POTASH, or POTASSA. This sub stance was so named from being prepared for commercial purposes by evaporating in iron pots the lixivium of the ashes of wood fuel. In the crude state called pot ashes, it consists, therefore, of such consti tuents of burned vegetables as are very soluble in water, and fixed in the fire. The potash salts of plants which originally con tained vegetable acids, will be converted into carbonates, the sulphates will become sulphites, sulphurets, or even carbonates, according to the Manner of incineration; the nitrates will be changed into pure car bonates, while the. muriates or chlorides will remain unaltered. Should quicklime be added to the solution of the ashes, a corres ponding portion Of caustic potassa will be introduced into the product, with more or lesslime, according to the care taken in decanting off the clear ley for evaporation.
On this continent, where timber is in many places an incumbrance upon the soil, it is felled, piled up in pyramids, and burned, solely with a view to the manu facture of potashes. The ashes are put into wooden cisterns, having a ping at the bottom of one of the sides under a false bottom ; a moderate quantity of water is then poured on the mass, and some quick lime is stirred in. , After standing for a few. hours, so as to take up the soluble matter, the clear liquor is drawn off, eva porated to dryness in iron pots, and final ly fused at a red heat into compact mass es, which are gray on the outside, and pink-colored within.
Pearlash is prepared by calcining pot ashes upon a reverberatory hearth, till the whole carbonaceous matter, and the great er part of the sulphur, be dissipated ; then lixiviating the mass, in a cistern having a false bottom covered with straw, evaporating the clear ley to dryness in flat iron pans, and stirring it towards the end into white lumpy granulations.
The best pink Canadian potashes con tain pretty uniformly 60 per cent. of ab solute potassa ; and the best pearlashes contain 50 per cent.; alkali in the former being nearly in a caustic state ; in the lat ter, carbonated.
All kinds of vegetables do not yield the same proportion of potussa. The more succulent the plant, the more it affords, for it is only in the juices that the vegetable salts reside, which are converted by inci neration into alkaline matter. Herbace ous weeds are more productive of potash than the graminiverous species, or shrubs, and these than trees ; and for a like rea son, twigs and leaves are more productive than timber. But plants in all cases are richest in alkaline salts when they have arrived at maturity. The soil in which they grow also influences the quantity of saline matter.
The following TABLE exhibits the aver age product in potassa of several plants:— In 1000 parts. Potaus.
Pine of fir 0-45 Poplar 0-75 Trefoil 0-75 Beechwood 1-45 Oak 1-53 Boxwood 2-26 Willow . 2-35 Elm and maple 3-90 Wheat straw 3-90 Barb of oak twigs 4-20 Thistles 5-00 Flax stems 5-00 Small rushes 5-08 Vine shoots 5-50 Barley straw 5-80 Dry beech bark 6-00 Fern 6-26 Large rush 7-22 Stalk of maize 17.50 Bastard chamomile (Anaemia cotica, L) 19-60 Bean stalks 20-00 Sunflower stalks 20-00 Common nettle 25-03 Vetch plant 27-50 Thistles in full growth 3.5-37 Dry straw of wheat before 47-00 Wormwood 73-00 Fumitory 79-00 Stalks of tobacco, potatoes, chestnuts, chest-nut husks, broom, heath, furze,tansy, sorrel, vine leaves, beet leaves, orach, and many other plants, abound in potash salts.
The purification of pearlash is founded upon the fact of its being more soluble in water than the neutral salts which debase it. Upon any given quantity of that sub in an iron pot, let one and a half times its weight of water be poured, and let a gentle heat be applied for a short time. When the whole has again cooled, the bottom will be incrusted with the salts, while a solution of nearly pure carbonate of potash will be found floating above, which may be drawn off clear by a syphon. The salts may be afterwards thrown upon a filter of gravel. If this ley be diluted with 6 times its bulk of water mixed with as much slaked lime as there was pearlash employed, and the mixture be boiled for an hour, the potash will become caustic, by giving up its carbonic acid to the lime. If the clear settled lixivium be now sy phoned off, and concentrated by boiling in a covered iron pan, till it assumes the ap pearance of oil, it will constitute the com mon caustic of the surgeon, the potassa faze of the shops. But to obtain potussa chemically pure, recourse must be had to the bicarbonate, nitrate, or tartrate of pot assa, salts which, when carefully crystal lized, are exempt from any thing to ren der the potassa derived from them im pure. The bicarbonate having been gently ignited in a silver basin, is to be dissolved in 6 times its weight of water, and the solution is to be boiled for an hour, along with one pound of slaked lime for every pound of the bicarbonate used. The whole must be left to settle without con tact of air. The supernatant ley is to be drawn off by a syphon, and evaporated in an iron or silver vessel provided with a small orifice in its close cover for the es cape of the steam, till it assumes, as above, the appearance of oil, or till it be nearly redhot. The fused potassa is now poured out upon a bright plate of iron, cut into pieces as soon as it concretes, and put up immediately in a bottle furnished with a well-ground stopper. It is hydrate of potassa, being composed of 1 atom of potassa 43, + l atom of water 9, = 57.