Professor Cleaveland, in noticing the saltpetre caves of the western country, observes, (Elementary Treatise on Mineralogy and Geology,) that one of the most remarkable of these caverns is in Madison county, on Crooked Creek, about sixty miles S. E. from Lexington. This cavern extends entirely through a hill, and affords a convenient passage for horses and wagons. Its length is six hundred and forty-six yards; its breadth is generally about forty feet; and its average height, about ten feet. One bushel of the earth of this cavern, commonly yields from one to two pounds of nitre; and the same salt has been found to exist, at the depth of at least fifteen feet; even the clay, a fact which seems rather remarkable, is impregnated with nitrate of lime. Kentucky also furnishes native nitre under a very different form, and constituting what is there called the rock ore, which is in fact a sand stone, richly impregnated with nitrate of potassa. These sand stones are generally situated at the head of narrow vallies, which traverse the sides of steep hills. They rest on calcareous strata, and sometimes present a front from sixty to one hundred feet high. When broken into small fragments, and thrown into boiling water, the stone soon falls into sand; one bushel of which, by lixiviation and crystallization, frequently yields ten pounds, and sometimes more than twenty pounds of nitrate of potassa. The nitre from these rocks contains little or no nitrate of lime. This account is corroborated by Dr. Brown,[13] to whom our author is indebted for his remarks.
In a memoir in the American Philosophical Transactions by Dr. Brown, then of Lexington Kentucky, we have a description of a nitre cave on Crooked Creek, with the process for extracting the saltpetre. From this memoir, the following extracts are made: The water which percolates through the cave in summer, as the walls and floor are dry in winter, condenses upon the rocks, and the substance thus formed, has the same properties as the salt obtained by lixiviating the earth of the floor. As far as the workmen have dug, the earth is strongly impregnated, every bushel of which, upon an average, furnishes one pound of nitre. The same earth will be again impregnated, if thrown into the cave. What length of time it requires to saturate it, is not known.
The workmen have different modes of forming an opinion with regard to the quantity of nitre, with which the earth may be impregnated. They generally trust to their taste; but it is always considered as a proof of the presence of the nitre, when the impression made one day on the dust by the hand or foot disappears the day following. Where there is a great deal of sand mixed with the dust, it is commonly believed that a small quantity of potash will suffice for the operation. The method of making saltpetre, usually practised in Kentucky, is as follows:
The earth is dug, and carried to hoppers of a very simple construction, which contain about fifty bushels. Cold water is poured on it for some time, and in a day or two, a solution of the salts runs into troughs placed beneath the hoppers. The lixiviation is continued as long as any strength remains in the earth. The liquor is then put into iron kettles, and heated to ebullition; it is afterwards thrown upon a hopper containing wood ashes, through which it is suffered to filtrate. As the alkaline part of the ashes is discharged before the nitrate passes through, the first runnings of this hopper are thrown back, and after some time, the clear solution of nitrate of potassa runs out, mixed with a white curd, which settles at the bottom of the trough. This clear liquor is boiled to the point of crystallization, then settled for a short time, and put into troughs to crystallize, where it remains twenty-four hours; the crystals are then taken out, and the mother water thrown upon the ash hopper, with the next running of the nitrate of lime. When the quantity of the nitrate of lime is too great for the portion of ashes employed, the workmen say their saltpetre is in the grease, and that they do not obtain a due quantity of nitre. If too much ashes are used, they say it is in the ley; and when it is left to settle previous to crystallization, a large quantity of salt will be deposited in the settling troughs, which they call cubic salts. These salts are again thrown upon the ash-hoppers, and are supposed to assist in precipitating the lime from the nitrate of lime, and in the opinion of the workmen are changed into pure saltpetre. To make a hundred pounds of good saltpetre at the great cave, eighteen bushels of oak ashes are necessary; ten of elm, or two of ashes made by burning the dry wood in hollow trees. The earth in some caves does not require half this quantity of wood ashes to decompose the earthy salts.
When wood ashes cannot be obtained in sufficient quantity, they make a lixivium of the earth, and boil it down, which they call thick stuff. This is put in casks, and transported to a place where ashes can be had. When dissolved and passed through wood ashes, it is changed, as in the former process, into saltpetre. Having thus given the Doctor's account, let us inquire, in the next place, into the theory of the process.
The theory is very evident. The mixed nitrate, consisting of variable proportions of nitrate of lime and nitrate of potassa, is extracted from the saltpetre earth by water, which dissolves it. Now, as the affinity of nitric acid for potassa is greater than for lime, and consequently potassa will decompose nitrate of lime, when the lixivium is passed through wood ashes, the potassa they contain will unite with the nitric acid, and the lime be separated, which remains in the hopper. The liquor holds in solution no other salt than nitrate of potassa, provided the quantity of alkali in the wood ashes be sufficient to effect the decomposition;—if more, it will pass through in an uncombined state; and if less, the liquor will contain nitrate of lime. As the alkali contains more or less carbonic acid, the decomposition is not a case of single but of double affinity, in which we form, at the same time, a carbonate of lime.
When the solution is boiled, and set aside in the troughs to crystallize, the nitre will form in a regular manner. The mother water, or the fluid which remains after the crystallization, may contain, from the circumstance before stated, either potash, or undecomposed nitrate of lime—hence it is thrown on the hopper in a subsequent operation.
The nitre, however, as made at the caves, is called rough or crude nitre. Before it is used for the manufacture of gunpowder, and other purposes, it is purified or refined. This operation, which we shall notice more fully hereafter, is nothing more than the separation of all earthy salts, and the alkaline muriates and sulphates; in other words, the conversion of the whole by the separation of foreign substances, into pure nitrate of potassa.
The mode of treating the rock ore, or sand rocks, which contain nitre, is the same as before given. It contains more nitrate of potassa, and therefore requires less potash, and in some instances, the nitre is perfectly pure. The sand rocks often yield twenty or thirty pounds per bushel. A mass of pure nitre, weighing sixteen hundred pounds, has been discovered. Smaller masses have also been found.
The rocks which contain the greatest quantity of nitre are extremely difficult to bore, and are tinged brown or yellow.
Saltpetre makers find it to their interest to work the rock ore in preference to the calcareous nitrate, as it yields more nitre.
It is a fact well known, that foreign saltpetre contains a variety of deliquescent salts, or those salts which attract and absorb moisture and also common salt. The efforts of European refiners are directed to their separation. The saltpetre of the Western country, Dr. Brown assures us, does not contain common salt.
Dr. Brown, in Silliman's Journal, i, p. 147, in a letter to professor Silliman, observes, that there exists a black substance in the clay under the rocks, of a bituminous appearance and smell. This black substance, it appears, accompanies the sand-rock nitre, and is the same as that found in Africa, which also accompanies nitre in that country. Animal matter seems to have existed in the nitre caves of Africa, forming, as Mr. Barrow expresses it, either a roof or covering; no such matter, however, has ever been found in or adjacent to the nitre caves of the Western country.
The observations of Mr. Barrow on the subject of the saltpetre of Africa may be interesting to the reader. He observes, (Southern Africa, p. 291,) that, about twelve miles to the eastward of the wells, (Hepatic Wells), in a kloof of the mountain, we found a considerable quantity of native nitre. It was in a cavern similar to those used by the Bosgesmans for their winter habitations. The under surface of the projecting stratum of calcareous stone, and the sides that supported it, were incrusted with a coating of clear, white saltpetre, that came off in flakes. The fracture resembled that