A more legitimate application of silicate of soda is to mix varying quantities of the concentrated solution with "neat " yellow or curd soaps. This treatment makes yellow soaps much atiffer, and in many cases, by hardening them, adds to their durability. About 5 per cent. of the solution at 1.700 sp. gr. is a suitable quantity, and has much the same effect 88 the addition of 5 per cent. of earb. soda crystals before described. Much larger quantities than 5 per cent. may be used, but soap so treated is apt to disintegrate unpleasantly in the hands of the consumer. Curd soaps are sold in England with which 15 or even 20 per ceut. of silicate of soda at 1.700 sp. gr. have been mixed. These large quantities considerably increase the " soda available for washing," as given by the alkoli metric test (see Soap Analyais, p. 1794).
Aluminate of soda.—As a detergent for mixing with soap, this substance is perhaps even more powerful than silicate of soda. It is chiefly obtalned from cryolite, a mineral found in great abun dance in Greenland, and may be readily prepared from it by boiling it with ; cryolitc, being a double fluoride of aluminium and sodium, gives up the whole of its fluorine to the lime, leaving a mixture, or compound, of alumina and soda. Like silicate of aoda, it ia not a definite chemical compound,—as will be seen by the following analysea of different samples:— The commeroial product ia an amorphous white substance, readily soluble in water, in which state it may be raixed with soap, like silicate of soda.
Bloc, Grey, and Bed Mottled Soaps.—These come under the head of silieated aoaps, and are thus made. Two coppers are required, an ordinary ateam copper for the first stage, and a fire-copper for the later atages. In the ateam-copper, the raw materials are killed, made, and fitted rather open. The fat-mixtures employed are usually vegetable oils, and almost alwaya (though not neces sarily) contain a fair proportion of coco-nut- or palm-kerael-oil. When first introduced from Germany, these soaps were made from well-bleached palm-oil and coco-nut-oil, in auch proportions as 3 palm- to 2 coco-nut-, or 2 to 1, or even 3 to /. Latterly, however, palm-kernel- has aup planted coco-nut-oil, and some of the palm- has been replaced by refined cotton-seed-oil. The choice of materials is very much guided by their cost. The fitted soap is shifted off its nigre into the fire copper, and, to every l000 lb. of it, is added about 250 lb. solution of silicate of soda at about 20° B.;
the exact ap. gr. depends chiefly upon the proportion of palra-kernel- to other mls. The whole is then boiled together with steam and fire, to thoroughly incorporate the mass; when this is complete, the steam is Mopped off, and the appearance of the copper is examined. Practice and experience, assisted by chemical analysia, can alone decide when the soap is in " mottling condition " ; in that state, it should have about 45 per cent. (or less) fatty acids, and 0.5-1'0 per cent. of sodium chloride, according to the raw materials employed. A good physical test is to take a layer out rapidly upon a cold trowel, and observe ita appearance, and the time required for it to " set." A shiny appearance on its surface indicates a deficiency, a frosty appearance, an excess, of mineral salts; if it sets too quickly, and shiny, more sodium chloride must be added ; if it appear frosty, and ia long in setting, enough mineral salts are present. So delicate is the process, that the addition of 1 lb. salt to a ton of soap at this stage will entirely alter the appearance of the mottling when cold. When it is in mottling condition, the mineral substance used as mottling is mixed with a small quantity of water, and sprinkled into the copper ; it is there thoroughly incorporated with the soap by a few minutes' boiling, and the soap is then transferred as rapidly as possible to wooden frames, which are carefully covered up when full, and kept as warm as possible, to allow time for the " mottle to strike." For blue mottle, 5-10 lb. artificial ultranaarine per ton of soap are used ; for grey, 1-3 lb. finely levi gated oxide of manganese. If the soap be cooled rapidly, it will be of a homogeneous blue or grey colour ; slow undisturbed cooling is essential to these soaps, and once in the frame, they should never be touched until they are quite cold ; it was for them that Gossage devised the pneumatic method of emptying coppers (p. 1781), but a centrifugal pump answers as well. It requires greater skill to make good " blue-mottled " soap than for any other kind, and the manufacture is in the hands of a few large firms. It may be observed here that in these soaps, the mottling, so difficult to produce, is a matter of appearance merely, and that soap with a plain white ground would wash just as well.