The principles involved in this process are briefly these:

The fat is partly saponified with weak lyes (usually those obtained from a previous boiling in the strengthening change are used), and salt is added to grain the soap. The ma.s.s is then allowed to settle into two layers. The upper layer is partly saponified fat; the lower layer, or spent lye, is a solution of salt, glycerine, and contains any alb.u.minous matter or any other impurity contained in the fat. This is known as the _killing_ or glycerine change. Strong lyes are now added and the fat entirely saponified, which is termed the _strengthening change_. The ma.s.s is then allowed to settle and the fluid soap run off above the "nigre." This operation is called the finish or _finishing_ change.

The method may be more fully ill.u.s.trated by a concrete example of the method of manufacture of a tallow base:

Charge--

Tallow 88 per cent.

Cocoanut oil 10 per cent.

Rosin w. w. 2 per cent.

Amount charge 10 tons

About five tons of tallow and one ton of cocoanut oil are pumped or run into the soap kettle and brought to a boil with wet steam until it briskly comes through the hot fat. The caustic soda (strengthening lyes from former boilings may be used here) is gradually added by the distributing pipe, any tendency to thicken being checked by the introduction of small quant.i.ties of brine ("salt pickle"). If the lye is added too rapidly the soap a.s.sumes a granular appearance, indicating that the addition of same must be discontinued. Water should then be added and the ma.s.s boiled through until it again closes. When the addition of the proper amount of caustic soda is nearing its completion the soap gradually thins. The steam is now cut down to about one turn of the valve, and brine is rapidly added or salt shoveled in. In ten to fifteen minutes the steam again breaks through and, from the appearance of the soap, it can be seen whether sufficient brine has been added. A sample taken out by means of a long wooden paddle should show the soap in fine grains with the lyes running from it clear. The steam is then shut off and the soap allowed to settle from one and one-half to two hours. In all settlings the longer time this operation is permitted to continue, the better will the subsequent operations proceed.

The mixture now consists of a partly saponified layer of fat above the spent lyes. The lyes are drawn off until soap makes its appearance at the exit pipe. The valve is then closed and the soap blown back into the kettle by steam. The lyes thus obtained are known as _spent lyes_, from which the glycerine is recovered. They should show an alkalinity of approximately 0.5 per cent. if the operation is carefully carried out.

The remaining tallow is now added and the above operations repeated.

After the spent lyes have been drawn off, the soap is closed with water and the proper percentage of rosin soap previously formed, or rosin itself is added to the ma.s.s in the kettle. More lye is then allowed to flow in until the mixture is up to "strength." This is usually tested by the "bite" on the tongue of a small cooled sample. After boiling until the steam comes through, the ma.s.s is grained with salt as before and allowed to settle one and one-half to three hours. These lyes, known as _strengthening lyes_ are run to storage to be used subsequently with fresh fat to take up the caustic soda contained therein.

The soap is now ready for finishing and is first boiled through and tried for strength. A drop of phenolphthalein (1 per cent.

phenolphthalein in 98 per cent. alcohol) is allowed to drop on the molten soap taken up on a trowel. The red color should be instantly produced and develop to a full deep crimson in a few seconds, or more lye must be added until this condition is realized. Should it flash a deep crimson immediately it is on the strong side. This cannot be conveniently remedied; it can only serve as a guide for the next boil, but in any case it is not of any serious consequence, unless it is too strong.

With the steam on, the soap is now examined with a trowel which must be thoroughly heated by working it about under the surface of the hot soap.

The appearance of the soap as it runs from the face of the trowel indicates its condition. It is not possible to absolutely describe the effect, which can only be properly judged by practice, yet the following points may serve as a guide. The indications to be noticed are the shape and size of the flakes of soap as the sample on the trowel breaks up and runs from the hot iron surface, when the latter is turned in a vertical position, as well as the condition of the iron surface from which the soap flakes have fallen. A closed soap will run slowly into a h.o.m.ogeneous sheet, leaving the trowel's surface covered with a thin layer of transparent soap; a grained ma.s.s will run rapidly down in tiny grains, about one-half an inch in diameter or less, leaving the hot trowel absolutely dry. The object of the finish is to separate the soaps of the lower fatty acids from those of the higher, and both from excess of liquid. A point midway between "open" and "closed" is required to arrive at this point.

Having arrived at the above condition, the soap is allowed to settle anywhere from one to three days and then run off through the skimmer pipes to the nigre and framed or pumped to the tank feeding the drying machine.

The stock thus obtained should be fairly white, depending upon the grade of tallow used and slightly alkaline to an alcoholic phenolphthalein solution. If removed at exactly the neutral point or with a content of free fat the soap will sooner or later develop rancidity. The soap thus obtained is an ordinary tallow base, and the one by far greatest used in the manufacture of toilet soaps. The percentage of cocoanut oil indicated is not fixed and may readily be varied, while in fine toilet soap the rosin is usually eliminated.

In the manufacture of full boiled soda soaps in which no glycerine is obtained as a by-product, it being retained in the soap itself, the soap formed is known as a "run" soap. The process is used most extensively in the manufacture of marine soaps by which the method may be best ill.u.s.trated. This soap is known as marine soap because of its property of readily forming a lather with salt water and is mostly consumed aboard vessels.

Marine soaps are manufactured by first placing in the kettle a calculated amount of lye of 25 deg. to 35 deg. B., depending upon the amount of moisture desired in the finished soaps, plus a slight excess required to saponify a known weight of cocoanut oil. With open steam on, the cocoanut oil is then gradually added, care being taken that the soap does not froth over. Saponification takes place readily and when the oil is entirely saponified the finished soap is put through the process known as running. This consists in constantly pumping the ma.s.s from the skimmer pipe back into the top of the kettle, the object being to prevent any settling of the nigre or lye from the soap, as well as producing a h.o.m.ogeneous ma.s.s. It is customary to begin the saponification in the morning, which should be completed by noon. The soap is then run for about three hours and framed the next morning.

After having remained in the frame the time required to solidify and cool, the soap is slabbed and cut into cakes. This process is difficult to carry out properly, and one not greatly employed, although large quant.i.ties of marine soap are purchased by the government for use in the navy and must fulfill certain specifications required by the purchasing department.

In making potash soaps it is practically impossible to obtain any glycerine directly because of the pasty consistency of the soap, and no graining is possible because the addition of salt to a soft soap, as already explained, would form a soda soap. Large quant.i.ties of soft soaps are required for the textile industries who desire mostly a strong potash soap, and the large number of automobiles in use at the present time has opened a field for the use of a soft soap for washing these. A soap for this purpose must be neutral so as not to affect the varnish or paint of automobiles.

A suitable soap for textile purposes may be made as follows:

Red oil 80 parts House grease 20 parts Caustic soda lye, 36 degs. B. 3 parts Carbonate of potash 5-1/2 parts Caustic potash 23-1/4 parts

Olive oil, corn oil, soya bean oil, olive oil foots or cottonseed oil may replace any of the above oils. A large quant.i.ty of cottonseed oil will cause the soap to fig.

To carry out the process, the caustic potash and carbonate of potash are dissolved and placed in the kettle together with the soda lye, and the oils added. This is most satisfactorily accomplished by being finished the day before the boiling is begun. The next day the boiling is begun and water added to bring the soap up to the desired percentage of fatty acid, due allowance being made for the water formed by the condensation of the open steam in boiling. Care must be taken that the soap in the kettle does not swell and run over during the saponification. A good procedure is to use open steam for a period of about two hours, then close the valve and allow the saponification to continue without boiling, and repeat this until it is entirely saponified. After the saponification has been completed the soap is briskly boiled all day and the proper corrections made; that is, if too alkaline, more oil is added, and if free fat is present, more potash. About 2 per cent.

carbonate of potash is the proper amount for a soap containing 50 per cent. fatty acid. The soap is sampled by allowing it to drop on a clean, cold gla.s.s surface. In so doing, the soap should not slide or slip over the gla.s.s surface when pressed thereon, but should adhere to the gla.s.s, or it is too alkaline. A sample worked between the fingers showing too much stringiness should have more strong potash and oil added. A sample taken out in a pail and allowed to cool over night will serve as a guide as to the body of the soap in the kettle. When the soap has thus been properly finished it is run into barrels.

For an automobile soap the following is a good working formula:

Corn oil 1,000 parts Potash lye, 31-1/2 degs. B. 697 parts

Proceed as in the directions just given for textile soap in placing charge in the kettle. When the kettle is boiling up well, shut off the steam and the saponification will complete itself. The soap may be run into the barrels the next day.

A heavy soap with a smaller percentage of fat may be made as follows:

Corn oil 1,000 parts Potash lye, 24-1/2 degs. B. 900 parts

Boil until the soap bunches, and shovel the finished soap into barrels.

Upon standing it will clear up. By the addition of more water the yield of soap per pound of oil may be run up to 300 per cent.

After soft soaps have been allowed to stand for some time the phenomenon known as "figging" often occurs. This term is applied to a crystalline-like formation, causing spots of a star-like shape throughout the soap. This is undoubtedly due to the stearine content of the soap crystallizing out as it cools, and forming these peculiarly-shaped spots. It more generally occurs in the winter and may be produced artificially by adding a small quant.i.ty of soda to the potash lye before saponification.

The oils usually employed in the manufacture of potash soaps are cottonseed oil, corn oil, soya bean oil, olive oil foots, red oil, cocoanut oil, grease and the various train oils. The usual percentage yield is from 225 per cent. to 300 per cent., based upon the weight of oil used. In calculating the weight of a soft soap it is to be remembered that since pota.s.sium has a higher molecular weight (56) than sodium (40), the corresponding soap formed is that much greater in weight when compared with a sodium soap. Rosin may be added to soft soaps as a cheapening agent.

COLD PROCESS.

The cold process for manufacturing soap is the simplest method of soap making, and the equipment required is small when compared to the other methods. All the more expensive equipment that is necessary is a crutcher, a tank to hold the lye, frames, a slabber or cutting table, and a press. Yet, in spite of the simplicity of thus making soap, the disadvantages are numerous for the production of a good piece of soap.

The greatest difficulty is to obtain a thorough combination of oil or fat and lye so that there will not be an excess of one or the other in the finished soap. At its best there is either a considerable excess of free fat which later exhibits itself in producing rancidity or uncombined caustic, which produces an unpleasant effect on the skin when the soap is consumed for washing. The latter objection, of course, can only be applied to toilet soaps.

Cocoanut oil is used very largely in the manufacture of cold-made soaps as it is well adapted for this purpose, although it is by no means true that other oils may not be employed. Since by this process of manufacture no impurity contained in the fat or oil is removed in the making of the soap, it is necessary that in order to obtain a fine finished product, any impurity contained in these may be removed if present, or that the fats be as pure as can be obtained. If inedible tallow is used for cold-made soap, it is advisable to bleach it by the Fuller's Earth Process.

The carrying out of this method is best ill.u.s.trated by an example of a cold-made cocoanut oil soap.

Charge:

Cochin cocoanut oil 846 parts Lye (soda), 35 degs. B. 470 parts Water 24 parts

The oil is run into the crutcher and the temperature of the oil raised to 100 degs. F. by dry steam. The lye and water are at room temperature.

After all the oil is in the crutcher, the lye and water are slowly added to prevent any graining of the soap. Toward the end the lye may be added more rapidly. When all the lye is in, the ma.s.s is crutched for about three hours, or until upon stopping the crutcher a finger drawn over the surface of the soap leaves an impression. If this condition is not realized, the soap must be mixed until such is the case. Having arrived at this point, the mixture is dropped into a frame which should remain uncovered. The heat produced by the further spontaneous saponification will cause the soap to rise in the middle of the frame. After having set for some days it is ready to be slabbed and cut into cakes.

A potash soap may be made by the cold process just as readily as a soda soap. Soaps of this type may be made by either of these formulae in a crutcher:

Olive oil foots 600 Potash lye, 18 degs. B. hot, 20 degs. B. cold 660

or

Corn oil 800 Rosin 200 Potash lye, 27 degs. B. 790 Water 340

Heat the oils to 190 degs. F., add the lye and crutch until the soap begins to bunch, when it is ready to be run into barrels where the saponification will be completed.

Semi-boiled soaps differ from those made by the cold process in temperature. In making semi-boiled soaps the fats are usually heated to 140 F. The addition of the lye raises the temperature to 180--200 F.

when saponification takes place.

CARBONATE SAPONIFICATION.

The method of the formation of soap by the utilization of the fatty acid directly, from which the glycerine has already been removed by some method of saponification other than with caustic soda, and neutralizing this with alkali, is becoming increasingly popular. The glycerine is more easily recovered from a previous cleavage of the fats or oils, but a soap made from the mixed fatty acids thus obtained is seldom white in color and retains an unpleasant odor. Since soda ash or sodium carbonate is cheaper than caustic soda and readily unites with a fatty acid, it is used as the alkali in the carbonate saponification. The process is similar to that already given under Rosin Saponification. About 19 per cent. by weight of the fatty acids employed of 58 per cent. soda ash is dissolved in water until it has a density of 30 degs. B., and the solution is run into the kettle, which is usually equipped with a removable agitator. The fatty acids, previously melted, are then slowly added while the mixture is boiled with open steam and agitated with the stirring device. The fatty acids instantly unite with the carbonate and rise in the kettle, due to the generation of carbon dioxide, and care must be exercised to prevent boiling over. After all the fatty acid has been added, and the ma.s.s is boiled through the saponification must be completed with caustic soda, as there is as yet no practical method known which will split a fat entirely into fatty acid and glycerine.

Thus about 10 per cent. of the fatty acids are true neutral fats and require caustic soda for their saponification. This is then added and the soap completed, as in full-boiled soaps.