These are made of wood, wrought iron, cast iron, or bricks, lined with glazed stone. Their dimensions vary, but the larger the kettle the better, as much labour, fuel, and ley are thus saved. The shape is cylindrical, widest at the top, having a faucet for the purpose of discharging the spent ley.
Brick Kettles are best in one respect, they retain heat the longest during the paste operation. The bottom of these can be composed of brick when steam is employed, in other cases a metallic bottom is necessary. If steam is employed, the superheated is preferable, as it can be introduced directly into the material, assisting the heating process, and causing a more forcible agitation of the ingredients than manual exertion can accomplish.
Cast-iron Kettles are used in small factories. In large establishments the lower portion is made of cast iron, and the upper of wood or brick. In purchasing kettles entirely of cast iron, the thinnest should be selected, as they are always composed of finer grain, and can be more easily filed than the thicker.
Sheet-Iron Kettles will last longer than cast iron. Those of the best soft sheet iron should be selected, the bottom piece being from 3/8 to 1/2 in. in thickness, and the sides from 3/16 to 1/4 in., according to the dimensions. A soft sheet-iron boiler, carefully cleaned after each operation, will last 5 or 6 years, or longer, without requiring any repairs.
In kettles for soap boiling the heat must be confined to the bottom, for if it is allowed to circulate round the sides, the ingredients will be burnt. In order to concentrate the heat, it is necessary that the grate is placed in the centre of the hearth and vertically below the kettle. The inside of the fireplace must be built of refractory bricks, so that the heat may be thrown back below the bottom of the kettle. The fuel employed must be that which produces the most heat and the least flame; hard coal should be selected. The openings through which the products of combustion enter the chimney should possess together the same surface as the grate; this is the best way to obtain a good draught and effect a complete combustion of the fuel.
Both ordinary and superheated steam are employed; the latter is preferable, because the heat can then be introduced directly into the material, whereas ordinary steam has to be condensed through a worm, or conveyed intermediately under a kettle with a double bottom, and a tub for the discharge of the condensed vapour. By applying superheated steam, both time and fuel are saved; high-pressure steam mingling with the fat increases the necessary agitation of all the ingredients, thus expediting saponification. A steam-boiler 8 ft. long and 3 ft. in diameter, with two atmospheres pressure, will manufacture weekly 100 cwt. of soap. Among other advantages of steam, not only can wooden vessels be used, but the temperature can be regulated by stopcocks; the fats combine more readily and rapidly with the alkalies; the boiling is uniform throughout the whole mass, and the soap never burns.
This operation is to produce a preliminary combination of fat and ley. Some soap-makers use during the whole operation a ley of the same strength, while others commence with a weak ley, then use one of middle strength, and finish, with a strong one. In the first case, a ley is employed of 10° to 15° B. In the second, of 7° to 10°,15° to 18°, and 18° to 25° B., successively. In some cases, as for red oil soap, very strong leys are employed, say of 25° to 30° B.; usually the fat is first put in the pan and then the ley is added. For the paste operation, no leys should be used containing foreign salts, such as are found in inferior kinds of soda, for it is then very difficult to form a union of the fats with the ley, and no good sud is obtained. But when the soap has been separated from the ley by salt, leys containing salt may be used. In saponifying red oil, salty leys may also be employed from the beginning. It is imperative in all operations that the ley should be caustic, because carbonate of soda will not unite with fat. For transforming 100 lbs. of fat into soap, about 14 lbs. of caustic soda are necessary, but generally more is employed, because the soda used is never a pure hydrate of soda.
The quantity of ley taken is also differently regulated by the manufacturers. Some add the whole amount of ley at the commencement, others add it gradually in small quantities. This last mode is preferable. From time to time, in order to test it, a drop of the paste should be put on the tip of the tongue, when, if there still is free alkali in it, a burning sensation will be produced, in which case the boiling must be continued until the soap gives a sweetish taste. More ley should then be added, under constant stirring, until the entire quantity is consumed. At this stage the contents of the kettle are transformed into a homogeneous, clear liquid, in which neither ley nor fat can be discovered. If the liquid is perfectly clear, it shows that the right proportion of fat and ley has been applied. Should saponification progress too slowly, a weak ley of from 1° to 2° B. may be added, and soap scraps will facilitate the combination of the fat with the alkali. By heating with an open fire, it sometimes happens that a portion of the paste, when it thickens, sticks to the bottom of the vessel and burns. This is indicated by a black smoke passing off here and there with the vapour.
When this occurs, the fire should forthwith be reduced, and some gallons of the strongest ley added to prevent farther mischief. By these means a slight separation of the soap from the ley is occasioned, and the contact between the former and the metallic surface destroyed. In all cases the paste operation is complete, when, on taking out the stirring rod, the paste no longer drops from it, but slides down in long threads.