Pot-Ash, denotes an alkaline salt, or, with more propriety, a compound of different salts, obtained by reducing large quantities of wood to ashes: this process is termed, by chemists, incinera-iion. The ashes are next boiled in water, so as to form a strong ley, which, alter being strained, is evaporated in an iron vessel almost to dryness ; the matter, which remains at the bottom, is then put into a crucible, liquefied over an intense heat, and poured out on iron plates, where it speedily cools, and assumes the form of solid potash.
Such is the method generally practised in making this alkaline salt; but ingenious chemists have contrived various modes of preparing it, with a view to prevent the exportation of the large sums annually paid to Russia, America, Sweden, and other northern countries. Hence, we shall state only the method followed by Dr. PeR-cival, for procuring pot-ash from the putrid water which runs from dung-hills; as being entitled to particular attention. His process is very simple : it consists in evaporating the fluid part, and in calcining the impure salt, till the foul or extraneous ingredients are almost entirely dissipated by the fire. From 24 wine-pipes of such liquor, Dr. P. obtained nine cwt. and 40 lbs. of saleable pot-ash, which was valued at 2l. 2s. per cwt.; the expence of the whole process amounted to 41. 9s. The salt, thus procured, has a greyish-white appearance; and is, when broken, of a hard, spongy texture : it is slightly affected by moist air; but, if it be kept in a dry apartment, near the fire, a powder is formed on its surface.—Lastly, this species of pot-ash contains, according to Dr. P.'s chemical analysis, such a proportion of pure alkali, as amounts to one-third part of its weight; while that imported from Russia, yields only one-eighth.
In the year 1796, a patent was granted to Mr. Hoakeslly, for bis method of making British pot-ash ; for the supply of all manufactures, in which the foreign salt or any alkaline matter is useful. The ingredients employed, consist of English, Welch, Irish, or Scotch Kelp; foreign barilla; and the salts obtained from soap-boilers' waste, whether by evaporation, or by calcination. The materials are pulverized, and thrown into a furnace of a peculiar construction, where they are, by intense heat, melted into a liquid, which is discharged through a channel into pots. When cold, the mass assumes the appearance of foreign pot-ash.
Barilla is said to furnish the best pot-ash, which is imported from Spain. It may also be advantageously prepared from kelp, salt., or glass-wort, fern, sea-wrack, and a variety of plants that are found both on the sea-coast, and in the interior parts of Britain.—This alkaline salt is employed in various manufactures, particularly in those of glass and soap ; likewise by dyers, in the scouring of cloths, etc. :—it pays, on importation, the low duty of 2s. 5 1/2d. per cwt.