saponified in the heat, with strong lye, without salting out, &c. A hard soap is formed, although much water may be present, and is serviceable on board ship, when condensed water is not available, on account of its solubility in salt water.
The operations subsequent to the soap leaving the 'copper' require little explanation. When no 'filling' substances are to be added the melted soap is run, or ladled, into large oblong boxes of wood or iron, called 'cooling frames.' After standing for several days to allow the soap to harden, the sides of the frames are removed, and the blocks of soap cut, first into slabs, by means of a thin steel wire, and then into 'bars' by a special machine, consisting generally of a strong frame or lever, carrying a number of wires stretched across it. The 'bars,' after having been stamped with the name of the maker, and the brand of the soap, are ready for being sent out.
The demand of the public for cheap soap has led to what is known as 'filling.' This consists in mechanically mixing with or 'crutching into' the soap, after it leaves the 'copper,' certain substances, added either with the view of increasing the detergent power of the soap, or simply to lessen the cost. Silicate of soda belongs to the first class, and is used with advantage in certain soaps. Such substances, however, as water, talc, clay, chalk, sulphate of baryta, &c. must be looked upon merely as adulterants.