F. _Use of wood-ash, charcoal, &c._ If the impurities prevent the liquid from passing freely through the filters, it is treated with glue, wood-ash, or, better, with a little animal charcoal, which seizes on the impurities and rises to the top as scum. G. _Crystallisation._ The mother liquid filters into the crystallising trough at 70.2° to 65.8° C. H. _Stirring the depositing solution._ The solution is kept in constant agitation by poles whilst cooling, in order that it may deposit in minute crystals, called _saltpetre flour_. Large crystals contain more or less of the impure mother liquid. I. _Washing and drying._ The agitation is discontinued at 25.8° C. and the mother liquid drawn off. The flour is drained on an inclined plane, transferred to a washing vat, where it is washed three times with cold water, and then finally dried. WHITEHORNE, 1560. A. _Preparation of grough from natural saltpetre._ On the bottom of a vessel pierced with “three or fower littell holes” is placed a linen cloth, “or else the end of a broom, or some straw.” A layer of nitrified earth, “a spanne thicknesse,” is laid on this, and on the earth “three fingers’ thicknesse” of a mixture of “two parts of unslacked lime and three of oke asshes, or other asshes.... And so, putting one rewe” of saltpetre alternately with one of the mixture, “you shall fill the tubbe ... within a spanne of (its mouth), and the rest you will fill with water.” The water, on percolating through the mass, drips into a brass cauldron which, when two-thirds full, is boiled “till it come to one-third part or thereabouts. And after take it off and put it to settell in a great vessell,” when it is to be “clarified and from earthe and grosse matter diligentlie purged.” B. _Boiling the solution of grough saltpetre._ The solution is then “taken and boyled of new.” F. _Use of wood-ash, animal charcoal, &c._ When the solution boils and throws up scum, it is treated with a mixture of “3 parts of oke asshes and 1 of lime, together with 4 lbs. of rock alum to every 100 lbs. of the mother liquid.” “In a little time you shall see it alaie, both clear and fair and of an azure colour.” C. _Removal of insoluble impurities._ The heavy impurities, which sink to the bottom, are got rid of by pouring the clarified mother liquid into another vessel. G. _Crystallisation._ “Take it out and put it in vessels of woode or of earth that are rough within, with certain sticks of wood, to congeal.” I. _Washing and drying._ “This same saltpeter being taken from the sides of the vessel where it congealed, and in the water thereof washed, you must lay it upon a table to drie throughly.” F´.[39] _Use of wood-ash, animal charcoal, &c._