Pota.s.sium chloride and pota.s.sium silicate find a limited use for intermixing with soft soaps.

It will be readily understood that hard and fast rules cannot be laid down for "liquoring" soap, and the correct solution to be employed can only be ascertained by experiment and experience, but the above suggestions will prove useful as a guide towards good results. A smooth, firm soap of clear, bright, glossy appearance is what should be aimed at.

_Filling._--Some low-grade soaps contain filling, which serves no useful purpose beyond the addition of weight. Talc is the most frequently used article of this description. It consists of hydrated silicate of magnesium and, when finely ground, is white and greasy to the touch. The addition of this substance to the hot soap is made by suspending it in silicate of soda solution.

Whatever filling material is used, it is important that the appearance of the soap should not be materially altered.

_Neutralising, Colouring and Perfuming._--The free caustic alkali in soap, intended for toilet or laundry purposes, is usually neutralised during the cleansing, although some soap manufacturers prefer to accomplish this during the milling operation. Various materials have been recommended for the purpose, those in most general use being sodium bicarbonate, boric acid, cocoa-nut oil, stearic acid, and oleic acid.

The best method is the addition of an exact quant.i.ty of sodium bicarbonate (acid sodium carbonate), which converts the caustic alkali into carbonate. The reaction may be expressed by the equation:--

NaOH + NaHCO_{3} = Na_{2}CO_{3} + H_{2}O Caustic soda Bicarbonate of soda Carbonate of soda Water

Boric acid in aqueous or glycerine solutions, and borax (biborate of soda) are sometimes used, but care is necessary in employing these substances, as any excess is liable to decompose the soap.

The addition of cocoa-nut oil is unsatisfactory, the great objection being that complete saponification is difficult to ensure, and, further, there is always the liability of rancidity developing. Stearic and oleic acids are more suitable for the purpose, but oleic acid has the disadvantage that oleates are very liable to go rancid.

A large number of other substances have been proposed, and in many instances patented, for neutralising the free caustic alkali. Among these may be mentioned--Alder Wright's method of using an ammoniacal salt, the acid radicle of which neutralises the caustic alkali, ammonia being liberated; the use of sodium and pota.s.sium bibasic phosphate (Eng.

Pat. 25,357, 1899); a substance formed by treating alb.u.men with formalin (Eng. Pat., 8,582, 1900); wheat glutenin "alb.u.minoses" (alb.u.men after acid or alkaline treatment); malt extract; and egg, milk, or vegetable alb.u.men.

The colouring matter used may be of either vegetable or coal-tar origin, and is dissolved in the most suitable medium (lye, water, or fat). The older types of colouring matter--such as cadmium yellow, ochres, vermilion, umbers--have been superseded.

In the production of washer household soaps, a small quant.i.ty of perfume is sometimes added.

_Disinfectant Soaps._--To the soap base, which must be strong to taste, is added from 3 to 4 per cent. of coal-tar derivatives, such as carbolic acid, cresylic acid, creosote, naphthalene, or compounds containing carbolic acid and its h.o.m.ologues. The incorporation is made in the crutching pan, and further crutching may be given by hand in the frames.

_Framing._--The object of framing is to allow the soap to solidify into blocks. The frames intended for mottled soaps, which require slow cooling, are constructed of wood, often with a well in the base to allow excess of lye to acc.u.mulate--for other soaps, iron frames are in general use. The frame manufactured by H. D. Morgan of Liverpool is shown in Fig. 9.

As soon as the frame is filled, or as soon as the crutching in the frame is finished, the soap is smoothed by means of a trowel, leaving in the centre a heap which slopes towards the sides. Next day the top of the soap is straightened or flattened with a wooden mallet, this treatment a.s.sisting in the consolidation.

[Ill.u.s.tration: FIG. 9.--Soap frame.]

[Ill.u.s.tration: FIG. 10.--Slabbing machine.]

The length of time the soap should remain in frames is dependent on the quality, quant.i.ty, and season or temperature, and varies usually from three to seven days. When the requisite period has elapsed, the sides and ends of the frames are removed, and there remains a solid block of soap weighing from 10 to 15 cwt. according to the size of frame used.

The blocks, after sc.r.a.ping and tr.i.m.m.i.n.g, are ready for cutting into slabs.

_Slabbing._--This may be done mechanically by pushing the block of soap through a framework containing pianoforte wires fixed at equi-distances (Fig. 10, which shows a machine designed by E. Forshaw & Son, Ltd.), or the soap may be out by hand by pulling a looped wire through the ma.s.s horizontally along lines previously scribed, or, for a standard sized slab, the wire may be a fixture in a box-like arrangement, which is pa.s.sed along the top of the soap, and the distance of the wire from the top of the box will be the thickness of the slab (Fig. 11).

[Ill.u.s.tration: FIG. 11.--Banjo slabber.]

All tallow soaps should be slabbed whilst still warm, cut into bars, and open-piled immediately; if this type of soap is cold when slabbed its appearance will be very much altered.

_Barring._--The slabs are out transversely into bars by means of the looped wire, or more usually by a machine (Fig. 12), the lower framework of which, containing wires, is drawn through the soap placed on the base-board; the framework is raised, and the bars fall upon the shelf, ready for transference into piles. It has long been the custom in England to cut bars of soap 15 inches long, and weighing 3 lb. each, or 37-1/2 bars of soap to the cwt., but in recent years a demand has arisen for bars of so many various weights that it must be sometimes a difficult matter to know what sizes to stock.

In another type of barring machine, portions of the slab, previously cut to size, are pushed against a framework carrying wires, and the bars slide along a table ready for handling (Fig. 13).

In cutting machines, through which "washer" household soap is being pa.s.sed, the bar is pushed at right angles through another frame containing wires, which divides it into tablets; these may be received upon racks and are ready for drying and stamping. It is needless to say that the slabs and tablets are cut with a view to reducing the amount of waste to the lowest possible limit. Such a machine, made by E. Forshaw & Son, Ltd., is shown in Fig. 14.

[Ill.u.s.tration: FIG. 12.--Barring machine.]

[Ill.u.s.tration: FIG. 13.--Bar-cutting machine.]

[Ill.u.s.tration: FIG. 14.--Tablet-cutting machine.]

_Open- and Close-piling._--As remarked previously, tallow soaps should be cut whilst warm, and the bars "open-piled," or stacked across each other in such a way that air has free access to each bar for a day. The bar of soap will skin or case-harden, and next day may be "close-piled,"

or placed in the storage bins, where they should remain for two or three weeks, when they will be in perfect condition for packing into boxes ready for distribution.

[Ill.u.s.tration: FIG. 15.--Soap stamp.]

_Drying._--"Oil soaps," as soaps of the washer type are termed, do not skin sufficiently by the open-piling treatment, and are generally exposed on racks to a current of hot air in a drying chamber in order to produce the skin, which prevents evaporation of water, and allows of an impression being given by the stamp without the soap adhering to the dies. It is of course understood that heavily liquored soaps are, as a rule, unsuitable for the drying treatment, as the bars become unshapely, and lose water rapidly.

_Stamping._--Bar soaps are usually stamped by means of a hand-stamp containing removable or fixed bra.s.s letters (Fig. 15), with a certain brand or designation of quality and the name of the manufacturer or vendor, and are now ready for packing into boxes.

A very large bulk of the soap trade consists of the household quality in tablet form, readily divided into two cakes. These are stamped in the ordinary box moulds with two dies--top and bottom impressions--the die-plates, being removable, allow the impressions to be changed. This type of mould (Fig. 16) can be adjusted for the compression of tablets of varying thickness, the box preventing the escape of soap. We are indebted to E. Forshaw & Son, Ltd., for this ill.u.s.tration.

[Ill.u.s.tration: FIG. 16.--Box mould.]

The stamping machine may be worked by hand (Fig. 17) or power driven.

Where large quant.i.ties of a particular tablet have to be stamped, one of the many automatic mechanical stampers in existence may be employed, the tablets being conveyed to and from the dies by means of endless belts.

Such a machine is shown in the accompanying ill.u.s.tration (Fig. 18).

If necessary, the soap is transferred to racks and exposed to the air, after which it is ready for wrapping, which is generally performed by manual labour, although in some instances automatic wrapping machines are in use.

Cardboard cartons are also used for encasing the wrapped tablets, the object being that these are more conveniently handled by tradesmen and may be advantageously used to form an attractive window display.

_Cooling._--Many attempts have been made to shorten the time required for the framing and finishing of soap, by cooling the liquid soap as it leaves the pan.

[Ill.u.s.tration: FIG. 17.--Soap-stamping machine, showing box mould.]

With milling base, this is successfully accomplished in the Cressonnieres' plant, by allowing the hot soap to fall upon the periphery of a revolving drum which can be cooled internally by means of water.

[Ill.u.s.tration: FIG. 18.--Automatic stamper.]

In the case of household soaps, where the resultant product must be of good appearance and have a firm texture, the difficulty is to produce a bar fit for sale after the cooling has been performed, as soap which has been suddenly chilled lacks the appearance of that treated in the ordinary way. Several patents have been granted for various methods of moulding into bars in tubes, where the hot soap is cooled by being either surrounded by running water in a machine of similar construction to a candle machine, or rotated through a cooling medium; and numerous claims have been made both for mechanical appliances and for methods of removing or discharging the bars after cooling. In many instances these have proved unsatisfactory, owing to fracture of the crystalline structure. Moreover, in pa.s.sing through some of the devices for solidification after chilling, the soap is churned by means of a worm or screw, and this interferes with the firmness of the finished bar, for, as is well known, soap which has been handled too much, does not regain its former firmness, and its appearance is rendered unsatisfactory.

A form of apparatus which is now giving satisfactory results is the Leimdoerfer continuous cooler (Fig. 19). This consists of a fixed charging hopper, A, a portable tank, B, containing tubes, and a detachable box, C, which can be raised or lowered by means of a screw, D. The bottom of the hopper is fitted with holes corresponding with the cooling tubes, _e_, and closed by plugs _c_, attached to a frame _b_, which terminates above in a screw spindle _a_, by means of which the frame and plugs can be raised and lowered so as to permit or stop the outflow of soap into the cooling tubes. The tubes are closed at the bottom by slides _d_, and the box B, in which they are mounted, is carried on a truck running on rails. The charging hopper can be connected with the soap-pan by a pipe, and when the hopper is filled with liquid soap the plugs _c_ are raised and the air in the box C exhausted, thus causing the soap to descend into the cooling tubes.

[Ill.u.s.tration: FIG. 19.--Leimdoerfer cooler.]

The slides _d_ are closed, the screw D released, and the box B moved away to make room for another. At the end of the rail track is an ejecting device which pushes the cooled soap out of the tubes, and the truck is run back on a side track to the machine for use over again. In this way the apparatus can be worked continuously, the soap being received from the cooling pipes on a suitable arrangement for transport to the press or store room.

A similar idea has been made the subject of a patent by Holoubek (Eng.

Pat. 24,440, 1904, Fig. 20). The soap is run into frames or moulds having open sides, which are closed by being clamped with screws and pressure plates between cooling tubes through which water circulates.

[Ill.u.s.tration: FIG. 20.--Holoubek's cooler.]