The pota.s.sium bichromate is dissolved in hot water and added to the crude palm oil, previously heated to 125 F. (52 C.), the requisite amount of muriatic acid being then run in and the whole well agitated by means of air. The bright red colour of the oil gradually changes to dark brown, and soon becomes green. The action having proceeded for a few minutes, agitation is stopped, and, after allowing to settle, the green liquor is withdrawn.

When sufficiently bleached the oil is finally washed (without further heating) with hot water (which may contain salt), to remove the last traces of chrome liquor.

If the above operation is carried out carefully, the colouring matter will be completely oxidised.

It is important, however, that the temperature should not be allowed to rise above 130 F. (54 C.) during the bleaching of palm oil, otherwise the resultant oil on saponification is apt to yield a soap of a "foxy"

colour. The bleached oil retains the characteristic violet odour of the original oil.

It has been suggested to use dilute sulphuric acid, or a mixture of this and common salt, in the place of muriatic acid in the above process.

_Crude Cotton-seed Oil._--The deep colouring matter of crude cotton-seed oil, together with the mucilaginous and resinous principles, are removed by refining with caustic soda lye.

The chief aim of the refiner is to remove these impurities without saponifying any of the neutral oil. The percentage of free fatty acids in the oil will determine the quant.i.ty of caustic lye required, which must only be sufficient to remove this acidity.

Having determined the amount of free acidity, the quant.i.ty of caustic soda lye necessary to neutralise it is diluted with water to 12 or 15 Tw. (8 or 10 B.), and the refining process carried out in three stages. The oil is placed in a suitable tank and heated by means of a closed steam coil to 100 F. (38 C.), a third of the necessary weak caustic soda lye added in a fine stream or by means of a sprinkler, and the whole well agitated with a mechanical agitator or by blowing a current of air through a pipe laid on the bottom of the tank.

Prolonged agitation with air has a tendency to oxidise the oil, which increases its specific gravity and refractive index, and will be found in the soap-pan to produce a reddish soap. As the treatment proceeds, the temperature may be carefully raised, by means of the steam coil, to 120 F. (49 C.).

The first treatment having proceeded fifteen minutes, the contents of the tank are allowed to rest; the settling should be prolonged as much as possible, say overnight, to allow the impurities to precipitate well, and carry down the least amount of entangled oil. Having withdrawn these coloured "foots," the second portion of the weak caustic soda solution is agitated with the partially refined oil, and, when the latter is sufficiently treated, it is allowed to rest and the settled coloured liquor drawn off as before. The oil is now ready for the final treatment, which is performed in the same manner as the two previous ones. On settling, a clear yellow oil separates.

If desired, the oil may be brightened and filtered, after refining to produce a marketable article, but if it is being refined for own use in the soap-house, this may be omitted.

The residue or "foots" produced during the refining of crude cotton-seed oil, known in the trade as "mucilage," may be converted into "soapstock" as mentioned in the preceding chapter, or decomposed by a mineral acid and made into "black grease" ready for distillation by superheated steam.

_Vegetable Oils._--The other vegetable oils come to the soap-maker's hand in a refined condition; occasionally, however, it is desirable to remove a portion of the free fatty acids, which treatment also causes the colouring matter to be preciptated. This is effected by bringing the oil and a weak solution of caustic lye into intimate contact. Cocoa-nut oil is often treated in this manner. Sometimes it is only necessary to well agitate the oil with 1-1/2 per cent. of its weight of a 12 Tw. (8 B.) solution of caustic soda and allow to settle. The foots are utilised in the soap-pan.

_Animal Fats._--Tallows are often greatly improved by the above alkaline treatment at 165 F. (73 C.). It is one of the best methods and possesses advantages over acid processes--the caustic soda removes the free acid and bodies of aldehyde nature, which are most probably the result of oxidation or polymerisation, whereas the neutral fat is not attacked, and further, the alkaline foots can be used in the production of soap.

_Bone fat_ often contains calcium (lime) salts, which are very objectionable substances in a soap-pan. These impurities must be removed by a treatment with hydrochloric or sulphuric acid. The former acid is preferable, as the lime salt formed is readily soluble and easily removed. The fat is agitated with a weak solution of acid in a lead-lined tank by blowing in steam, and when the treatment is complete and the waste liquor withdrawn, the last traces of acid are well washed out of the liquid fat with hot water.

_Rosin._--Several methods have been suggested for bleaching rosin; in some instances the const.i.tution of the rosin is altered, and in others the cost is too great or the process impracticable.

The aim of these processes must necessarily be the elimination of the colouring matter without altering the original properties of the substance. This is best carried out by converting the rosin into a resinate of soda by boiling it with a solution of either caustic soda or carbonated alkali. The process is commenced by heating 37 cwt. of 17 Tw. (11 B.) caustic soda lye, and adding 20 cwt. of rosin, broken into pieces, and continuing the boiling until all the resinate is h.o.m.ogeneous, when an addition of 1-1/2 cwt. of salt is made and the boiling prolonged a little. On resting, the coloured liquor rises to the surface of the resinate, and may be siphoned off (or pumped away through a skimmer pipe) and the resinate further washed with water containing a little salt.

The treatment with carbonated alkali is performed in a similar manner. A solution, consisting of 2-3/4 cwt. of soda ash (58), in about four times its weight of water, is heated and 20 cwt. of rosin, broken into small pieces, added. The whole is heated by means of the open steam coil, and care must be taken to avoid boiling over. Owing to the liberation of CO_{2} gas, frothing takes place. A large number of patents have been granted for the preparation of resinate of soda, and many methods devised to obviate the boiling over. Some suggest mixing the rosin and soda ash (or only a portion of the soda ash) prior to dissolving in water; others saponify in a boiler connected with a trap which returns the resinate to the pan and allows the carbonic-acid gas to escape or to be collected.

With due precaution the method can be easily worked in open vessels, and, using the above proportions, there will be sufficient uncombined rosin remaining to allow the resultant product to be pumped into the soap with which it is intended to intermix it, where it will be finally saponified thoroughly.

The salt required, which, in the example given, would be 1-1/2 cwt., may be added to the solution prior to the addition of rosin or sprinkled in towards the finish of the boiling. When the whole has been sufficiently boiled and allowed to rest, the liquor containing the colouring matter will float over the resinate, and, after removal, may be replaced by another washing.

Many other methods have been suggested for the bleaching, refining and treatment of materials intended for saponification, but the above practical processes are successfully employed.

All fats and oils after being melted by the aid of steam must be allowed to thoroughly settle, and the condensed water and impurities withdrawn through a trap arrangement for collecting the fatty matter. The molten settled fatty materials _en route_ to the soap-pan should be pa.s.sed through sieves sufficiently fine to free them from suspended matter.

CHAPTER V.

SOAP-MAKING.

_Cla.s.sification of Soaps--Direct Combination of Fatty Acids with Alkali--Cold Process Soaps--Saponification under Increased or Diminished Pressure--Soft Soap--Marine Soap--Hydrated Soaps, Smooth and Marbled--Pasting or Saponification--Graining Out--Boiling on Strength--Fitting--Curd Soaps--Curd Mottled--Blue and Grey Mottled Soaps--Milling Base--Yellow Household Soaps--Resting of Pans and Settling of Soap--Utilisation of Nigres--Transparent Soaps--Saponifying Mineral Oil--Electrical Production of Soap._

Soaps are generally divided into two cla.s.ses and designated "hard," and "soft," the former being the soda salts, and the latter potash salts, of the fatty acids contained in the material used.

According to their methods of manufacture, soaps may, however, be more conveniently cla.s.sified, thus:--

(A) Direct combination of fatty acids with alkali.

(B) Treatment of fat with definite amount of alkali and no separation of waste lye.

(C) Treatment of fat with indefinite amount of alkali and no separation of waste lye.

(D) Treatment of fat with indefinite amount of alkali and separation of waste lye.

(A) _Direct Combination of Fatty Acids with Alkali._--This method consists in the complete saturation of fatty acids with alkali, and permits of the use of the deglycerised products mentioned in chapter ii., section 2, and of carbonated alkalies or caustic soda or potash.

Fatty acids are readily saponified with caustic soda or caustic potash of all strengths.

The saponification by means of carbonated alkali may be performed in an open vat containing a steam coil, or in a pan provided with a removable agitator.

It is usual to take soda ash (58), amounting to 19 per cent. of the weight of fatty acids to be saponified, and dissolve it in water by the aid of steam until the density of the solution is 53 Tw. (30 B.); then bring to the boil, and, whilst boiling, add the molten fatty acids slowly, but not continuously.

Combination takes place immediately with evolution of carbonic acid gas, which causes the contents of the vat or pan to swell, and frequently to boil over. The use of the agitator, or the cessation of the flow of fatty acids, will sometimes tend to prevent the boiling over. It is imperative that the steam should not be checked but boiling continued as vigorously as possible until all the alkali has been absorbed and the gas driven off.

The use of air to replace steam in expelling the carbonic acid gas has been patented (Fr. Pat. 333,974, 1903).

A better method of procedure, however, is to commence with a solution of 64 Tw. (35 B.) density, made from half the requisite soda ash (9-1/2 per cent.), and when this amount of alkali has all been taken up by the fatty acids (which have been added gradually and with continuous boiling), the remaining quant.i.ty of soda ash is added in a dry state, being sprinkled over each further addition of fatty acid.

This allows the process to be more easily controlled and boiling over is avoided.

It is essential that the boiling by steam should be well maintained throughout the process until all carbonic acid gas has been thoroughly expelled; when that point is reached, the steam may be lessened and the contents of the vat or pan gently boiled "on strength" with a little caustic lye until it ceases to absorb caustic alkali, the soap being finished in the manner described under (D).

It is extremely difficult to prevent discoloration of fatty acids, hence the products of saponification in this way do not compare favourably in appearance with those produced from the original neutral oil or fat.

(B) _Treatment of Fat with Definite Amount of Alkali and no Separation of Waste Lye._--Cold-process soap is a type of this cla.s.s, and its method of production is based upon the characteristic property which the glycerides of the lower fatty acids (members of the cocoa-nut-oil cla.s.s) possess of readily combining with a strong caustic soda solution at a low temperature, and evolving sufficient heat to complete the saponification.

Sometimes tallow, lard, cotton-seed oil, palm oil and even castor oil are used in admixture with cocoa-nut oil. The process for such soap is the same as when cocoa-nut oil is employed alone, with the slight alteration in temperature necessary to render the fats liquid, and the amount of caustic lye required will be less. Soaps made of these blends closely resemble, in appearance, milled toilet soaps. In such mixtures the glycerides of the lower fatty acids commence the saponification, and by means of the heat generated induce the other materials, which alone would saponify with difficulty or only with the application of heat, to follow suit.

It is necessary to use high grade materials; the oils and fats should be free from excess of acidity, to which many of the defects of cold-process soaps may be traced. Owing to the rapidity with which free acidity is neutralised by caustic soda, granules of soap are formed, which in the presence of strong caustic lye are "grained out" and difficult to remove without increasing the heat; the soap will thus tend to become thick and gritty and sometimes discoloured.

The caustic lye should be made from the purest caustic soda, containing as little carbonate as possible; the water used for dissolving or diluting the caustic soda should be soft (_i.e._, free from calcium and magnesium salts), and all the materials carefully freed from particles of dirt and fibre by straining.

The temperature, which, of course, must vary with the season, should be as low as is consistent with fluidity, and for cocoa-nut oil alone may be 75 F. (24 C.), but in mixtures containing tallow 100 to 120 F.

(38 to 49 C.).

The process is generally carried out as follows:--

The fluid cocoa-nut oil is stirred in a suitable vessel with half its weight of 71.4 Tw. (38 B.) caustic soda lye at the same temperature, and, when thoroughly mixed, the pan is covered and allowed to rest. It is imperative that the oils and fats and caustic lye should be intimately incorporated or emulsified. The agitating may be done mechanically, there being several machines specially constructed for the purpose. In one of the latest designs the caustic lye is delivered through a pipe which rotates with the stirring gear, and the whole is driven by means of a motor.