Saponification by Agitation. Howes. - Twenty gallons of ley, of 1/125 sp. gr., are employed for every 100 lbs. of tallow. The apparatus consists of a cylinder 6 feet in diameter and 12 feet in length, and is capable of working 2 1/2 tons of tallow. Through the cylinder, lengthwise, a shaft extends, provided with radiating arms, to which an oscillating or rotatory motion is communicated. Convenient doors are attached for charging and emptying the cylinder. After charging the vessel agitation is continued for about 3 hours, when the whole is left undisturbed for a time, and ultimately removed into an open boiler, and completed in the ordinary way.

Gossage's Process

The boiling of the paste is effected by blowing steam into the bottom of the pan, and the mixture is treated with successive additions of stronger ley, undergoing between each a thorough boiling, until the fatty matter has taken up all the soda possible, and has thus become completely converted into soap; the excess of ley settles at the bottom of the pan, and is drawn off. The charge of soap is then drawn off from the pan without hand labour, by means of air pressure; the top of the pan is closed by a cover, the joint being made air-tight by an india-rubber packing ring, and compressed air is forced into the top of the pan by a pump, whereby the entire liquid mass of soap, amounting to as much as 20 tons, is expelled from the pan, being forced up through a discharge pipe passing through the cover, and flows through a long trough into the moulds. These are 45 inches long, 15 inches wide, and 52 inches high, each containing 1/2 a ton of soap, and are made simply of 4 cast-iron side-plates secured by clamps; the soap takes 3 days to cool and solidify, and the sides of the mould being then removed, the large block of ] soap is cut horizontally into slabs, which again are divided into bars by a wire frame.

The bars of the finer qualities are cut into cakes, which are stamped in a press having a heavy falling die lifted by a cam. The ley, or solution of caustic soda, is concentrated to the required strength for the soap-boiling pan by waste heat of the soda furnaces.

Silicated Soap

A solution of silicate of soda is employed in place of a portion of the tallow or oil used in the soap-boiling pans, thus producing a much cheaper soap with equal cleansing power. As ordinary soap owes its cleansing power to the fact that the soda, which constitutes the real detergent, is only in a state of weak combination with the tallow or other fatty substance; the latter can be to a considerable extent replaced by silicate of soda, in which soda exists only in weak combination with silica, thereby retaining its cleansing power, as in ordinary soap. The silicate of soda, known as soluble glass, is made by melting in a reverberatory furnace a mixture of fine white sand and soda ash; the melted charge is run out through a tap-hole, and solidifies in lumps of a kind of glass, which is soluble in water.

Quality Of Soaps

A good soap is easily soluble in alcohol, leaving scarcely 1 per cent. of solid residue, and forms a gelatinous liquid in boiling water. Hard or marbled soap should not contain more than 25 per cent. of water, rosin soap not more than 40, and a soft soap not more than 52. In cocoanut-oil soaps a larger amount of water than 52 per cent. may be allowed. In yellow soap a part of the fat may be replaced by 10 to 25 per cent. of rosin.

Household Soaps. Hard Soaps

Hard soaps are always soda soaps. There are grained soaps, those in which a separation of the under-ley has been made as described, and filled soaps, those in which the whole contents of the boiling pan are kept together and sold as soap. ' The cocoanut oil is especially employed for the manufacture of filled soaps, because it is easily soluble in brine, requiring a very large quantity to separate them, and then they become so hard that they can scarcely be cut with a knife. The more solid, constituents a fat contains, the harder the soap produced; the more oleine, the softer the soap. By mixing the fats in different proportions, soaps of any consistency can be obtained; this also depends upon the strength of the ley used in the process. Weak and middling strong leys will produce a light soap, while leys of 25° to 35° B. will produce a soap heavier than water. Sometimes a small admixture of sulphate of soda is employed in making soap, for the purpose of preventing its too great solubility when used in wash-ing. In the manufacture of soaps, 1/3 or 1/4 of fat is frequently substituted by rosin.

For the transformation of 100 lbs. of fat into soap, there are generally necessary 12 1/2 lbs. of solid caustic soda; this quantity must be more or less, in proportion to the nature of the fat.

Tallow Soaps

To saponify 1000 lbs. of fat, commence by putting the tallow into the boiler, and melt it with a slow heat, add 70 to 80 galls, of ley of 10° to 12° B., stir well, and keep a gentle fire for several hours. Should part of the fat separate from the mass, which is often the case, an oily liquid will be observed floating on the top. Then add, gradually, 35 to 40 galls, of ley of 15° to 18° B. By this addition the whole contents will soon form a homogeneous mass of a greyish-white colour. In order to establish the necessary consistency to the paste, boil gently for several hours, adding every hour 6 to 7 galls, of ley of 20° B. The time necessary for the first operation is from 10 to 12 hours for 1000 lbs. of fat. After this, pass to the cutting process, and operate as before described. It is essential that care be taken to stir the ingredients well while adding the salt. When the separation has taken place, leave altogether quiet for several hours, and then draw off the coloured under-ley; 90 galls, of ley of 25° should then be added; increase the heat, there being strong ley at the bottom of the pan, which preserves the soap from burning.