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.