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.