The agitation being complete, chemical action takes place with the generation of heat, and finally results in the saponification of the fats.

At first the contents of the pan are thin, but in a few hours they become a solid ma.s.s. As the process advances the edges of the soap become more transparent, and when the transparency has extended to the whole ma.s.s, the soap is ready, after perfuming, to be framed and crutched.

The admixture of a little caustic potash with the caustic soda greatly improves the appearance of the resultant product, which is smoother and milder.

The glycerine liberated during the saponification is retained in the soap.

Although it is possible, with care, to produce neutral soaps of good appearance and firm touch by this method, cold-process soaps are very liable to contain both free alkali and unsaponified fat, and have now fallen considerably into disrepute.

_Saponification under Increased or Diminished Pressure._--Soaps made by boiling fats and oils, under pressure and _in vacuo_, with the exact quant.i.ty of caustic soda necessary for complete combination, belong also to this cla.s.s. Amongst the many attempts which have at various times been made to shorten the process of soap-making may be mentioned Haywood's Patent No. 759, 1901, and Jourdan's French Patent No. 339,154, 1903.

In the former, saponification is carried out in a steam-jacketed vacuum chamber provided with an elaborate arrangement of stirrers; in the other process fat is allowed to fall in a thin stream into the amount of lye required for saponification, previously placed in the saponification vessel, which is provided with stirring gear.

When the quant.i.ties have been added, steam is admitted and saponification proceeds.

(C) _Treatment of Fat with Indefinite Amount of Alkali and no Separation of Waste Lye._--_Soft soap_ is representative of this cla.s.s. The vegetable fluid oils (linseed, olive, cotton-seed, maize) are for the most part used in making this soap, though occasionally bone fats and tallow are employed. Rosin is sometimes added, the proportion ranging, according to the grade of soap required, from 5 to 15 per cent. of the fatty matter.

The Soft Soap Manufacturers' Convention of Holland stipulate that the materials used in soft-soap making must not contain more than 5 per cent. rosin; it is also interesting to note that a patent has been granted (Eng. Pat. 17,278, 1900) for the manufacture of soft soap from material containing 50 per cent. rosin.

Fish or marine animal oils--whale, seal, etc., once largely used as raw material for soft soap, have been superseded by vegetable oils.

The materials must be varied according to the season; during hot weather, more body with a less tendency to separate is given by the introduction of oils and fats richer in stearine; these materials also induce "figging".

The most important material, however, is the caustic potash lye which should average 40 Tw. (24 B.), _i.e._, if a weak solution is used to commence saponification, a stronger lye must be afterwards employed to avoid excess of water in the soap, and these average 40 Tw. (24 B.).

The potash lye must contain carbonates, which help to give transparency to the resultant soap. If the lye is somewhat deficient in carbonates, they may be added in the form of a solution of refined pearl ash (pota.s.sium carbonate).

Caustic soda lye is sometimes admixed, to the extent of one-fourth, with potash lye to keep the soap firmer during hot weather, but it requires great care, as a slight excess of soda gives soft soap a bad appearance and a tendency to separate.

The process is commenced by running fatty matter and weak potash lyes into the pan or copper, and boiling together, whilst the introduction of oil and potash lye is continued.

The saponification commences when an emulsion forms, and the lye is then run in more quickly to prevent the ma.s.s thickening.

Having added sufficient "strength" for complete saponification, the boiling is continued until the soap becomes clear.

The condition of the soap is judged by observing the behaviour of a small sample taken from the pan and dropped on gla.s.s or iron. If the soap is satisfactory it will set firm, have a short texture and slightly opaque edge, and be quite clear when held towards the light. If the cooled sample draws out in threads, there is an excess of water present.

If an opaque edge appears and vanishes, the soap requires more lye. If the sample is turbid and somewhat white, the soap is too alkaline and needs more oil.

The glycerine liberated during saponification is contained in the soap and no doubt plays a part in the production of transparency.

_Hydrated soaps_, both smooth and marbled, are included in this cla.s.sification, but are _soda_ soaps. Soap made from cocoa-nut oil and palm-kernel oil will admit of the incorporation of large quant.i.ties of a solution of either salt, carbonate of soda, or silicate of soda, without separation, and will retain its firmness. These materials are, therefore, particularly adapted for the manufacture of marine soaps, which often contain as much as 80 per cent. of water, and, being soluble in brine, are capable of use in sea-water. For the same reason, cocoa-nut oil enters largely into the const.i.tution of hydrated soaps, but the desired yield or grade of soap allows of a variation in the choice of materials. Whilst marine soap, for example, is usually made from cocoa-nut oil or palm-kernel oil only, a charge of 2/3 cocoa-nut oil and 1/3 tallow, or even 2/3 tallow and 1/3 cocoa-nut oil, will produce a paste which can carry the solutions of silicate, carbonate, and salt without separation, and yield a smooth, firm soap.

The fatty materials, carefully strained and freed from particles of dirt and fibre, are boiled with weak caustic soda lye until combination has taken place. Saponification being complete, the solution of salt is added, then the carbonate of soda solution, and finally the silicate of soda solution, after which the soap is boiled. When thoroughly mixed, steam is shut off, and the soap is ready for framing.

The marbled hydrated soap is made from cocoa-nut oil or a mixture of palm-kernel oil and cocoa-nut oil with the aid of caustic soda lye 32-1/2 Tw. (20 B.). As soon as saponification is complete, the brine and carbonate of soda solution are added, and the pan allowed to rest.

The soap is then carefully tasted as to its suitability for marbling by taking samples and mixing with the colouring solution (ultramarine mixed with water or silicate of soda solution). If the sample becomes blue throughout, the soap is too alkaline; if the colour is precipitated, the soap is deficient in alkali. The right point has been reached when the marbling is distributed evenly. Having thus ascertained the condition of the pan, and corrected it if necessary, the colour, mixed in water or in silicate of soda solution, is added and the soap framed.

(D) _Treatment of Fat with Indefinite Amount of Alkali and Separation of Waste Lye._--This is the most general method of soap-making. The various operations are:--

(_a_) Pasting or saponification.

(_b_) Graining out or separation.

(_c_) Boiling on strength.

And in the case of milling soap base and household soaps,

(_d_) Fitting.

(_a_) _Pasting or Saponification._--The melted fats and oils are introduced into the soap-pan and boiled by means of open steam with a caustic soda lye 14 to 23.5 Tw. (10 to 15 B.). Whether the fatty matters and alkali are run into the pan simultaneously or separately is immaterial, provided the alkali is not added in sufficient excess to r.e.t.a.r.d the union.

The commencement of the saponification is denoted by the formation of an emulsion. Sometimes it is difficult to start the saponification; the presence of soap will often a.s.sist this by emulsifying the fat and thus bringing it into intimate contact with the caustic soda solution.

When the action has started, caustic soda lye of a greater density, 29 to 33 Tw. (18 to 20 B.), is frequently added, with continued boiling, in small quant.i.ties as long as it is being absorbed, which is ascertained by taking out samples from time to time and examining them.

There should be no greasiness in the sample, but when pressed between finger and thumb it must be firm and dry.

Boiling is continued until the faint caustic taste on applying the cooled sample to the tongue is permanent, when it is ready for "graining out". The pasty ma.s.s now consists of the soda salts of the fat (as imperfect soap, probably containing emulsified diglycerides and monoglycerides), together with water, in which is dissolved the glycerine formed by the union of the liberated glyceryl radicle from the fat with the hydroxyl radicle of the caustic soda, and any slight excess of caustic soda and carbonates. The object of the next operation is to separate this water (spent lye) from the soap.

(_b_) _Graining Out or Separation._--This is brought about by the use of common salt, in a dry form or in solution as brine, or by caustic soda lye. Whilst the soap is boiling, the salt is spread uniformly over its surface, or brine 40 Tw. (24 B.) is run in, and the whole well boiled together. The soap must be thoroughly boiled after each addition of salt, and care taken that too large a quant.i.ty is not added at once.

As the soap is gradually thrown out of solution, it loses its smooth transparent appearance, and becomes opaque and granular.

When a sample, taken out on a wooden trowel, consists of distinct grains of soap and a liquid portion, which will easily separate, sufficient salt or brine has been added; the boiling is stopped and the spent lye allowed to settle out, whilst the soap remains on the surface as a more or less thick ma.s.s.

The separated spent lye consists of a solution of common salt, glycerine, and alkaline salts; the preparation of crude glycerine therefrom is considered in chapter ix.

The degree of separation of water (spent lye) depends upon the amount of precipitant used. The aim is to obtain a maximum amount of spent lye separated by the use of a minimum quant.i.ty of salt.

The amount of salt required for "graining out" varies with the raw material used. A tallow soap is the most easily grained, more salt is required for cotton-seed oil soap, whereas soaps made from cocoa-nut and palm-kernel oils require very large amounts of salt to grain out thoroughly. Owing to the solubility in weak brine of these latter soaps, it is preferable to grain them with caustic soda lye. This is effected by adding, during boiling, sufficient caustic lye (32-1/2 Tw., 20 B.) to produce the separation of the granules of soap. The whole is allowed to rest; the separated half-spent lye is withdrawn and may be used for the pasting of fresh materials.

After the removal of the settled lye, the grained ma.s.s is boiled with sufficient water to dissolve the grain and make it smooth ("close" it), and is now ready for the next operation of "boiling on strength".

(_c_) _Boiling on Strength or Clear Boiling._--This is the most important operation and is often termed "making the soap". The object is to harden the soap and to ensure complete saponification.

Caustic soda lye (32-1/2 Tw., 20 B.) is gradually added until the soap is again opened or grained, and boiling continued by the use of the dry steam coil. As soon as the caustic soda lye is absorbed, another portion is slowly added, and this is continued until the caustic soda or "strength" remains permanent and the soapy ma.s.s, refusing to absorb more, is thrown out of solution and grained. The granular ma.s.s will boil steadily, and the boiling should be prolonged, as the last traces of neutral oil are difficult to completely saturate with alkali. Thorough saponification takes place gradually, and the operation cannot be hurried; special care has to be bestowed upon this operation to effect the complete combination of fat and alkali.

After resting for several hours, half-spent lye settles to the bottom of the pan. In the case of yellow soaps or milling bases the settled lye is removed to a suitable receptacle and reserved for use in the saponification of other material, and the soap is then ready for the final operation of "fitting".

(_d_) _Fitting._--If the operations just described have been properly performed, the fitting should present no difficulty. The soap is boiled with open steam, and water added until the desired degree of closing is attained. As the water is thoroughly intermixed throughout the ma.s.s the thick paste gradually becomes reduced to a smooth, thin consistence.

Samples are tested from time to time as to their behaviour in dropping off a hot trowel held sideways; the thin layer should drop off in two or three flakes and leave the surface of the trowel clean and dry. The soap is then in a condition to allow the impurities to gravitate. According to the required soap, the fit may be "coa.r.s.e" ("open") when the flakes drop off the trowel readily, or "fine" ("close") when the flakes only leave the trowel with difficulty.

If the dilution with water has been allowed to proceed too far, and too fine a fit is produced, which would be denoted by the layer of soap not leaving the trowel, a little caustic lye or brine may be very carefully added and the whole well boiled until the desired condition is obtained.

A good pressure of steam is now applied to the pan, causing the contents to swell as high as possible, this greatly facilitating the settling of impurities; steam is then turned off, the pan covered, and the boil allowed to rest for several days.

The art of fitting consists in leaving the contents of the pan in such a condition that, on standing, all the impurities precipitate, and the settled soap, containing the correct amount of water, is clear and bright.

The above is a general practical outline of the ordinary soap-boiling process. It may be modified or slightly altered according to the fancy of the individual soap-maker or the particular material it is desired to use. Fats and oils not only vary in the amount of alkali they absorb during saponification, but also differ in the strength of the alkali they require. Tallow and palm oil require lye of a density of 15 to 18 Tw. (10 to 12 B.), but cocoa-nut oil alone would not saponify unless the lye was more concentrated, 33 to 42 Tw. (20 to 25 B.).

Cotton-seed oil requires weak lyes for saponification, and, being difficult to saponify alone even with prolonged boiling, is generally mixed with animal fat.