The amount of salt required for "graining out" varies with the raw material used. A tallow soap is the most easily grained, more salt is required for cotton-seed oil soap, whereas soaps made from cocoa-nut and palm-kernel oils require very large amounts of salt to grain out thoroughly. Owing to the solubility in weak brine of these latter soaps, it is preferable to grain them with caustic soda lye. This is effected by adding, during boiling, sufficient caustic lye (32-1/2 deg. Tw., 20 deg. B.) to produce the separation of the granules of soap. The whole is allowed to rest; the separated half-spent lye is withdrawn and may be used for the pasting of fresh materials. After the removal of the settled lye, the grained mass is boiled with sufficient water to dissolve the grain and make it smooth ("close" it), and is now ready for the next operation of "boiling on strength". (_c_) _Boiling on Strength or Clear Boiling._--This is the most important operation and is often termed "making the soap". The object is to harden the soap and to ensure complete saponification. Caustic soda lye (32-1/2 deg. Tw., 20 deg. B.) is gradually added until the soap is again opened or grained, and boiling continued by the use of the dry steam coil. As soon as the caustic soda lye is absorbed, another portion is slowly added, and this is continued until the caustic soda or "strength" remains permanent and the soapy mass, refusing to absorb more, is thrown out of solution and grained. The granular mass will boil steadily, and the boiling should be prolonged, as the last traces of neutral oil are difficult to completely saturate with alkali. Thorough saponification takes place gradually, and the operation cannot be hurried; special care has to be bestowed upon this operation to effect the complete combination of fat and alkali. After resting for several hours, half-spent lye settles to the bottom of the pan. In the case of yellow soaps or milling bases the settled lye is removed to a suitable receptacle and reserved for use in the saponification of other material, and the soap is then ready for the final operation of "fitting". (_d_) _Fitting._--If the operations just described have been properly performed, the fitting should present no difficulty. The soap is boiled with open steam, and water added until the desired degree of closing is attained. As the water is thoroughly intermixed throughout the mass the thick paste gradually becomes reduced to a smooth, thin consistence. Samples are tested from time to time as to their behaviour in dropping off a hot trowel held sideways; the thin layer should drop off in two or three flakes and leave the surface of the trowel clean and dry. The soap is then in a condition to allow the impurities to gravitate. According to the required soap, the fit may be "coarse" ("open") when the flakes drop off the trowel readily, or "fine" ("close") when the flakes only leave the trowel with difficulty.