I

Dr Shaw's

*See PEARLAsh, and Trans

POTASH, the lixivious ashes of certain vegetables, used in making glass, soap, &c. See GLASS, SOAP, &c. For an account of the properties and combinations of potash, see CHEMISTRY. Potash was till lately considered as a simple substance; but it appears from the unexpected discoveries of Mr Davy in galvanism to be a compound of a peculiar metallic substance and oxygen. Soda is also a compound of a similar nature. For an account of Mr Davy's discoveries, see ZINC. Here we treat only of the manufacture of potash.

The method of making potash is directed by Dr method of Shaw as follows. Burn a quantity of billet-wood to making gray ashes; and taking several pounds of these ashes, potash. boil them in water, so as to make a very strong lixivium, or ley. Let this ley be strained through a coarse linen cloth, to keep out any black parts of the half-burnt wood that might happen to remain in the ashes; then evaporate this strained lye in an iron pan over a quick fire almost to dryness: then taking out the matter remaining at the bottom, and putting it into an iron crucible, set it in a strong fire till the matter is melted, and then immediately pour it out upon an iron plate, where it soon cools, and appears in the form of a solid lump of potash *. Much after this manner is potash made in the large way, for the service of the soap-boiler, glass-maker, fuller, &c. but according to the difference actions of of the wood, or combustible matter employed, with the Royal the manner of turning it, and conducting the process, Irish Aca- different kinds of potash are prepared. There are cerdemy, 1789, tain saline plants that yield this potash to great advantage, as particularly the plant kali; there are others that afford it in less plenty, and of an inferior quality, as bean-stalks, &c.: but in general, all vegetable subjects afford it of one kind or other, and may most of them be made to yield it tolerably perfect after the manner of the process already laid down, even the loppings, roots, and refuse parts of ordinary trees, vine clippings, &c. The fixed salts of all vegetables, excepting the kali and marine plants, when reduced to absolute purity, or entirely separated from the other principles, appear to be one and the same thing: whence it should seem, says Dr Shaw, that by a suitable management good saleable potash might be made in all places

art. 1. Class Science.

2

Mr Ste

paratu

Above half a century ago, Mr Stephens, encouraged Account of by the Society of Arts, &c. and by a parliamentary phen's ma grant of 30col. established a manufacture of potash in nufacture. North America, which produced such as was so perfectly good as to answer in bleaching and other uses the purposes of pearl-ash; and which at the same time afforded a very large produce. But the very great heat which his process required, occasioned the destruction of a very extensive apparatus; and other circumstances concurred to disappoint the hopes and check the spirit of the proprietors. The manufacture was, however, afterwards undertaken and prosecuted by others. Mr Stephens's apparatus was as follows: Fig. 1. A is the Plate bed of the kiln, which flies off about four feet by two fig. L from the grate, more or less according to the size; C is the ash-hole, 2 or 3 feet deep, Fig. 2. B repre- Fig. 2. sents quadrangular bars of iron, with their opposite 3 of his ap angles placed upwards and downwards, not above an inch asunder. Fig. 3. A, B, and C, are three steepers Fig. 3. five feet deep, and of any width from four to eight feet square, of the best white pine or cypress plank, with square joints and strong oak frames, placed each over a receiver, with a cock to let off the ley, and a vent just beneath the surface of the grating. E represents three receivers, standing each under, and projecting out, from its steeper. They must be made of the best stuff, carefully put together, and laid in tough clay, well rammed within the ground, their tops being level with the surface: they need not be so large as the steepers by six, eight, or twelve inches. Fig. 4. E represents a false Fig. 4 bottom or lattice of boards, eight inches deep and five square, with a hole in the under edge of every partition for the ley to pass into the steeper. Fig 5. A is the Fig. 5. vessel over the furnace in which the ley and ashes are mixed; B is a hole or funnel a few inches from the back of the furnace, with an iron socket to let the pipe through the hinder part of the arch, to reach down within two inches of the floor of the furnace. C is a

cast

Potash. cast-iron cauldron for boiling the ley to dryness when pearl-ash is made. D is a vessel whence the liquor is let into the cauldron as it evaporates. The mortar for building the furnace should be made of loam; the arch should be 18 inches thick, and the floor should be laid with tyles on a layer of sand an inch thick, with neat joints.

Process without using a kiln,

and with it.

Mr Stephens's process, both with and without the kiln, was as follows. Cut timber, felled at any season, into lengths of about eight feet: lay from three to ten of them lengthwise in a heap upon dry ground, and fill the vacancies between with smaller wood: the sooner it is burnt after felling, the better. Set fire to it by laying embers on the bottom logs at each end; and for burning the brush and lappings, with other smaller woods, lay them lengthwise on the ground, top to top, lapping over a little, with the butt ends outwards, and as close as a faggot; laying the larger woods on top till the heap is full four feet high; the length of the brush set against each other making the breadth of the heap. As to the choice of the timber, old hollow trees, if not dead, are best: pine, cypress, and cedar, are to be totally rejected.

As soon as the pile is burnt down, rake such ashes as lie round the outside a little in towards the middle; add no fresh fuel, nor throw on any brands. Let the ashes lie without stirring till you can just bear your hand in them; then carry them to a house, or under a shed, on a plank floor raised a little from the earth and well jointed; there wet them till brought nearly to the consistence of mortar in the first mixture of lime and sand, and ram them in a heap, in which they must lie full 20 days, or some months if you please; observing to be more sparing of water in winter, and ramming them closer, and sometimes wetting the top that it may never grow quite dry.

Wood may also be burnt in a kiln, as fig. 1. and 2.; and then it must be cut into such lengths as may be most convenient for carriage, and best suit the size of the kiln. The mouth of the ash-hole must be close stopped by daubing the joints of the lid with loam, or throwing a bank of sand or earth against it keep the bed of the kiln filled with wood up to the surface, but not above it, and let it burn incessantly till the ashes rise within six or eight inches of the grate. Draw them out whilst red-hot, and in that state sprinkle them with ley, from four to six caracts weight; weigh a small phial which holds about four ounces very exactly; then fill it with water and weigh that also: divide the weight of water into equal parts till you come tog of the whole, which is called a caract, two caracts, &c. until you have a weight equal to of the whole water, which is called 32 caracts: all which small weights, together with one equal to the phial filled with water, are to be kept for weighing the ley in the said phial till they are made damp; then ram them as before in a heap, but separate from the ashes made as above. N. B. By kilnburning a stronger ley may be more certainly procured than by the other way, where rain may chance to fall on the ashes before they can be removed.

The ashes thus prepared are to be put into vats or steepers, fig. 3. with a false latticed bottom, as fig. 4.; first putting coarse wheat or rye straw about a foot thick on the lattice or grating; on which put ashes to within four or five inches of the top, ramming them all the

way up, especially at the sides, with a small light rammer, as tight as you can, without bursting the vat. Form on the top of the steeper a hollow bason in the ashes four or five inches deep leaving the ashes four or five inches thick on the sides, by raising a small bank round the sides, so that the liquor may not overflow the edges of the ashes at top; keep this bason constantly filled with soft water in the steeper A, until the ashes will imbibe no more, which will be in 24 hours or more, according as it is rammed; then turn the cock, and let off what shall be soaked through into the receiver or lower chamber of the steeper, and no more; for if the several runnings are not kept separate, the ley will not be brought to its due strength. Follow that steeper with fresh water on the same ashes for several other runnings, which will each come off in a few days, till the liquor has neither smell nor taste; then heave out the ashes, and charge the steeper afresh.

Upon drawing off the first running from the steeper A, fig. 3. fill the steeper B with ashes as before, and put into its hollow at the top the ley so first run off, and the smaller or half leys also, till full, and draw off as directed for the steeper A: if this weighs 18 caracts or more, pump it into the cistern F as fit for use; if it be short of that, pass it off as half ley to the steeper C, and through fresh ashes till strong enough. With kilnashes only, from water passing through the first steeper, it will be strong enough for the cistern, if the ashes are well prepared. If your water be hard, let it stand two or three days exposed to the air and sun in a shallow back, and it will be soft. When you use kiln-ashes with others, lay them at bottom.

The ley must be conveyed from the cistern F, as it is wanted to the vessel A fig. 5.; where with every gallon of proof ley mix three ounces of fine light wood ashes; and to the ley that is one-fourth over-proof put six ounces of ashes; and if two-fifths over-proof 12 ounces, increasing or lessening according to the strength of the ley.

For evaporating the ley and melting the salt, heat a furnace till you bring it very near a white heat, of which the side-doors being red-hot is a mark. This will take 48 hours or more if the furnace be quite cold : when thorough hot, a little fuel keeps it so. Then, through the cock of the vessel A, pass the mixture by the funnel B into the furnace, not so as to reach much beyond the middle of the floor, before it changes from dark to bright red, letting the heat prevail towards front or back as you see necessary. When the mass begins to gather about the flues or in heaps, run in no more till the furnace is cleared by driving the fire backward. You must have two funnels, one soon choking; in an hour or less will issue out a red-hot stream of melted salt, which is potash, to be broken to pieces as soon as cold, and packed in tight close casks, being in no respect inferior to the best foreign ash whatever.

Potash.

6

The best potash is made from barilla, and comes from Spanish Spain. The plants from which it is procured are found potash the in great plenty about Carthagena, where they are indi- best. genous, and may be collected in a swamp called Almojar east of that place; the Sayones barilla is the best. They are found, besides all along that coast, on the borders of the Mediterranean for 60 leagues in length, and 8 in breadth. About 150,000 quintals of it are annually exported from Spain. It produces a revenue of 25,500l. Hh 2

a-year ;

Potash. a-year; each quintal paying a duty of 17 reals; yet Don Bernardo de Ulloa, A. D. 1740, says it was farmed at 18221. 45. 3d. M. Macdonnell has brought the manufacture of potash to its present perfection in Spain; bat its exportation is materially injured by the heavy tax on it. See Townshend's Travels, vol. iii. p. 131. Dr Ferci. See also BARILLA, KELP.

val's ac. count of making potash.

8

Other at

mpts.

In the 70th volume of the Philosophical Transactions we have an account of a method of procuring this salt from the putrid water which runs from dunghills. The process is very easy, consisting only in simple evaporation of the fluid, and calcinating the impure salt till most of the foulness is burnt out. From 24 wine-pipes full of this muck-water were obtained 9 cwt. I q. 12 lb. of saleable potash, valued at 42s. per cwt: the expence of manufacturing them being only valued at 41. 9s.

The potash thus made is of a grayish white appearance; deliquesces a little in moist air; but if kept in a dry room, near the fire, acquires a powdery surface. It is hard and of a spongy texture when broken, with many small crystals in its substance. The colour of its internal parts is dusky and variegated. To the taste it is acrid, saline, and sulphureous. It emits no smell of volatile alkali, either in a solid form, dissolved, or when added to lime-water; neither does it communicate the sapphire-colour to a solution of blue vitriol. Silver is quickly tinged black by it; a proof that it contains much phlogiston. Ten grains of this potash required II drops of the weak spirit of vitriol to separate it. The like quantity of salt of tartar required 24 drops: a strong effervescence occurred in both mixtures; and a sulphureous vapour exhaled from the former. A tea spoonful of the syrup of violets, diluted with an ounce of water, was changed into a bright green colour by five grains of the salt of tartar; but ten grains of this potash were necessary to produce the same hue in a similar mixture. Half an ounce of the salt dissolved entirely in half a pint of hot water; and when the liquor was cold, a large purple sediment subsided to the bottom; and it was found that this sediment amounted to about two-thirds of the whole quantity of ashes used.

Dr Percival, the author of this paper, concludes with observing, that this potash is a true fixed vegetable alkali, produced by putrefaction; that the quantity of alkali contained in it may be estimated at one-third of its weight, whereas the white Muscovy ashes are said to yield only one eighth part; that no quicklime appears to be contained in this potash, for a solution of it poured from its sediment remained clear though long exposed to the air; that it would be worth trying, whether the large purple sediment, which subsides when this potash is lixiviated, might not be applied to the manufacture of Prussian blue, or used in the manner recommended by Macquer for dyeing wool and silks; and that this manufacture will furnish the farmer with topdressing for his garden and land, of great fertilizing powers. See Phil. Trans. vol. lxx. p. 345.

These are the processes most essentially different from one another which have appeared concerning the manufacture of this useful salt. Some indeed have attempted to compose it on the supposition that alkali consisted of an earth combined in a peculiar manner with a certain acid. But the little success of all these attempts show that they have been built on a false principle. The only method of producing alkaline salts originally is from

the ashes of vegetables; and the vegetable substances Potash. which yield the largest quantity of them are tartar and marine plants. From the former the purest and strongest vegetable alkali is obtained, and from the latter the mineral alkali. From other vegetables, as fern, broom, bean-stalks, &c. an alkaline salt is produced, but so impure, and in such small quantity, that no manufacture of it can be established in this country with any reasonable expectation of profit.

9

from rock

Dr Watson (the present bishop of Landaff) suggests, On extractthat the investigation of a method of extracting its alka-ing its alline part from rock salt would be a most serviceable dis.kaline part covery. We have inexhaustible mines of rock-salt in salt. this country, which (he observes) the proprietors can afford at ten shillings a ton. A ton of rock-salt contains about half a ton of mineral alkali, which is for most purposes far preferable to potash. To those who have leisure to attempt such a discovery, he gives the following bint: whether the alkaline part of rock-salt may not be obtained by calcining it in conjunction with charcoal in open fires? His reason for this conjecture is founded upon the following experiment upon burning sea-wreck to a black coal, and stopping the process at that point, he has obtained great plenty of common salt, but no mineral alkali from the black ashes; though we are certain, that when the black ashes are thoroughly calcined, or reduced to white ashes, mineral alkali may be obtained from them. This makes it probable, that the common salt contained in the black ashes of sea-wreck is decomposed, and changed into a mineral alkali, during the burning of the black ashes. There are reasons to suppose, that the cinder of pit-coal would answer the purpose better than charcoal. Chem. Ess. vol. i. p. 136, &c.

10

The potashes of different countries vary much in qua- Dr Home's lity; and the experiments of Dr Home, in his Treatise experion Bleaching, seem to set forth their different proper-ments on ties in the clearest point of view. The different kinds the pot tried by him were,

ashes of

different

1. Blue pearl-ashes.-These appear to be a pure al- countries, kaline salt, mixed with a small quantity of vitriolated tartar and earth. Half a pound of this, filtered and evaporated, yielded 54 ounces of pure salt.-Here, however, we must observe, that though the quantity was so far diminished by this operation, yet we are not to imagine that the whole of this diminution was owing to impurities; for all salts are destroyed in some measure by solution in water and exsiccation.

2. White pearl-ashes are nearly of the same quality with the former; half a pound of them giving five ounces and seven drams of pure salt, with some vitriolated tartar and earth.

3. Russia or Muscovy ashes have very much the appearance of slaked lime, and are, like it, friable betwixt the fingers. They adhere to the tongue and their alkaline taste soon goes away, leaving in the mouth a strong taste of lime. Some small bits of charcoal are observable in their composition, and they never turn moist in the air. Half a pound of the salt lixiviated with water, and evaporated, gave only 10 drams 15 grains of very caustic salt. These consist therefore of a small quantity of alkaline salt united with a large quantity of lime.

4. Cashub-ashes are of the colour of iron-stone, and extremely hard, with many shining particles of charcoal

very strong ley was made, into which dry straw was Potash. dipped until it sucked up all the ley. This straw was afterwards dried and burnt, and gave him salts which he showed me, almost as good and pure as the pearlashes. This method I have several times tried; but could never burn the straw to white ashes, the salts diminishing the inflammability of the straw. It is a very expeditious method if it can be practised. But I can see no occasion for bringing the ley into a solid form, as the salts must again be dissolved in water before they can be used. The strength of the ley can easily be determined by the hydrostatical balance.

Potash. in them. They have a saline taste, with a considerable degree of pungency; feel gritty in the mouth when broke in pieces by the teeth, and will dissolve in water. To extract the pure salt, half a pound of the ashes were boiled in a pint of water; then that water poured off, and half a pint put on the ashes again; and so on, till the ashes tasted no more salt. This boiling took 24 hours, and the last water that came off had a strong taste of sulphur, and was blackish. A piece of silver put in the decoction was in a few minutes turned almost black; but though the decoction was evaporated considerably, it did not turn silver black more speedily than before. The whole, when totally evaporated, yielded only 10 drams of a brown salt having a strong caustic alkaline taste. Some Cashub-ashes powdered, and often washed in water, so that the salts were all carried off, were infused in water. After standing some time, there was a weak lime-water, with something of a saline taste, but no pellicle. Some of this residuum was put into a reverberatory furnace for two hours; after which it afforded good lime-water. Cashub-ashes then appear to contain an earth half vitrified, some lime, alkaline salts, and a quantity of sulpher.

II

"On manu

hem in his councy.

5. Marcoft ashes are of a paler colour than the former, with some small pieces of charcoal in their composition. They have a strong saline taste; and so great pungency, that they cannot be held long in the mouth. Half a pound dissolved in water, filtered and evaporated, yielded 11 drams one scruple and two grains of alkaline residuum. The decoction blackened silver, but not so strongly as the former; and by evaporation it quickly lost that quality.

Our author next proceeds to consider the probability acturing of manufacturing these ashes in this country. On which subject he has the following observations."The blue and white pearl-a-hes we have discovered to be pure alkaline salts, without any considerable mixture of heterogeneous bodies. Their purity shows the lixive to have been strained through some close substance, such as linen or flannel. The blue ashes show by their colour that they have sustained the most fire. But both of them are so much alike, that the one may be substituted for the other; and therefore we shall consider them in one view.

"Every one knows that alkaline salts, such as these, are got from all plants except the alkalescent, and from all trees except the most resinous, which afford them in very small quantity. These plants or trees, when sound, are pulled or felled in the spring, dried, and burnt to ashes. By the affusion of warm water the salts are dissolved, and, by straining, separated from the earth along with the water. This saline liquor, which is called a lixive, is evaporated over a fire; and what remains is an alkaline salt of the same kind with the pearl-ashes.

"I was informed by a skilful bleacher in Ireland, that he practised a more expeditious way of extracting the salts. He bought the ashes of different vegetables from the commonalty for 9s. a bushel. From these a

"Though I make no question, that the quantity of salt in plants of the same species, will vary in different soils and climates; yet it would be of advantage to have the proportion ascertained in general. Some trials of this kind I have made.

"Two pounds of fern which had been pulled August 16. were dried, and burnt to white ashes. These weighed 7 dr. and tasted very salt. When lixiviated, strained, and evaporated, they gave me 49 gr. of salt, about the eighth part of the ashes. If the fern had been pulled in April, it would have afforded more salt. Why then should we not prepare salts from this vegetable? There is more of it growing on our hills than would serve all our bleachfields. The Irish make great use of it.

"From 11 oz. of tobacco-ashes I had 1 oz. of salt.

Two ounces of peat-ashes afforded half a drachm of salt. Nettles, I am informed, afford much salt. Furze and broom, natives of this country, are very fit for this

purpose.

"But the kelp, as it grows in such plenty along our shore, and contains more salt than any other vegetable I know, would be the most proper, were it not for a mixture of some substance that renders it unfit for bleaching, at least of fine cloths, after they have obtained a tolerable degree of whiteness. It is observed by bleachers, that in these circumstances, it leaves a great yellowness in the linen. As these ashes are much used in Ireland, and as it is not uncommon to bleach coarse cloths with them in Scotland, a disquisition into their nature, and some attempts to purify them, may not be improper. There are no ashes sold so cheap as these ; for the best gives but 21. the 2000 weight (B). They may, therefore, allow of more labour to be expended on them, and come cheaper at long-run than the foreign salts.

"I dried some sea-ware, and burnt it, though I found that last operation very difficult. When I had kept it fused in the fire for two hours, it weighed 3 oz. I poured on the ashes an English pint and a half of cold water, that I might have as little of the sulphur as possibly. Thisley, after it had stood for some hours, was poured off clear, and had but a slight tendency to a green colour. I made a second infusion with milk-warm water, and poured it off from the sediment. This had a darker colour than the former; was kept separated from it, and evaporated by itself. There was a third infusion made;