COMMON HARD SOAP is of three kinds—yellow, white, and mottled. In making the yellow kind, the materials employed are tallow, grease, or kitchen and bone fat, palm-oil, resin, and the alkaline lye or solution of soda-ash, or crude carbonate of soda. The lye is prepared in the soda-vats, which are cast-iron cylinders about 6 feet wide by 4 or 5 feet in depth, and having perforated bottoms, below which is a large funnel for receiving the filtered lye. In the vat a layer of quick-lime is first placed, then successive layers of soda-ash and lime, through which water is allowed to percolate until it has dissolved about 2 per cent. of the alkali rendered caustic by the action of the quick-lime.
A ton of the fat melted in the boiler requires 200 gallons of the lye for the first process. They are gently boiled for four hours, and then allowed to cool; the partially formed soap subsides; and the lye is drawn off clear, and exhausted of its alkali. A second charge of fresh lye is then added, and the process repeated three times a-day for about six days. Previous to the last boil, the resin is added, as it is too liable to be liquefied by the alkaline lyes if exposed to the action of the whole series; the quantity of resin must always be less than half the fats employed, and before using must be reduced to coarse powder. Palm-oil is generally one of the fatty ingredients when resin is employed,
its pleasant violet odour and bright golden colour serving to disguise the resin. The great soaperies of Lancashire and Cheshire, which supply the vast export trade from Liverpool, produce most of the yellow soap made in England.
The soap is transferred from the boilers to the frames in a liquid state, and allowed to solidify by cooling. These frames are made of thick bars of wood, of the same width and length as an ordinary bar of soap; four of these are joined together, forming a square frame, and a number of these frames being placed one upon another, iron rods are passed through holes in the thickness of the wood, and being screwed up tight, form a sort of tank in which the liquid soap soon becomes solid. When this has taken place, the iron rods are withdrawn, and a thin piece of wire is inserted between the top frame and the one below it, and is made to cut through the soap in the line of the lower part of the frame. The top frame with its square of enclosed soap is then lifted off, and the process repeated until all the frames are separated. After a little exposure to the drying influence of the air, the soap is easily detached from the frames, and is then cut into bars and packed into boxes for sale.
Common White Soap is made in the same manner, with the exception of the palm-oil and resin, which are omitted, tallow being chiefly used, to which, in some sorts, cocoa-nut oil is frequently added.
Mottled Soap, which is almost peculiar to the London soaperies, is made of the same materials as the white soap, but previous to the last boil the mass is watered with a strong lye of crude soda, poured upon it by means of an ordinary gardener's watering-pot with a large rose. The mottling of the celebrated Castile soap is produced by a similar process, a solution of sulphate of iron being employed instead of the crude soda. In Southern Europe olive-oil is used instead of tallow, and forms a hard white soap.
The finer kinds of Hard Soap, or Toilet Soaps, are made of fine white tallow, suet, palm, olive, or almond oils, or a mixture of two or more of these, according to the quality required. The best Windsor soap is made with nine parts of fine tallow and one of olive-oil, the latter being preferred to suet, which is used for the inferior sort, in consequence of its slower saponification insuring a more perfect combination of the alkaline with the fat acids. The perfume used is a mixture of 6 lbs. of oil of caraway seed, 1½ lb. of oil of lavender, and 1½ lb. of oil of rosemary, to every 1000 lb. of the soap. The brown kind is coloured with burned sugar, or umber and Armenian bole; the latter colouring is used only for inferior sorts.
Honey Soap, now extensively used, is made by re-melting three other kinds, namely, palm-oil, olive-oil, and curd soaps, and scenting with the citronelle oil or essential oil of Andropogon citratus, which gives it its peculiar honey-scent. The other leading kinds of perfumed soaps are almond, rose, cinnamon, musk, and orange-flower soaps, all of which vary in their composition more or less, according to the taste of the manufacturer.