If no other than hard water is available, one of the various plans for softening it should be adopted under the guidance of a chemist, such as the addition to the water of milk of lime, either alone or with chloride of barium, or of silicate of soda.
Salt.—As will be seen in the sequel, common salt plays a very important part in soap-boiling, and what has been said with regard to water applies equally to it. The purest kind is rock-salt freed from insoluble matter ; next in order comes salt from brine-springs ; while sea-salt containe so many other salts besides sodium chloride, especially magnesium chloride, that it should be avoided if possible. When that is not the case, it should be dissolved in water, and the magnesia removed by the addition of silicate of soda, when the insoluble silicate of magnesia ehould be allowed to subside before the brine is used. In connection with salt, it will be found convenient to remember that strong brine or a saturated solution of salt contains one-fourth its weight or 25 per cent. of sodium chloride, that its sp. gr. is 25° B., and that if nothing else be present iu solution, every degree Baume of sp. gr. corresponds to 1 per cent. of salt.
It will be convenient here to consider a certain physico-chemical property of soaps, of great practical importance to the soap-boiler. The property referred to is the behaviour of soap to various saline solutions, and although not instrumental in the formatiou of soap, it is almost essential in separating the foreign matters that would otherwise render hard soap impure, and it ls also influential iu controlling the amount of water in soap. Although soda soaps are soluble in water, they are not soluble in a solution of common salt, nor of caustic soda. If, therefore, common salt be added to a solution of soap (or even of partially saponified fat) in water, the salt dissolves, and turns the soap out from its state of solution, in small fla.kes which collect together, and float on the surface of the salt solution, by virtue of their less sp. gr. The same thing happens when a strong solution of caustic soda is added to soap in an aqueous solution, or, more gradually when a solution of soap in water containing excess of caustic-soda, or some amount of sodium chloride, is concentrated by the evaporation of its water, as when a soap-copper is boiled by fire or close steam. The addition of salt (or of strong leys), therefore, to soap containing an excess of
water, removes the superfluous water, and in chemical language, precipitates the soap from it. Soap so precipitated contains 35-40 per cent. of water. When the fatty matter employed contains coco-nut- or palm-kernel-oils, more salt (or soda-leys) is required for this operation than when those oils aro not used.
It may also be noted here that if sodium chloride be added to a potash soap in eolution, an inlerchange of acids and bases takes place, soda soap being formed, and potassium chloride left in the solution. This process has been actually used in Germany for the fabrication of hard soaps.
MANUFACTIIRE.—Before proceeding to describe this on a large scale, where operations invulving conaiderable mechanical, physical, and chemical knowledge are conducted with a view to produce the best possible artide at the lowest cost, a short apace will be devoted to instructions for making small quantities of soap of inferior appearance, which will answer well for homely purposes, for the benefit of those living far from large towns, and who may yet have on their farms or stations many of the ingredients necessary for their production.
For those who have plenty of fat or oil at command, but no alkali, the small canisters of pure powdered caustic soda and caustic potash, sold by the Greenbank Alkali Co., St. Helen's, will be found very convenient. With these products, soapa can be made without any boiling. For a hard soap, dissolve 10 lb. of this soda in 4 gal. of water, and allow the leys to cool. Take 75 lb. of clean fat or oil, rendered fluid by heat if neceasary, and when it feels just warm to the hand, add the leys to it in an uninterrupted stream, stirring well all the time ; continue the affixing for 15 or 20 minutes, and then set aside in a warm place for a day. In this interval, the soda reacts upon the fat, and turns out its glycerine, which remains in the soap. Any impurities in the fat used, or any salt or other extraneous substance, will be apt to spoil the operation. For soft soap, use 20 lb. of this caustic potash, dissolved in 3i gal. of water, and mix as above with 81 gal. of cotton-seed, fish, or other (non-mineral) oil. For a harder soap, one or more of the gallons of oil may be replaced by 10 lb. (or a multiple thereof) of tallow.