Soap Manufacture and the Soap of Commerce.
BY ALFRED SMETHAM, F.C.S.
In a paper read before the Liverpool Chemists' Association, Nov. 8th, 1883, the author first briefly described the raw materials employed by the soap maker, and then proceeded to the processes adopted for their conversion into soap. The boiler first supplies himself with a weak solution of caustic and then melts in a pan a quantity of the fat to be operated upon. The specific gravity of the lye—as the solution of alkali is called—should not in the first instance exceed 1.050—1.060. The heat is maintained by means of, steam, the direct use of a fire being now practically obsolete. The first action of the caustic is to produce an emulsion, and when this is properly formed more alkali is added, the strength of the lye being gradually increased. The reason why a weak lye must be used in the first instance is that soap is insoluble in a strong solution of caustic, and the particles of fat would, by the use of a strong lye, become encased in an insoluble layer of soap, which would prevent further action from taking place. The lye is added in until an excess of caustic is found in the pan. More oil or fat, or, where required, rosin is then added, and the fat or rosin saturated by subsequent additions of lye. When the operator, by examining the texture of the soap, considers the reaction complete, the watery solution of soap and glycerin is decomposed, by the addition of salt, in a solution of which soap is insoluble. The soap then rises to the surface in a finely divided state, and after complete separation the spent lye is removed. After the removal of the lye the soap is again heated, and, if necessary, some weak lye added, so that the soap may assume a "close" texture, as it is called. When this is complete the soap is removed, usually by pumping, to another vessel, where it is "crutched." This consists essentially in stirring the mass by rotating arms moved by machinery, by which means the soap is brought into condition, and if of too great consistency more water is added. It is now ready for the frames, composed of slabs of wood or iron placed together in a rectangular form, and made in such a manner that when the soap has solidified the sides may be removed. The soap is left in the frames until it is completely set. When this has occurred, the block is taken out and cut into slabs by means of a wire pulled through it in a horizontal direction. The slabs thus formed are placed upon a table with a movable arm, across which wires are stretched, and the slabs are by this means again divided into bars. It is then, if it be a pure soap, ready for packing. Sometimes, however, it is desired to make up the soap in tablets of given weight. It is then cut up into pieces of the requisite weight and stamped in a press with dyes, the presses usually being worked by hand.
This, then, is a brief outline of the manufacture of a genuine soap; but the exigencies of the case render it necessary to produce a variety of soaps, at cheap prices. This has been brought about by competition and the inability of the public to discriminate between a well-made and a common soap, and consequently it is impossible for any firm manufacturing only pure soaps of high quality to hold its place in the struggle for existence. The cheaper soaps, being more readily soluble in water, produce a lather more quickly than a pure soap, and as the public does not as a rule make comparative trials as to the lasting powers, and is almost invariably led away by a cheap article, the sale of the best soaps has of late fallen off considerably, and the cheaper kinds have taken their place.
Although I have used the term pure soap to represent a soap manufactured from fat and alkali alone, it would be unfair to designate the common soaps adulterated, as we shall see on considering their nature.
A pure tallow soap will only take a certain proportion of water, and it becomes necessary to mix other substances with it if the percentage of water is increased. A substance which is useful in this respect, and which at the same time has detergent properties, is silicate of soda. This is the substance known as soluble glass, but it is usually sold to the soap boiler in solution. It is composed of silicic acid and soda, in various proportions, and is formed of two kinds, the neutral and the caustic. The neutral has a specific gravity of about 1.370, and contains about 65 per cent. of water. The proportion of silicic acid is about 26 per cent., and the remainder is soda and impurities. The caustic silicate is a much heavier solution, and has a specific gravity of about 1.700. It contains about 43 per cent. of water, 33 per cent. of silicic acid and the remainder alkali and impurities.
These solutions are either used alone, or in combination, and are added to the soap before finishing. It is necessary to "crutch" well to insure the complete mixing, and the crutching should be continued until the soap is about to set. The silicated soaps generally contain a larger proportion of water than pure soaps, besides the actual weight of silicate, and they can, therefore, be produced at lower prices. The detergent power of these soaps is greater than would be indicated by the pure soap contained in them, and in many districts this variety finds a market more readily than the better qualities. I should here point out that the value of a soap is not altogether determined by the composition. A pure soap may be produced from a discolored tallow or oil, which as a rule injures the appearance and causes it to command a less value in the market.
A form of silicated soap which obtains a large sale is the mottled. This differs essentially from the mottled soap manufactured a few years back, which was pure and necessarily of a high standard. It is usually manufactured from bleached palm oil, or from palm nut oil or cocoa nut oil as the chief ingredient. It is usually run with silicate to a considerable extent and contains a variable amount of fatty acids—the quantity depending on the quality it is desired to make. The mottling is produced generally by the addition of ultramarine, which gives to the soap a bright appearance.
Some samples, sold at low prices, have come under my notice which have not only been run with silicate but contain from 6 to 8 per cent. of common salt and not more than one quarter of their weight of fatty acids. The salt is of no value as a detergent agent, and must be looked upon simply as a "make weight." It is only with soaps made from palm nut and cocoa nut oil that the salt will combine properly. The peculiar behavior of these two soaps in salt water renders them valuable for marine purposes. Very considerable experience is required in making a soap of low quality which shall be firm to the touch and present the appearances of a good soap, and the difficulty is increased in the case of common mottled soap, where it is necessary to have the mottling equally distributed throughout the mass. The methods by which this is attained are kept, as a rule, as trade secrets, but no great difficulty is experienced when the matter is approached on scientific principles.
In the common soaps which are usually used for scouring, etc., the proportion of soda in excess of the fatty acids may be greater than in those used for finer purposes or for toilet use. The choice of the fat must also be regulated by the purposes to which the soap is intended to be put.
The details of the manufacture require careful attention, and can only be mastered after long experience, but it is necessary that all the processes should be carried on in the lines I have indicated. The peculiar behavior of each kind of soap would occupy more time than is at my disposal, nor would it serve to elucidate the processes; but it is important that the manufacturer should be conversant with their properties. As a rule, the larger the amount of stearin or palmitin there is in the fat operated upon the harder will be the soap.
Before closing the remarks upon the manufacture I may just refer to two methods which are occasionally resorted to, to improve the appearance of common soaps. The first of these consists in placing the soap in an oven or stove, so that it may become surface died and present a hard "skin." The second of these consists in dipping the soap in a strong solution of brine or other liquid. The salt has a great affinity for water and removes it from the surface of the soap but the soap itself is quite insoluble. This process improves the the appearance considerably, and prevents the soap having a sticky consistency on the exterior of the bar.
In treating of the second part of my subject, it may be well to preface my remarks with a brief account of the methods by which I have arrived at the results which I propose to state, as showing the quality of the soaps found in the market. I have now in my possession upwards of three hundred analyses of soap from different sources, which have been submitted to me at various times. In analyzing these I have found the following processes the most convenient and accurate.
The water is determined by drying in an air-bath a weighed portion of the soap at a temperature of 120°C. At this temperature the soap swells up and the water is soon expelled without any loss of the fatty matters or danger of losing the substance. The weight is taken after about three hours, and subsequent weights are made at intervals of about an hour until the weight is constant.
To obtain the percentage of fatty acids I find it best to weigh out about 3 grams of the soap in a porcelain or platinum basin, including in the weight of the basin a small stirring rod about 3 inches long. The soap is then dissolved in a small quantity of water in the basin, and when completely dissolved, about 5 cc. of dilute sulphuric acid are added. This decomposes the soap, setting free the fatty acids and forming sulphate of soda. The solution is then gently warmed—preferably on a water-bath—until the whole of the fatty acids have risen. It is then allowed to cool, and the fatty matter will usually form a solid cake. If this does not occur a weighed quantity of purified wax must be added and the whole re-melted. When the cake is formed it is simply moved a little from the side, and the liquid from below, which should contain no fat, is poured off. The cake is re-melted with distilled water and allowed to settle as before. This is continued until the washings are free from acid. The cake is then melted in a water oven and again allowed to cool, and the water which still adheres is removed by gently touching with filter paper, and the basin is again placed in the water oven and weighed until the weight is constant. From the figures obtained the percentage of fatty and resinous acids is calculated.
The soda is determined by adding to the filtered solution from a given weight of soap an excess of standard acid and titrating back the excess of acid by means of standard alkali, using cochineal as indicator.
The percentage of silicate is obtained from the silicic acid found. To obtain this I prefer to ignite about 2 grams of the soap in a platinum dish until the volatile matters are dispersed. After cooling, the ash is covered with a glass and treated with an excess of hydrochloric acid. It is then evaporated to dryness, taken up with dilute acid, well washed and then ignited and weighed.
These are the constituents which it is usually necessary to determine, but it is sometimes required to make a more complete analysis. When this is desired it is a good plan to dissolve the soap in alcohol and filter. By this means most of the adulterating materials are separated. The clilorine is best estimated after decomposing the soap with nitric acid and allowing the fat to solidify, as in the estimation of fatty acids, by precipitating with nitrate of silver and weighing the resulting chloride.
The percentage of free alkali is important. It can be obtained by precipitating the clear alcoholic solution with carbonic acid, but I prefer to titrate the solution with standard acid, using phenolphthalein as indicator. The results are good.
In making out the analysis of a soap it must be remembered that the fatty constituents actually exist as fatty anhydrides and not as fatty acids, and if, therefore, we determine the whole of the constituents of a soap and include the fatty matters as the estimated acids we shall find that the figures will add up to about 103 per cent. This is due to the absorption of water by the fatty anhydrides in decomposition. The actual percentage of fatty acids should always be placed as a foot . note.
In making a choice of the soaps usually found in the market is difficult to know which to take as representative, but it will, perhaps, be sufficient to divide them into two classes, the pure and the silicated. The analyses given of the average qualities of these soaps show the following:
|Soaps.||Fatty Acids.||Soda.||Hydrated Silicate of Soda.||Water.|
—Phar. Jour. and Trans., Jan. 5, 1884, pp. 534-537.