770

SOAP.— Manufacture.

behaviour of soap with a solution of this salt wasnoticed by Liebig :—•

If a piece of common hard soap be placed in a solu-tion of salt at ordinary temperature, it floats upon thesurface without ever being moistened, and if the liquidbe heated to boiling, it separates without foam intogelatinous floccuke, which collect on the surface, andupon cooling unite into a solid mass, from which thesolution flows off like water from fat. If the flocculsebe taken out of the hot fluid, they congeal on coolinginto an opaque mass, which may be pressed betweenthe fingers into fine laminse without adhering tothem. If the solution is not quite saturated, thesoap then takes up a certain quantity of water, andthe flocculse separate through the fluid on boiling.But even when the water contains only ^-J^-th partof common salt, ebullition does not produce solution.If the soap is boiled in a dilute and alkaline solutionof salt, and suffered to cool, it again collects on thesurface of the fluid in a more or less solid state,depending on the greater or less degree of concen-tration of the solution; that is, on the quantity ofwater taken up by the soap. By boiling the dilutesolution of salt with soap for a considerable time,the aqueous flocculse intumesee, and the mixtureassumes a foamy appearance, but still they are notdissolved, as the solution separates from them. Theflocculse, however, have become soft and pasty evenafter cooling, and their pastiness depends on thequantity of water they have imbibed. By still con-tinued boiling this character again changes, and inproportion as the water evaporating renders thesolution more concentrated, the latter again extractsthe water from the flocculse; the liquid, however,continues to foam, but the bubbles are larger. Atlength a point is attained at which the solutionbecomes saturated ; the larger iridescent bubblesformed just before disappear, and the liquid continuesto boil without froth ; all the soap collects as a trans-lucent mass on the surface, and now the solution andsoap cease to attract water from each other.

If the plastic soap be now removed and cooled,while the solution is pressed out, it becomes so solidas scarcely to receive an impression from the fingers.In this state it is called grain soap. The addition ofsalt, or its solution, to a concentrated alkaline men-struum of soap in water, precipitates the soap ingelatinous floccuke, and the mixture behaves preciselyas solid soap boiled with a dilute solution of salt.Caustic potash and carbonate of potash act like saltin separating soap from the alkaline fluid. Theapplication of these facts to the manufacture of soapis obvious. The fat is kept boiling in an alkaline lyeuntil all pasty matters disappear; but the lye shouldhave such a strength that the soap may be per-fectly dissolved in it. Thus tallow may be boiledfor days in a caustic potash solution, of specificgravity 1-25, without being saponified. If the lye bestronger a partial saponification ensues; but beinginsoluble in the fluid, the soap floats on the surfaceas a solid mass. By the gradual addition of waterwith continued boiling, the mass at a certain pointbecomes thick and clammy, and with more water

an emulsion is formed; on continued heating thisbecomes perfectly clear and transparent, if a sufficientquantity of alkali be present. In this state it may bedrawn out into long threads, which on cooling eitherremain transparent, or are milky and gelatinous. Aslong as the hot mass, suffered to drop from a spatula,exhibits a milkiness or opalescence, the boiling iscontinued, or more alkali added. When excess ofalkali is present, the milkiness arises either from im-perfect saponification or want of water; the formeris known by dissolving a little in pure water, whichbecomes perfectly clear when the whole is saponified.

If the lye contains lime the mixture is also turbid,but the addition of an alkaline carbonate causes theturbidity arising from this cause to disappear instantly.In order to separate the soap from water, free alkali,and glycerin, a large quantity of salt is graduallyadded to the boiling mass, waiting after each additiontill that portion is completely dissolved. The firstaddition increases the consistency of the mass, whileeach successive portion renders it more fluid, till itloses its adhesive character, and drops from thespatula in short thick lumps. As soon as the con-gelation is complete—that is, when gelatinousflocculse separate from a clear watery liquid—thefire is extinguished, the soap allowed to collect onthe surface, and cooled either on the liquid, orladled out and allowed to get solid.

In being separated by means of salt from a verydilute solution, soap suffers an alteration in compo-sition, the soap so thrown down being a distearate,dipalmitate, or dioleate of the alkali, instead of theneutral compound originally in solution ; but whenthe solution is of moderate strength, the soap is separ-ated unaltered in composition. The same results asthose obtained by the use of common salt are alsoproduced, although in a less energetic manner, bychloride of potassium, carbonates of the alkalies,sulphate of soda, acetate of potash, and chloride ofammonium. Of these, sulphate of soda and chlorideof potassium have but a very slight action. Con-centrated caustic lyes also separate soap from itssolution, in the same manner as with common salt;in weak caustic lye, on the contrary, soap is perfectlysoluble. On this account soap-boilers, especially atthe commencement of the operation, except in thecase of cocoa-nut oil, always use weak lyes, as thestronger would prevent the necessary amount ofcontact amongst the ingredients, and very muchretard the process of saponification. Not only allforeign matters other than soaps, but also the baseof the oils—glycerin—is completely separated fromsoap by means of caustic or saline solutions, in whichit is perfectly soluble. The separation of the smallquantities of earthy and metallic soaps, alwaysformed during the process of saponification by meansof impure alkalies, requires a distinct operation,termed the fitting process, which will be presentlyalluded to.

Manufacture. — The boilers used in the soap manu-facture are formed of cast-iron or wrought-iron plate.Their capacity varies according to the quantity ofsoap to be produced—some being as large as 15 feet