The time required increases with the amount of materials operated on at one time. The ehief points needing attention are, to use pure materials, to avoid exeesa of alkali, and so to manage the temperature and the stirring as to make a complete mixture of the melted fat and leys, that will not separate before saponification takes place.
I, b, 8.—In this class, are the soaps produced by boiling under pressure. This proeess has Leen the subject of uumerous patents at various times, all having for their object the shortening of the timo occupied in the ordinary methods of open boiliug (I. c.), and the saving of the salt employed therein. In this ease, also, the quality of alkali employed, whether caustic or carbonate, is accurately adjusted to the fat to be saponified, and the glycerine is retained in the ultimate product ; mixtures of any sapouifiable fats and resins may be employed. The kind of apparatus used is shown in Fig. 1245 ; and it consists of a steam boiler provided with a man-hole and safety valve, with a feed-pipe A and discharge-pipe C, and with a long thermometer B, in a pocket filled with paraffin. The proper quantities of fat and caustic leys are let in through A ; all taps are elosed; a fire is kindled, and maintained until the thermometer rises to about 154°-1 (310° F.), equivalent to a steam pressure of 63 lb. a sq. in. When it has remained at this point for an hour, the tap at C may be opened, and the contents discharged into a cooling-frame D, by the steam pressure in the boiler. For a good yellow soap, 7 ewt. tallow, 3 cwt. palm-oil, 3 cwt. rosin, and 140-150 gal. eaustic soda leys at 21° B. are recoinmended by the inventor, Dunn. Another formula is 800 lb. tallow, 200 lb. palm-oil, 400 lb. rosin, 175 gal. caustic soda leys at 25° B., for one hour at 122°•2 (252° F.), or 17 lb. steam pressure. It is obvious that this will make a drier soap, since lcys at 25° B. eontain le,ss water than leys at 21° B., and that the quantity of water desired in this product ean thus be regulated to a nicety. In 1865, Bennett and Gibbs, of Buffalo, New York, took out a patent for effecting this operation with carbonated alkali, thus avoiding the expense of caustieizing. Their boiler is sitnilar to that shown in Fig. 1245, but is placed horizontally, and provided with an ag,itator similar to that for Hawes' boiler. The process requires a higher temperature and pressure than the previous one, ranging from 176°• 6 to 204°•4 (350-404° F.), or 220-280 lb. a sq. in. The outlet-pipe is provided with a safety-valve, and the inventors state that if this be loaded to about 250 lb. a sq. in., and the raw materials be pumped
in at one end, the process may be made coutinuous, finished soap coming from the outlet, produced in less than one hour from the introduction of the raw materials. The fermula recommended is for every 100 lb. of saponifiable fat, 30-33 lb. of soda-ash of 48 per cent. dissolved in 100 lb. water. In the early stages of the process, the liberated carbonic acid is allowed egress by one of the safety-valves, and if any liquid escapes before a temperature of 163° (325° F.) is reached, it should be returned to the cylinder. The following advantages over ordinary processes are claimed :— (1) Rapidity of manufacture, (2) improvement in quality, (3) increased yield of soap, (4) economy of labour, (5) saving of fuel, (6) use of cheaper fatty material, (7) saponification of the whole of it, (8) uniform certainty of results, (9) retention of glycerine, and improvement of product thereby, (10) ability to use carbonated alkali. It is obvious, however, that the risk of explosion is not slight, and the practical difficulty of working the agitator at that temperature and pressure must be considerable.
I, c, a. Soft Soaps.—The production of these is the simplest case of the process most usually adopted in the fabrication of soap, viz. boiling in open vessels, technically termed " coppers," with the aid of steam (either wet or superheated) or of fire, or of the two simultaneously. It will be convenient, therefore, to describe here the construction and fittings of the various kinds of soap coppers (or soap-pans), and the different modes in which steam and fire are applied to boil their contents.
In the days when there was an excise duty upon soap, " coppers " were usually of what is now considered a. very small size, and were constructed of cast iron ; they consisted of hemispherical pans, upon which were mounted as many cylindrical rings as were necessary to make the copper of a suitable depth, usually about twice its diameter ; the rings were joined to each other, and to the hemispherical bottom, with cement joints. Tbere was no limit to their size, except the difficulty of making large castings, and they were usually encased in masonry, and fitted with fire-places aud flues in the manner to be presently described for modern wrought-iron " coppers.'' In the case of those boiled by flro (tho only method until steam-boiling was introdnced), the hemispherical bottoms were very apt to crack from overheating, and from many other causes, which it is scarcely necessasy to detail, as these pans are fast becoming obsolete.