The history of the Royal-Society of London for the improving of natural knowledge by Tho. Sprat.
Sprat, Thomas, 1635-1713., Cowley, Abraham, 1618-1667. To the Royal Society.
The manner of making SALT-PETER.
IN the first place you must be provided of eight or ten Tubs, so large, that they may be able to contain about ten Barrows full of Earth, each of them. These Tubs must be all open at the top; but in the bottom of every one of them, you must make a hole near to that side you intend to place outermost, which hole you must fit very well with a Tap and Spigot on the outside downward. On the inside of the Tub, near the tap-hole, you must carefully place a large wad of straw, and upon that a short piece of board, which is all to keep the earth from stopping up the tap-hole. When you have placed your Tubs on their stands, at such a distance one from the other, that you may come with ease between them, then fill them up with such Peter-earth as you have chosen for your work, leaving only void about a spans breadth between the Earth and the edge of the Tub; then lay on the top of Page 269 the Earth in each Tub, as near as you can to the middle, a rundle of Wicker, like the bottom of a Basket, and about a foot in diameter, and by it stick into the earth a good strong Cudgel, which must be thrust pretty near the bottom; the Wicker is to keep the Water, when it is poured on, from hollow∣ing and disordering the Earth, and the Cudgel is to be stirred about, to give the Water ingress to the Earth upon occasion: Then pour on your Earth common cold Water, till it stand a hands breadth over the Earth: When it hath stood eight or ten hours loosen the Spigots, and let the Water rather dribble, than run into half Tubs, which must be set under the taps: This Lixivium the Workmen call their Raw-liquor; and note that if it come not clear at the first drawing, you must pour it on again, and after some little time draw it off, till it come clear, and of the colour of Urine.
If you are curious to know how rich your Li∣quor is before boyling, you may take a Glass-vial, containing a quart, fill it with the common Water you use, then weigh it exactly; next fill the same Glass with your Liquor, and find the difference of weight, which compared with the quantity of all your Liquors, will give you a very near ghess, how much Salt-peter you are like to make by that boyl∣ing.
Then pour on again, on the same Earth, more common Water, that it may bring away what is re∣maining in the Earth of the former Liquor. This second Liquor is of no other use, but to be poured on new Earth, instead of common Wa∣ter, because it contains some quantity of Salt-peter in it.
When this is done, turn out the useless insipid Earth out of the Tubs, which you must fill with new Earth, and continue this Operation, till you have in the same manner lixiviated all the Earth: Then fill your Copper with your Liquor, which Copper, for one of the Profession, must be about two hundred weight, and set strongly in a Furnace of brick-work; besides, on one side of your Fur∣nace, you are to place a Tub full of your Liquor, which at a tap below may dribble as fast into the Copper, as the force of the Fire doth wast your Liquor, which Invention is only to save charges in Fewel. When you have boyled it up to that height, that a little of it, flirted off the finger on a live Charcoal, will flash like Gun-powder (which for the most part falls out to be after two dayes and a nights boyling) at what time, upon tryal, a hun∣dred weight of the Liquor contains about five and thirty pound weight of Peter. But the Work∣men seldom make use of any further indication, than by finding the Liquor hang like oyl on the sides of the Brasen-scummer, when 'tis dipped into it, which is a sign it is fit to be passed through the Ashes, which is done in this manner.
You must prepare two Tubs fitted after the man∣ner of the first, where you put your Earth, saving that at the bottom of these Tubs, you must lay Reeds or Straw a foot high, over them place loose boards, pretty neer one another, over them, a little more Straw (which is to keep the Ashes from the top, and to give the Liquor room to drein the bet∣ter from them:) Then fill up your Tubs with any sort of Wood-ashes to half a foot of the top; Then pour on the foresaid Liquor, as it comes scal∣ding Page 271 hot out of the Copper, on the Ashes contain∣ed in the first Tub; then after a while draw it off at the top; and so continue putting on and draw∣ing off, first at one Tub of Ashes, then at the other, till your Liquor grow clear, and lose the thick tur∣bid colour it had when it went on.
When all the Liquour hath in this manner past through the Ashes of both Tubs, that by this means all its greasie oyl is left behind in the Ashes, you must keep it for the second boyling in a vessel by it self: in the mean time pour upon your Ashes a suffi∣ent quantity of common Water very hot, once or twice, to bring away what is remaining of the Li∣quor in the Ashes.
When you begin the second boyling, put first into the Copper the Water that went last through your Ashes, and as that wasteth, let your strong Li∣quor drop into the Copper, out of the Tub above described, standing on the side of the Furnace, till the Liquor in the Copper be ready to shoot or chrystallise.
Note that toward the end of your boyling, there will arise great store of Scum and Froth, which must be carefully taken off with a great brass Scummer, made like a Ladle, full of little holes, and usually about that time it lets fall some common Salt to the bottom, which you must take up with the said Scummer, and lay it aside for another use.
To know when the Liquor is ready to shoot into Peter, you need but drop a little of it on a knife, or any other cold thing that hath a smooth superfi∣cies, and if it coagulate, like a drop of tallow, and do not fall off the knife when it is turned down∣ward, which also may be judged by its hanging like Page 272 oyl to the sides of the Scummer. When the Liquor is brought to this pass, every hundred weight of it containeth about threescore and ten pound weight of Peter.
When you find your Liquor thus ready to shoot, you must with great Iron Ladles lade it out of the Copper into a high narrow Tub for that purpose, which the Workmen call their settling Tub; and when the Liquor is grown so cool, that you can en∣dure your finger in it, you shall find the common or cubick Salt begin to gravulate and stick to the sides of the Tub, then at the tap, placed about half a foot from the bottom, draw off your Liquor into deep wooden Trays, or Brass-pans, and the cooler the place is where you let them stand to shoot in, the better and more plentifully will the Salt-peter, be produc'd; but it will be of no good colour till it be refined, but will be part white, part yellow, and some part of it blackish.
The Salt which sticketh to the sides and bottom of the setling Tub is (as I have sayd) of the na∣ture of common Salt; and there is scarce any Peter to be found but is accompanied with it, though no doubt some of this is drawn out of the Ashes by the second Liquors: If it be foul they refine it by it self, and about London sell it at good rates to those that salt Neats Tongues, Bacon, and Collar-Beef, for besides a savory taste, it gives a pleasing red co∣lour to most Flesh that is salted with it. Pliny sayes Nitrum obsonia alba & deteriora reddit Olera viri∣diora, whether Salt-peter doth so, I have not yet tryed.
When the Liquor hath stood two dayes and two nights in the Pans, that part of the Liquor which is Page 273 not coagulated but swims upon the Peter, must be carefully poured off, and being mingled with new Liquors must again pass the Ashes before it be boyl∣ed, else it will grow so greasy it will never generate any Salt.