When fats or oils are heated with caustic leys, a combination of fatty acids with alkali is formed; this is designated saponification. Soaps are divided into hard and soft, the former having soda, and the latter potash, for their bases. The former, however, is the most extensively manufactured, whilst the demand for the latter is limited. Acids decompose soaps, combining with their base and expelling the fatty acids, for these, being insoluble in the former, float on the surface of the liquid. By this means soaps are easily analyzed.
Vegetable oils have been divided into two classes, the drying and*the fluid oils. Of the first-named are oil of linseed, hempseed, and poppy oil. Of the second, olive oil, palm oil, sweet almonds, and cocoanut oil. According to the mode of obtaining oils, they are distinguished as oils of the first and second pressure. Those of the second pressure are more serviceable to the soap manufacturer, for though less liquid and often mucilaginous, they contain more stearine, and the richer the oils are in stearine, the harder are the soaps they yield.
Six fatty acids have been discovered in the cocoa butter, most of which being solids, accounts for the great firmness of the soaps it forms. This fat is also remarkable as uniting with soda leys in any proportion, without separating from them. Owing to this property, this fat is used in large quantities for the making of filled soaps. It is very slow to unite with ley by itself; it is therefore usually applied in combination with tallow or palm oil, increasing their emollient properties, and also giving to the tallow soaps a brilliant whiteness.
This is of an orange colour, and when not rancid, of a violet odour. Palm oil is employed both in the bleached and in the natural state. In the bleached state it produces a soap of most beautiful whiteness, and rich with the characteristic odour of the oil.
The bleaching of 1000 lbs. requires 5 lbs. red chromate of potassa, 10 lbs. strong hydrochloric acid, and 2 1/2 lbs. sulphuric acid. First, the chromate of potassa is pulverized and dissolved in hot water. The palm oil should then be transferred to a wooden tank, and heated with steam to 120° Fahr. The steam is turned off and a portion of the solution of the chromate of potassa is added, agitated, and a proportional portion of hydrochloric acid used; at last the sulphuric acid. After thoroughly agitating this mixture for a "few minutes, the oil changes in colour, becoming first black, then dark green, and soon afterwards light green, when a thick froth appears on the surface, an indication of the completion of the process. If a sample, when taken out and allowed to settle, does not appear sufficiently bleached, an additional portion of the bichromate of potassa, with muriatic and sulphuric acids, should be added. The whole has to be left quiet for 1 hour, so that the solution of the resulting salts may settle. The clear oil is then drawn off into a wooden cask, mixed with some water, and heated again by the introduction of steam. It is again left alone for some time, and the fat subsequently drawn off.
In making soaps palm oil is usually employed with tallow, in the proportion of 20 to 30 of the former to 100 parts of the latter. It is also employed in making rosin soap, to correct the flavour of the rosin and brighten the colour.
There are three kinds, namely, the virgin oil, obtained by a gentle pressure of the fruit; a second kind, gained by submitting them to the Action of hot water and pressing them between metallic plates previously heated; and the third, an inferior kind, is the product of this residuum when boiled in water. Only the two latter kinds serve in the manufacture of soaps; they yield an excellent soap, esteemed for its fresh and agreeable odour. It is very extensively used by soap manufacturers in Marseilles and for Windsor soap.
There is a great difference in the consistency of animal fats, the richer they are in solid constituents the higher is their melting point. In the class of whale fishes the fats are generally fluid; in the carnivorous animals, soft and rank-flavoured; nearly scentless in the ruminants; usually white and copious in well-fed young animals; yellowish and more scanty in the old. The fat of the kidneys is generally harder and more compact than that found in the cellular tissues and in the bowels of animals. The colour and odour of the fats, of course, affect the manufacture of soaps.
This is the most used of all animal fats; it has a yellow tint, due to colouring matter, separable by several washings in hot water, and is firm, brittle, but not so white as mutton suet. That rendered by steam is generally the whitest.
Mutton fat is richer in stearine than beef tallow, and is consequently much sought after by tallow as well as stearine candle manufacturers. Saponified with soda ley it yields a beau-liful white soap, but being so rich in stearine it is liable to become too hard and brittle. In order, therefore, to obtain a more unctuous product it is generally mixed with about 20 per cent. of lard or cocoanut oil, whereby a superior soap is obtained.
Lard is an excellent material for soap manufacturers; it forms a white, sweet, and pure soap. For the purpose of rendering it more frothing it is saponified either with tallow or cocoa-nut oil.