It was their discovered that the addition of carbonate of soda, or " sal soda," has a remarkable effect in stiffening and hardening soaps to which it is added in a state of strong solution ; it also increases chemically its detergent power. This process is very largely employed in America ; the amount of soda used depends upon the raw materials from which the soap was made, and upon the quality of the desired product ; a very usual proportion is 1 cwt. carbonate of soda crystals (melted) to every tou of soap. Not unfrequently, a solution of pearl-ash (impure carbonate of potash) and conamon salt, mixed in varying proportion!, at a sp. gr. of about 30-35° B., is used. for a similar purpose.
Silicated Soaps.—The discovery of methods of manufacturing on a large scale soluble silicates of soda and potash, gave a very important impetus to the soap-trade, since these substances are peculiarly suitable for the purposes now being described. The first application on a large scale was the production of soaps by Christr. Thomas & Bros., of Broad Plain Soap and Candle Works, Bristol, containing both silicate and sulphate of soda, and by these means they were able to produce in 1856 a soap of very great detersive power, which could be sold retail at less than the duty on soap, which had been removed a few years previously. It is usually supposed, however, that the value of silicated soaps was first publicly and officially recognized at the international Exhibition of 1862, when a prize medal was awarded to W. Gossage and Sons, Widnes Sospery, for their samples.
It will he convenient, therefore, to describe here shortly the process of manufacture and pro perties of the silicates of soda and potash, These compounds, known also as soluble glass or water glass, may he prepared either by the dry or wet methods. The first is usually adopted by Gossage, Crosfield, and others ; it depends on the fact that, at high temperatures, silica plays the part of a very strong acid, capable of displacing acids much stronger than it at ordinary temperatures. On the clean hearth of a reverberatory furnace, 9 parts of soda-ash at 50 per cent. soda are fused with 11 parts clean white sand, or (for the potash salt) equal parts of carbonate of potash and sand. The product may be sold in the dry state, or may be dissolved in boiling water ; not unfrequently boiling under pressure is necessary to effect complete solution. If the solution be too alkaline, it may be boiled with rosin, or a fatty acid, or it may be treated with a mineral acid, either liquid or gaseous. Instead of carbonate of potash, a mixture of " salt-cake " (sulphate of soda) and coal may be fused with sand, and the mixture decolorized by arseniate of soda (i. e. a mixture of
white arsenic, nitrate of soda, and soda-ash), hut a much higher temperature is required in this case, and the wear and tear of the furnace is very great.
For purposes where uniformity of composition is important, it is far better to employ the wet method, as is used by Itansome for artificial stone, and by Christr. Thomas & Bros., Bristof In this case, white sand or calcined flint is put into a Papin's digester, with a solution of caustic soda at about 12° B. Steam is turned into the jacket, and maintained there at about 25-30 lb. a sq. in.; occasional samples are drawn off by a try-cock, and when all trace of causticity has disappeared, steam is turned off, and the contents are " blown out " into tanks where a few hours' subsidence deprives the solution of all suspended impurity. It is then about 24° B., and may be concen trated, if desired, as far as sp. gr. 1.700. Any mechanical arrangement that moves the flints about, facilitates their solution. Made in this way, the silica and the soda bear to each other a very simple, but a very constant, ratio, viz. 2 to 1, and hence great uniformity of composition is obtained, which is uot always the case when soluble silicates are made in the furnace. The com pound is usually sold in solution at 140° Tw. (sp. gr. 1.700), and should scarcely vary from this composition :— Solutions of silicate of soda, containing a larger proportion of silica than 2 to 1, cannot be concen trated so far, but are very auitable for many aoaps ; those containing less silica than 2 to 1 are unsuitable for all soaps, and should be carefully avoided.
Silicate of soda may be mixed with almost any kind of soap, but the strength of the solution employed must be varied according to circumstances. Very wea,k solutions are often added to " neat yellow soaps," and when employed in this way, it is a good general rule, cetcris paribus, to increase the sp. gr. of the solution with the percentage of it employed. Thus, if it be desired to increase the quantity of water in a " neat " soap by 4-5 per cent., a solution at 5° B. will be suitable ; while if the quantity of water is to be increased to a total of 50 per cent., a stronger solution (10-12°) is required. This kiud, technically known as "run soap," waa at one time largely made in America, and still is in England under the name of "London pale." Such soaps arc of the consiatency of thin treacle when mixed, at say 160-170° F., and are apt to disappear rapidly in hot water, as well as to lose weight when kept.