“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.” “LIBER IGNIUM,” _cir._ 1300. A. _Preparation of grough from natural saltpetre._ If natural saltpetre is dissolved in boiling water, cleansed, and passed through a filter, and boiled for a day and a night; the (grough) saltpetre will be found deposited in crystals at the bottom of the vessel. The original is as follows:— “Nota, quod sal petrosum est minera terræ et reperitur in scrophulis contra lapides. Hæc terra dissolvitur in aqua bulliente, postea depurata et distillata per filtrum et permittatur per diem et noctem integram decoqui, et invenies in fundo laminas salis conielatas cristallinas.”[40] HASSAN ER-RAMMAH, 1275-95. A. _Preparation of grough from natural saltpetre._ “Take white, clean, bright (natural) saltpetre _ad lib._, and two new (earthen) jars. Put the saltpetre into one of them, and add some water. Put the jar on a gentle fire until it gets warm” (and the saltpetre dissolves. Skim off) “the scum that rises” (and) “throw it away. Stir up the fire until the liquid becomes quite clear. Then pour it into the other jar in such a way that no scum remains attached to it. Place this jar on a low fire until the contents begin to coagulate. Then take it off the fire, and beat (the crystals) gently.” F. _Use of wood-ash, animal charcoal, &c._ “Take dry willow wood, burn it, and plunge it into water according to the recipe for its incineration. Take three parts by weight of the saltpetre” (just obtained), “and the third of a part of the wood-ash, which has been carefully pulverised, and put the mixture into a jar—if made of brass, so much the better.” B. _Boiling the solution of grough saltpetre._ “Add water and apply heat, until the ashes and saltpetre no longer adhere together. Beware of sparks.” The original is as follows:—[41]