This is done by stirring into the ingredients of the soap-kettle either soda ley containing salt, or a solution of salt, or dry salt. The separation is founded upon the insolubility of the soap in brine or strong caustic leys, whereas weak leys would dissolve it. Of all soaps the cocoanut oil is the most remarkable, for, being dissolved by a brine solution, it is peculiarly serviceable for washing in salt water, whence its name, marine soap. ] This soap becomes so hard, that when separated from the glycerine it cannot be cut with a knife, and consequently the salting operation should not be per- formed, but the soap boiled in strong ley with one water. The following is the method by which the salting operation is effected. One workman gradually adds the brine or dry salt, while another agitates the paste with a stirring rod from below upwards. This is done under gentle boiling. It is essential to add the salt in the right proportion; the whole amount requisite should not be stirred in at once, but in portions of about one-sixth. After half of it has been dropped in, the soap should be allowed to boil for about 10 minutes before any addition is made.
According to concentration, 12 to 16 lbs. of salt are necessary for 100 lbs. of fat, to separate the formed soap from the surplus of water. The separation is perfect, when the aqueous portion is observed to run off from the curdy mass; when a sample is taken with a spatula, it is not of an adhesive character whilst hot; and when, on placing some in the palm of the hand, and rubbing it with the thumb, it hardens into firm scales. The termination of the process is also indicated when the surface splits up into several fields, separated from each other by deep furrows, in which there is not the fresh and soft appearance of froth, but of dry slabs. The fire should be extinguished when the soap, hitherto covered with froth and bubbles, suddenly sinks, and the froth breaks up into roundish massive grains, distinctly separated from each other and from the saline solution. The salting being completed, let the mass remain quiet for several hours, and then the under-ley may be drawn off by the faucet.
This operation is to obtain hardness, consistency, and complete neutrality of the soap. Commence to boil the paste gently with tolerable strong leys. Some manufacturers proportion the quantity of ley to be used, and having put in the first, boil for 8 hours or so, then draw off the ley, put in the second, boil again, draw off, and so on. Should the soap, during the intervals, become too liquid, which may happen if a too weak ley has been applied, some handfuls of salt must be added, or the soap boiled with a weak ley containing salt. After each addition of ley, there should be, in taking up a portion by the spatula, some difficulty in running off the ley. Should this not be the case, water must be added, whereupon a quicker union of the alkali with the fat will be obtained. The process is terminated when large, regular, and dry scales appear on the surface, and when these give elastic, brilliant, white scales, and are easily pulverised by rubbing them in the palms of the hands. The soap should then be covered, left for some time, and eventually removed in the ladles.
The formation of veins in the soap is produced, either as the effect of the ley itself, or by the addition of foreign substances to the soapy paste. Some kinds of sodas employed in the manufacture of soaps contain both the sulphuret of iron and sodium. In saponification a chemical combination takes place between these and the fatty acids. These diffuse themselves throughout the mass, together with black sulphuret of iron, and being held in intimate suspension, form bluish veins in the white ground, thus giving to the soap the appearance of marble. In Castile soap these in course of time, after exposure to the atmosphere, assume a brownish colour, a change caused by oxidation. If the soda employed does not contain those constituents in itself, sulphate of protoxide of iron, or copperas, previously dissolved, is introduced into the soapy paste, say 4 oz. of the dry substance to 100 lbs. of fat. By the chemical union of this oxide with the sulphuret of sodium, always existing in the crude soda, the colouring principle of marbling is produced. Mottled soap, made as above, contains necessarily less water than any other soap, as a superabundance of water would have precipitated the colouring matter, and rendered vein-ing impossible.
For successful marbling, a thorough practical knowledge is absolutely requisite. The essential point is to run the soap into the frames as soon as it presents the indications necessary for obtaining a good marbling. The eye is the best guide in this respect, as there are no precise regulations for this operation. The interspersion of the blue with the red veins is effected by stirring some pulverised colcothar into the soap, after marbling in the ordinary way.
When crystallized sulphuret of sodium is brought together with neutral fats, they are saponified at ordinary temperature and in a very short time. A mixture of equal parts of crystallized sulphide of sodium, olive oil, and water, produces after 10, sometimes after 5 or 6 days a thoroughly saponified paste, consisting of soap, glycerine, sulph-hydrate of sodium, and the surplus of monosulphuret of sodium. When subjected to heat, sulphuretted hydrogen will escape, and soap remain. In this case, one equivalent of sulphide of sodium produces the same quantity of soap as one equivalent of pure caustic soda, but it is not at all necessary to make use of crystallized and chemically pure sulphide of sodium, as that which is obtained by decomposing the sulphate of soda by charcoal can be employed. It is much cheaper than the caustic soda. The appearance of the soap made in this way is exactly the same as that made in the ordinary way; but it retains a disagreeable smell not easily destroyed. For ordinary purposes, however, such as scouring woollen fabrics, this kind of soap may well be used.