Soap-Making Manual A Practical Handbook on the Raw Materials, Their Manipulation, Analysis and Control in the Modern Soap Plant.

Raw Materials Used in Soap Making.

Soap is ordinarily thought of as the common cleansing agent well known to everyone. In a general and strictly chemical sense this term is applied to the salts of the non-volatile fatty acids. These salts are not only those formed by the alkali metals, sodium and potassium, but also those formed by the heavy metals and alkaline earths. Thus we have the insoluble soaps of lime and magnesia formed when we attempt to wash in "hard water"; again aluminum soaps are used extensively in polishing materials and to thicken lubricating oils; ammonia or "benzine" soaps are employed among the dry cleaners. Commonly, however, when we speak of soap we limit it to the sodium or potassium salt of a higher fatty acid.

It is very generally known that soap is made by combining a fat or oil with a water solution of sodium hydroxide (caustic soda lye), or potassium hydroxide (caustic potash). Sodium soaps are always harder than potassium soaps, provided the same fat or oil is used in both cases.

The detergent properties of soap are due to the fact that it acts as an alkali regulator, that is, when water comes into contact with soap, it undergoes what is called hydrolytic dissociation. This means that it is broken down by water into other substances. Just what these substances are is subject to controversy, though it is presumed caustic alkali and the acid alkali salt of the fatty acids are formed.

OILS AND FATS.

There is no sharp distinction between fat and oil. By "oil" the layman has the impression of a liquid which at warm temperature will flow as a slippery, lubricating, viscous fluid; by "fat" he understands a greasy, solid substance unctuous to the touch. It thus becomes necessary to differentiate the oils and fats used in the manufacture of soap.

Inasmuch as a soap is the alkali salt of a fatty acid, the oil or fat from which soap is made must have as a constituent part, these fatty acids. Hydrocarbon oils or paraffines, included in the term "oil," are thus useless in the process of soap-making, as far as entering into chemical combination with the caustic alkalis is concerned. The oils and fats which form soap are those which are a combination of fatty acids and glycerine, the glycerine being obtained as a by-product to the soap-making industry.

NATURE OF A FAT OR OIL USED IN SOAP MANUFACTURE.

Glycerine, being a trihydric alcohol, has three atoms of hydrogen which are replaceable by three univalent radicals of the higher members of the fatty acids, e. g.,

OH OR C_{3} H_{5} OH + 3 ROH = C_{3} H_{5} OR + 3 H_{2}O OH OR

Glycerine plus 3 Fatty Alcohols equals Fat or Oil plus 3 Water.

Thus three fatty acid radicals combine with one glycerine to form a true neutral oil or fat which are called triglycerides. The fatty acids which most commonly enter into combination of fats and oils are lauric, myristic, palmitic, stearic and oleic acids and form the neutral oils or triglycerides derived from these, e. g., stearin, palmatin, olein. Mono and diglycerides are also present in fats.

SAPONIFICATION DEFINED.

When a fat or oil enters into chemical combination with one of the caustic hydrates in the presence of water, the process is called "saponification" and the new compounds formed are soap and glycerine, thus:

OR OH C_{3}H_{5} OR + 3 NaOH = C_{3}H_{5} OH + 3 NaOR OR OH

Fat or Oil plus 3 Sodium Hydrate equals Glycerine plus 3 Soap.

It is by this reaction almost all of the soap used today is made.

There are also other means of saponification, as, the hydrolysis of an oil or fat by the action of hydrochloric or sulfuric acid, by autoclave and by ferments or enzymes. By these latter processes the fatty acids and glycerine are obtained directly, no soap being formed.

FATS AND OILS USED IN SOAP MANUFACTURE.

The various and most important oils and fats used in the manufacture of soap are, tallow, cocoanut oil, palm oil, olive oil, poppy oil, sesame oil, soya bean oil, cotton-seed oil, corn oil and the various greases. Besides these the fatty acids, stearic, red oil (oleic acid) are more or less extensively used. These oils, fats and fatty acids, while they vary from time to time and to some extent as to their color, odor and consistency, can readily be distinguished by various physical and chemical constants.

Much can be learned by one, who through continued acquaintance with these oils has thoroughly familiarized himself with the indications of a good or bad oil, by taste, smell, feel and appearance. It is, however, not well for the manufacturer in purchasing to depend entirely upon these simpler tests. Since he is interested in the yield of glycerine, the largest possible yield of soap per pound of soap stock and the general body and appearance of the finished product, the chemical tests upon which these depend should be made. Those especially important are the acid value, percentage unsaponifiable matter and titer test.

A short description of the various oils and fats mentioned is sufficient for their use in the soap industry.

Tallow is the name given to the fat extracted from the solid fat or "suet" of cattle, sheep or horses. The quality varies greatly, depending upon the seasons of the year, the food and age of the animal and the method of rendering. It comes to the market under the distinction of edible and inedible, a further distinction being made in commerce as beef tallow, mutton tallow or horse tallow. The better quality is white and bleaches whiter upon exposure to air and light, though it usually has a yellowish tint, a well defined grain and a clean odor. It consists chiefly of stearin, palmitin and olein. Tallow is by far the most extensively used and important fat in the making of soap.

In the manufacture of soaps for toilet purposes, it is usually necessary to produce as white a product as possible. In order to do this it often is necessary to bleach the tallow before saponification. The method usually employed is the Fuller's Earth process.

FULLER'S EARTH PROCESS FOR BLEACHING TALLOW.

From one to two tons of tallow are melted out into the bleaching tank. This tank is jacketed, made of iron and provided with a good agitator designed to stir up sediment or a coil provided with tangential downward opening perforations and a draw-off cock at the bottom. The coil is the far simpler arrangement, more cleanly and less likely to cause trouble. By this arrangement compressed air which is really essential in the utilization of the press (see later) is utilized for agitation. A dry steam coil in an ordinary tank may be employed in place of a jacketed tank, which lessens the cost of installation.

The tallow in the bleaching tank is heated to 180deg. F. (82deg. C.) and ten pounds of dry salt per ton of fat used added and thoroughly mixed by agitation. This addition coagulates any albumen and dehydrates the fat. The whole mass is allowed to settle over night where possible, or for at least five hours. Any brine which has separated is drawn off from the bottom and the temperature of the fat is then raised to 160deg. F. (71deg. C).

Five per cent. of the weight of the tallow operated upon, of dry Fuller's earth is now added and the whole mass agitated from twenty to thirty minutes.

The new bleached fat, containing the Fuller's earth is pumped directly to a previously heated filter press and the issuing clear oil run directly to the soap kettle.

One of the difficulties experienced in the process is the heating of the press to a temperature sufficient to prevent solidification of the fat without raising the press to too great a temperature. To overcome this the first plate is heated by wet steam. Air delivered from a blower and heated by passage through a series of coils raised to a high temperature by external application of heat (super-heated steam) is then substituted for the steam. The moisture produced by the condensation of the steam is vaporized by the hot air and carried on gradually to each succeeding plate where it again condenses and vaporizes. In this way the small quantity of water is carried through the entire press, raising its temperature to 80deg.-100deg. C. This temperature is subsequently maintained by the passage of hot air. By this method of heating the poor conductivity of hot air is overcome through the intermediary action of a liquid vapor and the latent heat of steam is utilized to obtain the initial rise in temperature. To heat a small press economically where conditions are such that a large output is not required the entire press may be encased in a small wooden house which can be heated by steam coils. The cake in the press is heated for some time after the filtration is complete to assist drainage. After such treatment the cake should contain approximately 15 per cent. fat and 25 per cent. water. The cake is now removed from the press and transferred to a small tank where it is treated with sufficient caustic soda to convert the fat content into soap.

Saturated brine is then added to salt out the soap, the Fuller's earth is allowed to settle to the bottom of the tank and the soap which solidifies after a short time is skimmed off to be used in a cheap soap where color is not important. The liquor underneath may also be run off without disturbing the sediment to be used in graining a similar cheap soap. The waste Fuller's earth contains about 0.1 to 0.3 per cent. of fat.

METHOD FOR FURTHER IMPROVEMENT OF COLOR.

A further improvement of the color of the tallow may be obtained by freeing it from a portion of its free fatty acids, either with or without previous Fuller's earth bleaching.

To carry out this process the melted fat is allowed to settle and as much water as possible taken off. The temperature is then raised to 160deg. F. with dry steam and enough saturated solution of soda ash added to remove 0.5 per cent. of the free fatty acids, while agitating the mass thoroughly mechanically or by air. The agitation is continued ten minutes, the whole allowed to settle for two hours and the foots drawn off. The soap thus formed entangles a large proportion of the impurities of the fat.

VEGETABLE OILS.

Cocoanut Oil, as the name implies, is obtained from the fruit of the cocoanut palm. This oil is a solid, white fat at ordinary temperature, having a bland taste and a characteristic odor. It is rarely adulterated and is very readily saponified. In recent years the price of this oil has increased materially because cocoanut oil is now being used extensively for edible purposes, especially in the making of oleomargarine. Present indications are that shortly very little high grade oil will be employed for soap manufacture since the demand for oleomargarine is constantly increasing and since new methods of refining the oil for this purpose are constantly being devised.

The oil is found in the market under three different grades: (1) Cochin cocoanut oil, the choicest oil comes from Cochin (Malabar). This product, being more carefully cultivated and refined than the other grades, is whiter, cleaner and contains a smaller percentage of free acid. (2) Ceylon cocoanut oil, coming chiefly from Ceylon, is usually of a yellowish tint and more acrid in odor than Cochin oil. (3) Continental cocoanut oil (Copra, Freudenberg) is obtained from the dried kernels, the copra, which are shipped to Europe in large quantities, where the oil is extracted. These dried kernels yield 60 to 70 per cent oil. This product is generally superior to the Ceylon oil and may be used as a very satisfactory substitute for Cochin oil, in soap manufacture, provided it is low in free acid and of good color. The writer has employed it satisfactorily in the whitest and finest of toilet soaps without being able to distinguish any disadvantage to the Cochin oil. Since continental oil is usually cheaper than Cochin oil, it is advisable to use it, as occasion permits.