Tuesday, October 13, 2009

How To Make Saltpetre

The process of making Saltpetre from the earth of the lime-stone caves in the Southern Confederacy is so simple that any one residing in the neighborhood of a cave in a lime-stone rock – and nearly all the caves are in such rock – can, without any expense, make at least a few pounds of the salt every day, and with assistance could make it a very profitable business at the price which the government is now paying. To furnish the practical information required, in plain language, to such persons, so as to enable each one to add to the production of an article so indispensable to the military operations of our country, now struggling for its free existence, induces the writer to publish these notes; he would earnestly appeal to his countrymen who may live near any cave, to put themselves, if need be, to some inconvenience, in order to aid in the invaluable production. We cannot be too thankful that this gigantic war was entered upon with large supplies of ammunition and the materials for its fabrication, but little of which has yet been expended; but in a contest of such magnitude, where we have to apply the fiery food for some two thousand mouths of large dimensions – some of which consume not less than three-fourths of a keg of powder at each charge, it will readily be seen that the most abundant stores must fail sooner or later, unless care be taken in time to replenish the demands of consumption.

But the nitre is still in the earth, and it behooves us to extract it in time, before we commence to feel a pressure in this direction. It is true, we are receiving daily from a few caves what would be considered a very large amount in ordinary times, but the times are extraordinary, and hence requires extraordinary supplies; thus the individual who makes a pound of saltpeter each day, contributes in fact more to the ultimate success of his country, than if he shouldered his musket and marched with all his sons to the tented field.

Gunpowder is made of over three fourth parts of nitre (purified saltpeter,) fourteen parts of charcoal and ten parts of sulphur, all by weight; hence, the nitre is much the largest portion of gunpowder material, requiring consequently the largest daily supply.

The crude saltpeter from the caves called grough saltpetre in commerce, requires to be purified before it can be used for gunpowder, and for this purpose government has established a refinery at Nashville, capable of refining daily 5,000 pounds of grough saltpetre into pure nitre, as white as snow, and ready for the powder mills. In the extensive Government Powder Works, now in course of rapid erection in Georgia under the direction of the writer, over five tons of saltpetre will be refined each day, if required, and converted into gunpowder.


One ordinary iron pot, for boiling; three or four tubs, pails, or barrels cut off; two or three small troughs; some course bags or a wheel barrow to bring the earth from the cave, and four strong barrels, with one head each – empty vinegar, whiskey or pork barrels are very good – are about all the articles required for a small saltpetre manufactory. – To these, however, must be added some ash barrels to make potash lye, as it is better that this should be made at the same time and place, the ashes from the fire under the pot for boiling assisting the production.


First bore a hole about the size of the finger through the head or end of each barrel near one side, and fit a wooden plug to each hole – then set the barrels on some pieces of timber near each other, the heads down and the whole of each projecting over the timber. Put some twigs into the bottom of each barrel, and on these place straw or hay about half a foot thick when pressed down; then having brought some of the earth from the cave and broken up all the lumps, fill each barrel full without pressing it down. Put the plugs into the wholes tightly and fill up the barrel with as much water (hot water is best in winter) as it will hold; allow the whole to remain until next day, then pull out the plugs, having placed a tub or pail under each, and pour all the water from the first barrel into the second barrel, and the water or liquor which drains from this barrel must be poured on top of the earth of the third barrel, and finally, the liquor which drains from this last barrel must be poured into a tub or other vessel. Now having previously made some strong lye from wood ashes, pour a small stream of it into the tub and stir it well; immediately the clear liquor will become muddy, and as long as the lye continues to curdle or cloud the liquor, it must be poured in; of course you will have to wait, now and then for the liquor to settle to se if it requires more lye. No more must be used than is necessary, for it not only wastes the lye, but is an impurity which the refinery must afterwards get rid of. We will now suppose that the proper quantity of lye has been used, and the liquor allowed to settle or drain through cloth until it becomes clear; it is then poured into the pot and boiled away until a drop taken up on the end of a stick becomes hard or solid when let fall upon cold metal or upon a plate.

The liquor is now to be dipped out of the pot and poured into a cloth placed over a tub or barrel, and allowed to strain through into the tub below and become cold. As soon as the liquor begins to cool, crystals of saltpetre will commence forming, and when cold the liquor left – called mother liquor – must be poured off from the slat peter back into the pot with the fresh liquor for boiling, as it still has a considerable saltpeter in it. There will be found at the bottom of the pot after the liquor is dipped out, when the boiling is completed, some earthy salts which after draining, can be thrown away as impurities; if however, some long needle-shaped crystals should be seen in it when cold, it contains some saltpetre, and about a quart of hot water should be added, and then poured off after a time when it will have dissolved all the saltpetre left among the earthy salts; this wash water can then be put back into the pot after the impurities shall have been cleaned out.

The saltpetre, formed by the foregoing process, must be first allowed to drain well, and then placed on cloths stretched before the fire or out in the sun to dry; when the drying is completed, it is to be put into sacks or barrels, and is ready to be transported to the nearest ordinance officer, who will pay for the same on receiving the bills of its shipment on the railroad.

If the crystals of saltpetre are wet and brown, and will not keep dry, it is because too much lye from the wood ashes has been used; this can be removed by nearly filling a tub or barrel with the saltpetre and pouring cold water on it, as much as the tub will hold and after remaining about one hour the water can be drained off from the bottom, when it will carry with it most of the lye; this wash water must be poured into the lye of the wood ashes so as not to loose the saltpetre which it contains.

The foregoing process evidently contains all that is required in principle for the making of saltpetre on a large scale, since nothing more is to be done than increase the number of barrels and boilers. Casks would be better, perhaps, than barrels in such case, and vats made by placing the lower ends of pieces of plank about four feet long into a trough and opening or spreading out the upper ends about three feet, then making ends to the vat, is an economical and convenient arrangement which may be used on a small as well as a large scale, instead of casks or barrels. In making use of these vats, strips of wood should be placed over the edges of the planks on the inside, and a thick layer of twigs and straw should be placed at the bottom between the planks on the inside, as well as along the sides and ends, to about one foot of the top of the vat and kept there by pieces of wood leaning against the sides, whilst it is filled with earth from the cave; if this is not done the liquor will in many cases drain through very slowly, and time lost to no purpose. A hollow or channel about a foot deep should be made along the centre of the earth in the vat to collect the water poured in.

Whether vats, casks or barrels be used, the same principle must be carried out of the passing the leached (or drained) liquor from the first vessel into the second, and from the second to the third before boiling, otherwise there will me much time and fuel lost in useless boiling of a weak liquor; this is a common error at the caves, and causes the saltpeter to cost more than necessary in time, labor and fuel.

We will now follow the process of leaching more particularly. Suppose it takes eight gallons of water to fill up the barrel after the earth has been put in even with its top, or nearly so, then about one half or four gallons only will drain off generally; we must now refill the barrel with four gallons more of water and this time four gallons will drain or leach out, because the earth has already been charged with water. Again we refill the barrel the third time, putting in four gallons of water more, and after four gallons of liquor (or as much as will drain away) has again leached off the earth must be thrown out, and the barrel refilled with fresh earth from the cave.

It will now be explained how to proceed so as to have a regular rotation of the barrels, as they shall be empted one after the other of the old earth and refilled with fresh earth from the cave. It will be supposed at first that the work is on a small scale, then there will be required four barrels, if it be desired to proceed economically; to make it clear, we will suppose that these four barrels are placed round in a circle near each other, and three of them are filled with fresh earth, the fourth remaining empty. Now when the earth of the first barrel has been exhausted of saltpetre by the three washings, it will be thrown out, but instead of filling up this barrel with fresh earth we fill up the fourth or empty barrel, and this can be going on during the leaching. Thus we have three barrels working as at first, the fresh barrel being the third in the new series, and receiving the leached liquor form the one next to it. The first barrel of the new arrangement, however has already been washed twice before the new barrel was filled with earth; hence after washing it once more with the four gallons of water, which it has just received from the barrel just emptied – which quantity drains off and is poured into the one next to it – the earth is removed and this barrel left empty. Barrel number one, which we first emptied of the old earth, having now been refilled with earth from the cave, becomes the third in the new arrangement, and so on.

The liquor of the first of the three working barrels being always poured into the second barrel and the liquor which leaches form this to be always poured into the third barrel, and finally the liquor which drains form this barrel is to be put into the vessel, where the lye from the ash barrel is mixed with it, and the whole allowed to settle. When the liquor has become clear by settling or being strained through a cloth, it is ready to be placed into the kettle for boiling down. Thus there is a continual rotation of the work of the barrels with out disturbing anything, and constantly providing strong liquor for the kettle to be boiled without losing any saltpetre in the earth thrown away. It will generally take two days for the liquor to drain off from each barrel, but the time will very with the nature of the earth, as a sandy one may take but a few hours, whilst a clayed one may take three or more days. In this case it would be better to mix sand with the earth, leached ashes or gravel or even hay or straw, than to lose so much time. If vats be made deeper than a barrel, more time will be required to leach them off, which of course is to be avoided as nothing is gained by taking two or three weeks to leach off a large vat, whilst the same earth in smaller vats or casks may be leached off in two days.

With regard to mixing the lye of wood ashes with the liquor of the third vat or barrel, the proper way is to take a certain quantity, say one pint of the liquor, and put it in a clear glass, then gradually add the lye, and stir well. So long as the lye curdles or clouds the liquor more must be added. – When sufficient lye has been used, allow the liquor to settle and become clear, then add a few drops more of lye; if it no longer clouds the liquor, sufficient has been used, and if adding the lye has been done carefully, no more has been employed than was just necessary to precipitate the impurities. – We will suppose one gill of lye has been used to the pint of liquor, then it would take eight gills or one quart of lye to eight pints or one gallon of liquor – hence knowing the number of gallons of liquor, it is easy to see at once how many gallons of lye must be added without further trial.

The above experiment may be employed to ascertain if any earth contains saltpetre; for if the lye of wood ashes causes a curdling, or muddies the water in which a considerable quantity of the earth has been mixed, and then drained off, we may presume there is saltpetre present, and the quantity will, in general, be in proportion to the amount of curdling. A slight clouding the liquor may be produced by other salts being present instead of saltpetre; but if there is much curdling, it is a pretty sure sign of saltpetre. If there be any doubt, however, allow the muddy liquor to settle, the draw it off and boil it down until it thickens, then dip a slip of paper into it and dry it well, touch a coal of fire to the paper and it burns rapidly and sparkles, you may be sure saltpetre is present.

In making the lye from wood ashes it is well to remark, that the leaves, bark, branches and limbs of the tree contain more potash than the trunk, and that the oak and ash are generally the best woods to get ashes from. In leaching the ashes a similar arrangement may be used, as for the saltpetre liquor, that is, four barrels may be used together in the same way, keeping three filled with ashes and the fourth empty, and passing the lye from one to the other, as before explained. By this means a strong lye is always on hand to be used, and the saltpetre liquor is not watered too much by a weak lye which has to be evaporated away at an expense of fuel and labor.

I will now speak to the economy of labor in the operations; thus, if the cave is sufficiently large and light enough, or can be lighted cheaply by fire – the ashes of which may be used – it is evidently cheaper to carry the vats and boilers into the cave just where the earth is, or near by, than to carry the earth out in bags to the outside of the cave to be leached. Again, it is more economical to bring water to the earth than to take the earth to the water, when both are outside of the cave, as is sometimes done, because there is much less weight of water used. By pursuing the method pointed out no time or material is lost – each day has its regular recurring operations to be performed – no surplus water has to be boiled away, and the lye is wasted, rendering the saltpetre impure.

To give some idea as to the quantity of saltpetre that can be made, I will state that twelve barrels of the earth of the caves will, in general make not less than one hundred pounds of saltpetre, and this will take from twelve to fifteen bushels of ashes.

If the twelve barrels are arranged in four circles or rows, with an extra or fourth barrel to each row, then a barrel of the leached earth can be emptied from every other row each day, (or two barrels a day) and the same number filled with fresh earth, thus in six days the twelve barrels will have been worked through, and this can be done by one man, whilst a second man boils away the liquor and attends to the vats or barrels; a third man can more than supply the ashes used, and can assist in filling the barrels. Thus in six days we have the labor of three men, which as abundantly ample to make over one hundred pounds of saltpetre, unless the earth is difficult to be procured. If the earth is richer than that supposed, or if the leaching of each barrel takes but one day, instead of two, as will frequently be the case then two hundred pounds of saltpetre may be procured in the same time, but in the latter case will require additional labor. This is at present worth seventy dollars, being the price which government now pays for a limited time, at thirty-five cents per pound, to encourage is production and remunerate individuals for first cost of apparatus.

Those who manufacture saltpetre on a considerable scale will find it convenient to have two or more casks or cisterns sunk in the earth to receive the mother liquor from the evaporating kettle, where it is left twenty-four hours to crystalyze its saltpetre.

In boiling the liquor form the vats or barrels, after it has settled or been strained from the sediment formed by adding the lye, a think skum will rise to the surface, which must be skimmed off as it forms, and thrown on the top of one of the vats, so as not to loose the saltpetre that my be dissolved from it.

The bottom of the pot or boiler, after a time will become foul from the lime and earthy salts deposited on it, which can be prevented, if thought necessary, to a considerable degree, by sinking in the kettle a small pot with a wide mouth. The sediment will collect in this pot, and can be removed from time to time, because the liquor remains comparatively still within it, and allows the salts to settle whilst the agitation of the boiling prevents the sediment falling to the bottom of the kettle.

Saltpetre made after the foregoing directions will not have above five per cent of impurities; but, if carelessly made, it will have much more, and as these have to be separated at the refinery, before it can be used for gunpowder, such saltpetre is not worth so much to government.

Tasting the earth to see if it will yield saltpetre is not a very accurate way of determining the fact, because the lime saltpetre has less taste than the potash saltpetre, and the former is the one mainly in the earth, which lye converts into ordinary, or potash saltpetre. Thus, an individual might be deceived into rejecting earth which may yield a sufficient quantity, if worked.


In order to call attention to the very considerable loss sustained by imperfect working of the earth of the caves, I will state that a certain cave in Georgia, which was examined by Prof. Pratt, of the Oglethorpe University, who kindly furnished me with the result of his analysis – it appeared that in earth which actually contained not less, on the average, than 90 pounds to the barrel – much of it containing 120 pounds – only about 67 pounds were obtained. Thus, one fourth at least of the entire amount of saltpetre was lost, or about eight dollars to each barrel; also the amount of labor employed was abundantly ample to have obtained and leached daily twice the quantity of earth that was done, of which there was sufficient within 200 yards of the mouth of the cave to furnish forty-five thousand lbs. of saltpetre. Hence, the loss at this cave was as follows for each 12 barrels of earth actually leached at this time:

Twelve barrels of earth or 90 pounds of saltpetre lost to one barrel leached which, at 30 cts., amounts to $31.50

On-quarter of the saltpetre lost to each leached barrel, or 22½ pounds, at 35 cts. Amounts to $7.87½

Total loss, $39.37½

Here was an actual loss to the proprietor of nearly forty dollars; thus he received for each 12 barrels worked, obtaining 67 pounds only, $23.45

Should have received with proper working, with the same amount of labor and capital, in the same time 24 barrels, or 180 pounds, $63.00

The above case is a sufficient demonstration of the necessity of pursuing the method laid down in these notes for the making of saltpetre.

– Published in The North Carolina Weekly Standard, Raleigh, North Carolina, Wednesday, April 16, 1862, p. 2

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