The method of manufacture, as usually carried on, is of the simplest possible description. The plants, or timber, are arranged in pits some 3 or 4 feet deep, or piled upon the ground and burned, fresh material being added from time to time until a' sufficiently large heap of ashes is obtained. Dining this process whatever organic salts of potassium and sodium are present are converted into carbonates and hydrates, and remain behind with the other potash, &c., salts when the combustion is completed. The ash obtained is sifted carefully from half-burnt wood or charcoal, and placed in a series of wooden vessels for lixiviation. This part of the process is conducted in a variety of ways. Commonly the tanks are ranged in three rows one above another. Each is provided with a false perforated bottom loosely covered with cinders or straw. Below the false bottom a pipe communicates with the tank placed immediately below it. The ashes are filled in upon the false bottoms of the uppermost row of tanks, and water at a temperature of about 110° run upon them. A small quantity of slaked lime should be added to render the dissolving out of the carbonate and hydrate of potassium more certain and complete. The strong liquors are run off, as soon as each tank fills, to the evaporating pans, fresh water being run on meanwhile. When the liquors begin to come over weak, the lye is run upon the second set of tanks placed below, in which fresh ashes have been placed, and again, when sufficiently strengthened, run off to the pans. This operation is continued at will, the fresh water and weak lye always coming upon unexhansted ash. The final weak liquor from each tank may be run to waste if a particularly good product is required. It will be seen that this method of lixiviation corresponds almost exactly with the tank operations of the Le Blanc soda process.
The lye, of a light-brown colour if good, is evaporated in the iron pans mentioned, until it assumes a thick consistency. Sometimes the operation is continued until the mass is almost solid— being in the state of the "black salts" of the corresponding soda process—sometimes the thick liquors are boiled down in a separate vessel to dryness. A pretty constant agitation should be kept up during evaporation. This crude product consists chiefly of carbonate and sulphate of potassium, with about 12 per cent. of water, also various impurities and empyreumatic bodies. From the drainers into which it is raked, or the pots, it is spread upon the sole of a reverberatory furnace and thoroughly calcined, the heat being kept under until the salts are thoroughly dried, but finally urged to redness. During calcination the mass of salt is carefully paddled and turned over, so that every portion is exposed to the action of the heat. The loss of weight amounts usually to about 20 per cent., all carbonaceous matters being burned out, potassium hydrate converted into carbonate, and the various lower sulphur salts oxidized into sulphate. The product is then known as "pearl ash." Commercial " pot-ash" is the crude product of the pans or pots—the evaporated lye, before the impurities have been removed in the carhonator, as described. The "pot-ashes," known as "firsts," "seconds," "thirds," and " unbrandables," according to quality, are usually of a grey colour. If the carbonating process has been carefully conducted, the " pearl-ashes " are white, or
very nearly so. The latter may be subjected to a second refining process, being redissolved, con centrated, evaporated, and furnaced in similar apparatus to that used in the first process. The product then corresponds to the "refined alkali" of the soda manufacture. It is sometimes called " salt of tartar." • The very finest quality is made by washing out the soluble carbonate from the calcined pearl-ash, and evaporating the solution in iron pans, keeping the mass well stirred. It then assumes a granular form, and a beautifully white colour.
There is a considerable variety of both pot-ashes and pearl-ashes, the products of the different localities where they are manufactured having their own distinctive characteristics. The American pot-ashes are usually grey, or reddish grey, and of three qualities, containing respectively 35 to 40 per cent. of potash, 48 to 52 per cent., and 54 to 58 per cent. They are usually packed in oak barrels, weighing from 3 to 10 cwt. each. The American pearl-ash is of good quality, very fairly white, and ranging up to 58 per cent. In Russia and Poland the carbonate is often manufactured from the ash of straw, the product being inferior, and going by the name of " potasse de paille." The best Russian pot-ashes, often called Bann, contain from 48 to 52 per cent. of potash. French pot and pearl-ashes are of superior quality, and are often manufactured from the lees of wine and vine stalks. The Tuscan are of three qualities-60 per cent.; 50 to 55 per cent., white ; 50 to 55 per cent., blue. The following Tables show the composition of average samples of the various kinds of pot-ashes :— Large quantities of American pot-ashes aro still imported into this country, chiefly from the ports of Montreal, Quebec, and New York, and are consumed in the mannfacture of soft-soap. The quantity is becoming smaller year by year, less timber being felled now than formerly for this purpose, and also because the demand for potash is now almost entirely supplied by the carbonate made from the sulphate by Le Blane's method, and by that obtained from the French hcet-re,,t vinasses. The amount of potash obtained from timber is exceedingly small, es compared with the quantity of the latter consumed. Pine-wood contains about 1.25 per cent. of ash, of which about 0'12 per cent. is potash ; hence, in order to produce 1 ton of commercial pot-ashes, containing 65 per cent. of total potash, about 520 tons of timber would have to be burned. In the course of a few years, it is probable that timber, as a source of commercial potash, will he entirely superseded by the newer European methods. The following table represents average samples of Montreal pot-ashes of different qualities :— American pot-ashes contain, as a rule, more caustic potash than other qualities. This is owing to the use of lime in tho lixiviating tanks ; and, for many purposes, as, for example, soap-making, where the pot-ash has ultimstely to be causticized, it is a positive advantage, saving lime in the after process.