Classification of Soap-Making Methods.
In the saponification of fats and oils to form soap through the agency of caustic alkalis, as has been stated, the sodium or potassium salts of the mixed fatty acids are formed. Sodium soaps are usually termed hard soaps, and potassium soaps soft. There are, however, a great many varieties of soaps the appearance and properties of which depend upon their method of manufacture and the oils or fats used therein.
The various methods adopted in soap making may be thus classified:
1. Boiling the fats and oils in open kettles by open steam with indefinite quantities of caustic alkali solutions until the finished soap is obtained; ordinarily named full boiled soaps. These may be sub-divided into (a) hard soaps with sodium hydrate as a base, in which the glycerine is recovered from the spent lyes; (b) hard soaps with soda as a base, in which the glycerine remains in the soap, e. g., marine cocoanut oil soaps; (c) soft potash soaps, in which the glycerine is retained by the soap.
2. Combining the required amount of lye for complete saponification of a fat therewith, heating slightly with dry heat and then allowing the saponification to complete itself. This is known as the cold process.
3. Utilizing the fatty acid, instead of the neutral fat, and combining it directly with caustic alkali or carbonate, which is incorrectly termed carbonate saponification, since it is merely neutralizing the free fatty acid and thus is not a saponification in the true sense of the word. No glycerine is directly obtained by this method, as it is usually previously removed in the clearage of the fat by either the Twitchell or autoclave saponification method.
In the methods thus outlined the one most generally employed is the full boiled process to form a sodium soap. This method of making soap requires close attention and a knowledge which can only be obtained by constant practice. The stock, strength of lyes, heat, amount of salt or brine added, time of settling, etc., are all influencing factors.
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 mass 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 albuminous 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 mass 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 illustrated 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 quantities of brine ("salt pickle"). If the lye is added too rapidly the soap assumes a granular appearance, indicating that the addition of same must be discontinued. Water should then be added and the mass 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 mass 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 mass 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 homogeneous sheet, leaving the trowel's surface covered with a thin layer of transparent soap; a grained mass 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 illustrated. 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 mass 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 homogeneous mass. 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 quantities 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 quantities 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 quantity 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 glass surface. In so doing, the soap should not slide or slip over the glass surface when pressed thereon, but should adhere to the glass, 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 quantity 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 potassium 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.