The saltpetre obtained by this process is not sufficiently pure. The purification is completed by washing it with water saturated with nitre, which dissolves the foreign substances. This washing is completed in a vessel, the bottom of which has been pierced with holes. The nitre, however, is left some hours in contact with the water, when the latter is permitted to run out. When the solution is of the same degree of concentration as that of the saturated water, the operation is finished. The nitre is dried for use.
The old process of refining saltpetre is thus described: Put into a copper, one hundred pounds of nitre, and fourteen gallons of water; let it boil gently half an hour, removing the scum as it forms; then stir it, and before it settles put it into filtering bags, which must be suspended from a rack. Put under the filters glazed earthen pans, to receive the liquor; in which place sticks for the crystals to form on. In two or three days, it will all crystallize.
In some saltpetre works, sulphate of potassa is used with advantage. This salt is furnished in abundance, by the combustion of a mixture of nitre and sulphur, in the manufacture of oil of vitriol. It forms the residue after the combustion. It is likewise produced in the preparation of nitric acid, in the decomposition of nitrate of potassa, by sulphuric acid. It may, therefore, be obtained in quantity, from the oil of vitriol manufacturers, and the aquafortis distillers. It is usually called _vitriolated tartar_.
It is known that sulphuric acid forms, with lime, an almost insoluble compound, called sulphate of lime, or gypsum; and hence, when sulphate of potassa is mixed with a solution of nitrate of lime, nitrate of potassa is formed, which remains in solution, and sulphate of lime is precipitated. The same effect takes place with all earthy nitrates. For the application of sulphate of potassa, in this way, we are indebted to M. Berard. It might be advantageously employed in decomposing the calcareous nitrate of the nitre-caves of the western country.
M. Longchamp has recommended the use of sulphate of soda, or Glauber's salt, for decomposing the muriate of lime, which exists occasionally in impure nitre. These two salts reciprocally decompose each other; sulphate of lime is precipitated, and muriate of soda remains in solution. The latter is separated by evaporating the nitrous solution.