As neither saponifioation, nor the decomposition of a soap by a stronger acid, in order to liberate its fatty-acids, can take place except in the presence of water, the elements of water-11,0—play a very prominent part in all reactions relating to neutral fats, and the preparation of soaps and fatty-acids from them.
It was soon discovered by Chevreul and his collaborateurs that the removal of glycerine from natural fats, i. e. their conversion into fatty-acids, enormously increased their hardnese and illumi nating power, so that candles made from the naixture of stearic and oleic acida, reeulting from the removal of glycerine from tallow, by the process indicated above, were lees greaey, and gave much more light, than candles made from the eame tallow untreated, though they had not so nice a colour. The next step was the separation of the harder from the softer portiona of the fatty acide, and it was found that when this was effected by preesure, the oleic acid, in flowing away, carried with it in solution the whole of the colouring matter of the mass, leaving the crude stearie aeid tolerably white. To make it absolutely so, little else was found necesSary than repeated pressings at various temperatures, the series of operations, after the removal of the glycerine, being purely mechanical. In carrying out this on a rnanufacturing scale, the expensive alkalies soda and potash were soon replaeed by lime, and the preparation of stearie acid by this process is now conducted as follows :—The tallow to be purified is placed in a large, slightly conieal, wooden tan, which will be more particularly described hereafter. In this tuu, the tallow ie mixed with 16 per cent. of good slaked lime, made into a thin cream with water. After tightly closing the tuo, steam is iotroduced from a pipe bclow, and the contente are boiled for four houra. During the boiling, the mixture is kept constantly agitated by means of a wooden shaft bearing three horizontal arms, worked by steam power. The action of lime upon the constituents of tallow has the effect of decomposing them, glyeerine being set at liberty, while stearate, and oleate, of lime are formed. The formation of these salts, which, when mixed together, constitute an insoluble soap, greatly facilitates the subsequent eeparation of the solid and liquid constituents of the tallow. To ascertain when the operation is complete, a small portion of the boiling mixture ie drawn out in a ladle, and cooled. When cold, the eainplo should appear perfectly smooth and solid, and should be very brittle, powdering finely in a mortar. When the operation ia complete, the steam is shut off and the agitator is stopped, the whole contents etanding until cool, and the fatty matters and lime form a solid mass at the bottom. They aro then dug out aud removed to another tun, similar in all respects to the last. Here they are treated with four parts of strong
sulphuric acid for every three parts of Hine previously added, and are then heated and agitated in the same manner as before. During the operation, the lime salts are decompoeed by the acid, sulphate of lime falling to the bottom, and the soapy fat rising in a thicas layer to the eurface. Again, the whole is permitted to stand ; when cool, the fat is ekimrued off and placed in a third wooden veesel, where it is well washed with water and by steam blown into it. The washed fat is next heated to the inelting-point, and run into dishes or troughs made of tin ; these are placed in a room, the tem perature of which is kept at from 20° to 30° (68° to 86° F.), and left for two or three days, or until the contents have assumed a granular or crystalline strueture, when they are removed from the dishes, and cut into shreds by machinery. The shreds are then placed in canvas or woollen bags, or between large, square sheets of canvas, and are carefully deposited between the plates of a powerful hydraulic press. Pressure is exerted gently at first, and is gradually increased until the flow of the liquid oleic acid ceases. The press is then unlocked, and. the hard, thin cakes of crude stearic aeid , are thrown into another wooden tun similar to the others. Here they are melted down by blowing in steam, which is continued for some hours. After eettling, the fatty matters are drawn off into tin dishes, and placed aside to cool. The temperature of the room in which the cooling is conducted should be slightly higher than the previous one, or about 30° (86° F.). The dishea should remain here until the contents assume a crystalline etructure, when they may be emptied. The blocks are then eut up into lumps, and ground to a mealy powder by n3eans of a rasping machine, worked usually by steam. This powder is gathered into bags, rnade either of hair or of wool, or both, and is then submitted to a eeemad pressure in another hydraulic press, differing from the former one by having a heating apparatus attached ; the plates should also be heated before the press is used. The necessity for heat in this second pressure is due to the extreme difficulty experienced in eliminating the last portions of oily matter from the fat. When the full pressure is being exerted, the press is left for about fifteen minutes before being unlocked. The cakes thus obtained are cleaned with a knife, the pariogs being added to the next batch. They are again melted by steam, a little wax being sometimes added, in order to destroy the crystalline texture of the etearic acid, which renders it unfit for use in eandle-rnaking. This finishes the process, and the stearine is melted into blocks ready for use.