After clear boiling it is usual practice to give the soap one or two brine washes to remove entangled lye. The soap then undergoes (4) finishing or fitting (in the U.S.A. called "settling change"), which consists in once more closing the soap by the addition of just sufficient steam and water, and boiling until the soap is brought to the right condition of hydration and openness. The workman in charge adjusts the coarseness of the "fit" according to the type of soap handled ; the art consists in bringing the soap to such a condition that impurities shall separate in the subsequent "settling" (3-7 days).
The material in the pan settles into four distinct layers : (I) a "fob" or crust of solidified soap foam at the surface, which is skimmed off and added to the next batch fitted: (2) a layer of clean "neat soap" ("settled" soap) comprising 75-80% of 'the contents of the pan, testing 63-64% fatty acids and containing about 30% of water. Such a soap is termed "genuine soap," since in practice it is not possible to produce a commercial soap in the pan (without drying) containing less water: (3) a layer (about 15-20% of the total) of darker discoloured soap termed "nigre," containing the separated impurities, chiefly metallic soaps, (of iron, etc.), excess alkali and salt. Nigres from several batches may be united and worked up separately as a lower quality soap, but are usually added to the charge for the next lower grade of soap. Bleaching of the nigres is occasionally practised : (4) a small amount of alkaline liquor containing sodium carbonate, chloride and traces of hydroxide.
In America the practice of skimming off the crust is not common. The "genuine" soap, or settled soap, is pumped off, bringing the crust and the nigre together. Nigres are also gen erally added to the next charge of the same grade of soap rather than to a lower quality soap. If the kettle to which the nigre is added appears too dark, it is "pitched" or settled so that the nigre from this contains the most of the impurities. This nigre, only obtained once or twice a year, is added to a soap of lower quality.
The pan is "cleansed" by running off the liquid hot neat soap into frames, or into crutching pans for the addition of rosin soap (washer soaps), or medicaments, perfume, colouring matter, etc. (toilet soaps, q.v.), or for "running," "liquoring" filling" (household soaps). Running or liquoring genuine soap consists in the addition of alkaline solutions; soda and sodium silicate are commonly used, and pearl ash is often added to improve the appearance. These materials exert a certain cleansing action and cannot be considered as sheer adulterants such as the "fillers," of which sodium sulphate, talc, chalk, starch, barytes, etc., are typical. In the presence of silicate and salt it is possible to produce a firm soap containing more than the 30% of water char acteristic of genuine soap. Rosin may be added to the charge, preferably during the strength boil, but it is more usual to saponify it separately, and to add the resultant rosin soap in the crutching pan; its presence imparts good lathering power to the soap and it possesses a fair detergent action.
After crutching, the soap is run into the frames to cool. Machin ery to effect more rapid cooling has been introduced recently and has an increasing vogue, especially on the Continent ; the pro duct of rapid chilling is not quite identical with, nor so satisfactory for some uses as, the soap obtained by slow spontaneous cooling. The cold soap is cut into bars and slabs (by wires stretched on frames). The bars are air-dried for a day, to form a sort of crust, which is sufficiently firm to enable the brand, etc., to be stamped on the tablet. This process completes the manufacture of soap for household and technical use.