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A. Preparation of grough from natural saltpetre.38

Natural saltpetre is dissolved in boiling water, the insoluble impurities removed, and the solution evaporated by the sun or artificial heat. The solid residue is grough saltpetre, and contains 1 to 10 per cent. of impurities, consisting of the chlorides of potassium and sodium, sulphates of potash, soda, and calcium, vegetable matter, sand, and moisture.

B. Boiling the solution of grough saltpetre.

The grough saltpetre is placed in an open copper with a false bottom; water is added, and heat applied until the mixture boils at 110° C.

C. Removal of the insoluble impurities.

The scum which rises to the surface during this operation is removed by ladles; the sand and heavy impurities fall upon the false bottom, which is removed just before the mixture boils. The boiling is continued until the scum ceases to rise.

D. Second boiling of the solution.

Cold water is added; the solution is boiled for a few minutes, and then allowed to cool somewhat.

E. Filtration.

At 104.5° C. the mother liquid is transferred to a tank with holes in its bottom, closed by filters.

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.