To make an unsophisticated soft soap, a suitable quantity of oil is run into the copper, not exceeding its total capacity; at the eame time, potash leys at 9°-11° B., not absolutely caustic, but retaining some potassium carbonate, are let in, and the steam is turned on, or the fire kindled, or both; the fan may also be adjusted at the height beyond which the soap is not to boil, and the whole is carefully watched. If the copper has not boiled until a volume of leya has run in equal to that of the fat, the edema] of leys should be stopped, and started again when the ebullition commences. If the oil and leys do not appear to have combined, the fire should be checked, the stream of leys stopped, and gentle steam-boiling continued until this is the case. It is very difficult, especially with rape-oil, to get the alkali to combine, but when once the process has begun, it goes on with tolerable rapidity with subsequent additions of leys. From time to time, small aamples of the soap are dropped upon a glass plate, and after cooling at a temperature aa near 8° (46°• 4 F.) as can be obtained, are carefully examined. The soap is good if it is clear and translucent; a fatty border indicates deficiency of alkali; while if the sample be granular, grey, and lustreless, too much ley has been added, a fault that must be corrected by the addition of more oil, previously mixed with leys at 2° B. Should the sample separate on the glass into soap and clear liquor, the quantity of leys is excessive. If the combination be good, and alkali deficient, stronger ley (at first 15°-16°., then 23°--25° B.) may be cautiously added ; a sign of saturation, or rather alight excess of alkali, is the appearance of a etiiped skin, or ley-veins, on the surface of the sample. When it is judged that enough alkali is present, the steam is turned off, and 8. certain amount of water is evaporated by boiling the copper with fire, during which operation the bubbles get larger, the soap being almost laminated, and make so much noise in their escape that the boiler's phraae.is " the soap talks." When finished, a small sample must not glide or he elippery ou the glass, nor must it draw into threads when worked up between the fingers and thumb; a very snaall ring should appear in the sample in 12-15 min., indicating the necessary slight excess of potash. 'fhe soap is filled into barrels while quite hot, and to pron3ote rapid cooling of the mass, cold soap is often added.
A eomewhat shorter method, saving the evaporation in the later stagea, has been introduced of late. For every 100 lb. oil, 200 lb. lop at 20° B. are required ; when liquid fate are used, this quantity is run in at the commencement of the operation ; with solid fats, may be taken, and when thoroughly incorporated, the rest may be added, aud the soap boiled as preN iously described.
If it be desired to make a soft-soap in which some of the potash as replaced by soda, the proportions of the two hays must be accurately adjusted to each other, and to that of the fat used. The process was first worked out by Gentele, and improved by Cristiani, who recommends for the saponification of oil by R. potash and * soda the following formula:-5000 lb. oil, 2674 lb. potash ley at 20 B., 740 lb. potash ley at 25° B., and 2353 lb. soda ley at 20° B. If enough steam be not condensed iu the boiling, water may be added to make the whole weigh 12,500 lb.
To produce a graiued soft-soap (or " fig"), it is essential to use pure potash leye, and to employ some hard fat, the stearic or palmitie acids of which form potash salts whose crystallization produces the grains, within somewhat narrow limits of temperature, viz. 9° (48°•2 F.) and 15° (50° F.). The following fat mixtures will produce it :--(a) 55 palm-oil, 45 oleic acid; (b) 55 palm oil, 15 tallow, 30 linseed-oil; (e) 70 palm-oil, 30 linseed-oil. An artificial grain is given by clay starch, &o.
Two kinds of genuine soft-soaps occur in commerce, whose composition is :— The question of admixtures with genuine soft-eoap, after its fabrication has been completed, is one that dernands the serious attention of both manufacturer and consumer. They may be divided into two classes : (1) those intended to increase the detersive power ; and (2) those added solely to cheapen the precinct. To the latter, belong clay, starch, fecula, glue, and a number of other fraudulent admixtures, whose detection will be dealt with under the head of soap-analysis. The first class demands o. few explanatory words, and contains chiefly two substances, rosin, and silicate of soda or potash; the manufacture of the latter, and their use in hard soaps, are described on pp. 1786-7. For soft-soap intended for household and laundry purposes, rosin may be substituted for part of the saponifiable material (to the extent of 10-15 per cent. upon the total oil used) without injury, and, in some cases, with actual benefit ; in the kame class of soaps, the addition of silicate (or carbonate) of potash or soda certainly increases the detersive power, especially where hard water is employed. Most of the soft-soaps made, however, are used by woollen ntanufacturers, for wool-washing, fulling, scouring, and sizing, and there is no doubt that the best soap for these purposes is a genuine potash-oil-soap. Experience has shown that the addition of rosin has an undesirable effect upon the fibre, and that the presence of soda in any form is absolutely injurious to it. Wool in its natural state is lubricated by " suint," which contains nearly 50 per cent. carbonate of potash, and scarcely a trace of soda ; it is evident therefore that in discarding soda, and using potash, the manufacturer follows the teachings of nature. The use of silicate of potash is injurious, since it attacks the fibre of the wool, and in some cases hy its decomposition, even deposits silica therein, greatly to the detriment of the ultimate fabric. So much injury has been done by the use of unsuitable soaps, that many woollen manufacturers have been driven to make their own, thereby, as they think, ensuring purity. This, however, is also a hazardous proceeding, and it would be really more to their interest to state their exact wants, and to pay a proper and fair price for a soap carefully made with all the appliances and knowledge of a large soap-factory, than to run the risk of using a product in which, from want of practice or knowledge, a s( rious oversight had occurred. The excessive desire for cheapness on the part of purchasers has done more than anything else to depreciate the quality of soft-soaps (and of others). Further general remarks on this subject, and upon the desirability of purchasers buying soaps whose composition is guaranteed by analysis, will be found on pp. 1793-4.