Then after all this, take of the best Wine Vinegar, about one pound thereof, and pour it into the said water, and you will presently see a blackish spume, or filth arise in plenty at the top thereof, and when you have left it so a while, it may be easily taken off, because of the thickness it contracts.
The boiling being yet a while longer continued, pour in the same quantity of Vinegar again, and diligently separate the Scum as afore: This pouring on of Vinegar, and separating of the Scum, may be repeated yet a third time, and oftner, whereby the purgatory water may be made altogether pure, and that there appears not any more spume.
When all this is done, cast in upon this water two or three Ounces of Burnt-Alum, most finely powdered, stir all well with a Spatula, whereby the Salt-petre may shoot into longer Crystals.
Then after this, pour out the purgatory water into the aforementioned deep Vessels, and cover them over with Linen Cloths, that the Liquor cool not. Let it stand thus one hour or two at most, and a yellow slime will settle to the bottom: Then let the water while it is yet warm flow out hence, into other Vessels made of wood, and especially of Elm, and which are large enough; or else, you may let it run into Copper Pans set deep in the Earth, for the better coolings sake, which cover over very neatly with Cloths, that so the Crystallization may begin from the bottom, and not at the upper part, as we see it wont to happen, where it is not kept by Coverings from the External cold: And we also observe therein too, that the Crystals shoot not into that length, as when the Crystallization begins from the bottom.
Let them stand thus for two or three days, until you see no more Crystals generated, which you may on this wise try.
Put some of the Lixivium in your (little) probatory Vessel, wherein, if you find that there are no more Crystals generated, you may take out the rest of the Lixivium: Then take out the Petre with an Iron Spatula (or Truel) and put it into a Vessel that is full of holes in the bottom, that so all the Lixivium may be the better separated therefrom. And you shall have your Salt-petre pure and white, and freed from its Salt.
Forasmuch as the Lixivium here separated hath as yet some saltness in it, it may be mixed with other strong Lixivium to be (agen) boil’d up, for the production of the Crude Salt-petre is usually holpen forward there-withal.
Some there are that for the better purifying the Petre, do put Calx vive in together (with it) whereby the water becomes most white like Milk, and the Salt-petre gets a greater fairness.
You must withal observe that during the time that the purifying water is contained in the Copper, your fire must be most gentle, least by overmuch boiling you get some damage, for a little fire is able easily to cause (a too vehement) ebullition, and when it has got strength (or the start of you) it is hardly allayed again. The scum which you take off from it you may pour upon ashes as if it were a Lixivium, that so nothing (from whence any profit can be hoped for) may be lost.
There is sometimes seen to stick to the bottom of the Copper (especially when it is used about many boilings) a certain stony and hard substance like Tartar, which unless it be taken away, your Copper will easily suffer damage under that same matter and be burnt.