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water. The upper part, bbb b, which never comesinto contact with the fire, and is intended to affordspace for the soap to rise, expands in the form of acone. The fire-place, f is separated from the ash-pit, 7i, by the grate, r. The fire, after having heated
the bottom of the pan, passes by the flue, xxx, halfround the side of the pan into the chimney, e. Thisis accessible for the purpose of cleaning by the door,P. The soot is thrown into the pit, K. A tube,with a cock, ?/, leads from the lowest part of the panfor the removal of the lye. The whole of the panis sunk into the floor of the boiling house in such amanner that the brickwork of the upper part projectsabout 3 feet above the ground.
These pans are made to contain 240 cwts. of soap.The upper part is frequently constructed of oakstaves, and sometimes a siphon is used instead ofthe cock and tube, y. The violent ebullition of adirect boiling is considered so advantageous as tocounterbalance loss from occasional burning.
A boil of 120 to 160 cwts. of oil passes throughthree stages. At first a lye of from 8° to 11° B. isused. This is prepared in the boiler by mixing thefirst and last lyes above mentioned. As soon asthis has come to the boil the whole of the oil is runin, and soon an emulsion is formed, which thickensas the evaporation of the water proceeds. If oilfloats on the surface more lye is added. The soapmust remain in solution; if it separates an extraquantity of weak lye is added: 6 to 8 lbs. of greenvitriol are added for the purpose of mottling. Thenext stage is the addition of salt, or rather a stronglye containing salt. This is poured gradually intothe pan, with constant stirring until the soap beginsto congeal and separate in flocks. In the course ofthree hours the lye below it may be drawn off. Thesoap is next scooped into one pan to finish: 280gallons of saline lye of 18° to 20° are added and theboiling proceeded with. The further assimilation ofsoda is now dependent upon the concentration of saltlye in the pan. When the lye appears no longercaustic, that which is in the lower part of the pan isdrawn off and replaced with fresh soda lye. Theoperation is repeated from four to six times, during
which the soap acquires great consistence, and thealkalinity of the lye is less easily destroyed.
At length the lye retains its alkalinity, and thesoap begins to boil in a jerky fashion; the frothymass forms itself into granules, and the smell of oilis no longer perceptible. The curd when pressedinto the palm of the hand should form a firm granularcake. The fire is kept up for from ten to eighteenhours after the last portion of lye has been added.
The curd has a slate colour, produced by thesulphide of iron and iron soap. In order to dis-seminate the colouring matter as veins throughthe soap, the consistence of the soap (at presenttoo great) is diminished to the proper amountby sprinkling the surface of the mass with weaklye, at the same time that workmen rake andstir it about with stirrers in such a manner asto bring the lower parts of curd to the top,thus equalizing the lye and colouring matters.
The soap is then scooped into moulds, whenin eight or ten days it cools down to mottledsoap. The reddish tint is due to exposure tothe air.
Soft Soap. —This variety of soap differs inmany essential particulars from those already de-scribed. The alkali used in its production is potash,and the oil, either in whole or in part, a drying oil,as that of hemp seed, poppies, &c., or a fish oil, aswhale or seal oil. The reaction of potash upon fatsand oils is precisely the same as in the case of soda.
Soaps with potash are, however, manufactured onan entirely different system to those of soda or hardsoap In the latter, the soap is withdrawn from thelye when only a portion of the oil has been saponified, and fresh lyes are added until saponification iscomplete.
In the production of potash or soft soaps no lyesare separated during the whole course of operation;the only object being the combination of the potashwith the fat or oil. Potash soap, therefore, of coursecontains the whole of the base of the oils or glycerin,which is expelled from combination by the strongerbase, as well as the impurities always existing in thelye, as carbonate of and caustic potash, chloride ofpotassium, sulphate of potassium, and in small quan-tities many other salts, which in the manufacture ofhard soaps are removed in the exhausted lyes. Pot-ash soap, also, always contains a much greater pro-portion of water than hard soaps, the quantity beingin the majority of instances from 50 to 60 per cent.If desired, the glycerin of the fat and saline impuri-ties of the lye might easily be, removed by the appli-cation of strong lyes towards the completion of theprocess, but on the large scale such a system is neverpursued. A much larger proportion of alkali is con-sumed by a given weight of oil or fat, when saponi-fied by potash, than when soda is used, 1 part ofpure potash being taken up by from 41, to 5 parts ofoil; and hence the quantity of real soap producedfrom a given weight of oil or fat when saponified bypotash is considerably greater than when the alkaliused is soda; and this quantity is still further aug-mented by the large proportion of water, together