SOAP, a composition of caustic fixed alkaline salt and oil, or other grease. It is sometimes hard and dry, sometimes soft and liquid ; much used in washing, and other purposes, as well in the arts and manufactures as in domestic pur poses. The manufacture of soap in and near London first began in the year 1524 : before this our countrymen imported the best soap from foreign parts, though they were supplied with an inferior sort from Bristol.
The materials used in soap-making are, oil of any kind, vegetable or animal; and fixed alkali, either soda or potash. These (that is, oil and alkali) enter into the com position of every soap, and, besides, lime is essential to give the alkali the re quisite degree of causticity: common salt is also employed in most of the potash soaps.
The gei:eral process of soap-making is on the whole very simple ; and con sists, first, in making a caustic, or partly caustic, ley, with the alkali and lime ; next, of boiling the ley with the oil till they are perfectly united into a smooth uniform soap ; and lastly, of drying the soap till it is become of a proper consist ence for use. But though the general process is simple, and success to a cer tain degree may be insured by any one who puts the proper ingredients together in the plainest manner, there are a con siderable number of precautions, and many minute attentions, required, to make the best and most perfect article with the greatest economy. The sim plest, and on the whole the most beauti ful soap, is the fine white soap prepared from olive oil and soda, extracted from the best barilla, which is manufactured very largely in the countries where the olive grows, especially in the south of France, in some parts of Italy, and in Tripoli.
Soap is manufactured in this country principally from tallow, or any other fat ; and the alkali employed is either barilla or pearl-ash, or a mixture of the two, ac. cording to the price and practice of the manufacturer. But as potash alone will not make a stiff soap, recourse is had to the action of common salt, which, when added after the potash and oil are united, produces a separation of the compound from the water incorporated with it, hardens it, and renders it equal to the soda soaps. The following is the usual method of making the common white and yellow soap used in domestic pur poses. White hard soap is generally made with three separate charges of ley. The potash (supposing this to be the alkali) is previously dissolved with water in a small boiler, with a little fire, and the solution is poured over a vat, contain ing common wood ashes mixed with lime, which mates the first and strongest ley.
As soon as this has run off, the ashes are turned, more lime is added, and water is pumped on, which forms the second, or weak ley. The large boiler is then charg
ed with the tallow, and about two-thirds of the strong ley being added, a mode rate fire is kept up, to incorporate the materials, which is known by their run ning into a stiff glue. If this does not take place in about seven hours (with 29 cwt. of tallow in the boiler) more alkali must be added. The tallow is then kill ed, or saturated, and the fire is drawn, and the materials allowed to remain at rest for a short time. Common salt is then thrown in, and stirred up with long poles till it is thoroughly incorporated, and till the mater changes from a dark coloured glue to a thin soapy substance. A brisk fire is then made, and the ma 4erials boiled for a few minutes, when the fire is again drawn, and the materials in the boiler allowed to settle for an hour and a half, during which the spent lees sink to the bottom of the boiler, and are pumped off. The second operation be gins with raising the fire, and adding to the soapy mass the weak Icy, which is to be managed the same as at first, and again brings it to the state of a glue, which a very little salt will restore to the sapo naceous state, and after boiling and cooling the second lees are pumped off. In the third operation, the third part of the strong ley which was reserved is added, which, as before, changes the mass to a thick glue, that must be grained with salt as before. The contents are then boiled strongly for three hours, more or less, till, by taking samples oc casionally with a trowel, the soap feels sufficiently hard and dry to the touch, and the Icy will be seen to run quite clear from the soap on the trowel, leav ing this latter in round lumps. The boil of soap is then finished, by pumping off the spent ley, and scraping off a quantity of light froth front the top of the soap, and the soap is fit for framing.
Soap is soluble in alcohol. The solu tion of soap is decomposed by all the acids, which curdle it, or separate the oil in the form of white lumps. An ex cess of acid re-dissolves the curd, and the oil is transferred to the acid ; but on boiling this solution, the oil separates en tirely, and rises to the surface. The solu tion is also curdled by lime, barytic, or strontian water, and in this case the curd consists of the oil united with the earth, and concreted by this All the soluble salts of these and the other earthy bases equally decompose soap water, and form a curtly precipitate, a Lich is the reason why hard water, that Awl') s con tains sulphate of lime, and often other earthy salts, immediately curdles soap, instead of forming an uniform solution. The metallic salts will produce the same effect, and a combination of the oil and metallic oxide is produced. See Aikin's Diet.