F´.[39] Use of wood-ash, animal charcoal, &c.
“Minding to have (saltpetre) above the common use, for some purpose, more purified, &c. (which for to make exceeding fine powder, or aqua fortis, is most requisite so to be):—take of the aforesaid mixture (F) ... and for every barrel of water you have put in the cauldron ... you must put into it five potfulls” of the mixture. “In the same quantity of water so prepared, put so much saltpeter as it will dissolve.”
D. Second boiling of the solution.
Boil the whole until it “resolve very well.”
E. Filtration.
When the scum rises, transfer the mother liquid to a tub with holes in the bottom, on which is laid a linen cloth covered with a layer of sand four finger-breadths deep.
D´. Third boiling of the solution.
The filtered liquid is boiled again “in order to make the greater part of the water seeth away.... Make it boil so much until you see it ready to thicken, pouring in now and then a little of the mixture” (F).
G´. Final crystallisation.
The mother liquid is then transferred to wooden troughs “to congeal,” for which three or four days are allowed. “After this sort thou shalt make the saltpeter most white and fair, and much better than at the first setting.”