Saltpetre being the principal ingredient in fireworks, and a volatile body, by reason of its aqueous and aërial parts, is easily rarified by fire; but not so soon when foul and gross, as when purified from its crude and earthy parts, which greatly retard its velocity: therefore, when any quantity of Fireworks are intended to be made, it would be necessary first to examine the saltpetre; for if it be not well cleansed from all impurities, and of a good sort, your works will not have their proper effect, neither will it agree with the standing proportions of compositions: but to prevent accidents I shall proceed with the method of refining it. How to refine SALTPETRE. Put into a copper, or any other vessel, one hundred weight of rough nitre with about fourteen gallons of clean water, and let it boil gently for half an hour, and as it boils take off the scum; then stir it about in the copper, and before it settles put it into your filtring bags, which must be hung on a rack, with glazed earthen pans under them in which must be sticks laid across for the crystals to adhere to; it must stand in the pans for two or three days to shoot, then take out the crystals and let them dry: the water that remains in the pans boil again for an hour, and strain it into the pans as before, and the saltpetre will be quite clear and transparent; if not, it wants more refining, to do which proceed as usual, till it is well cleansed of all it’s earthy parts. N. B. Those who do not chuse to procure their saltpetre by the above method, may buy it ready done, which for fireworks in general will do equally as well. How to pulverise SALTPETRE. Take a copper kettle whose bottom must be spherical, and put into it fourteen pound of refined saltpetre, with two quarts or five pints of clean water; then put the kettle on a slow fire, and when the saltpetre is dissolved, if any impurities arise, skim them off, and keep constantly stirring it with two large spatulas, till all the water exhales; and when done enough, it will appear like white sand, and as fine as flour; but if it should boil too fast, take the kettle off the fire, and set it on some wet sand, which will prevent the nitre from sticking to the kettle. When you have pulverised a quantity of saltpetre, be careful to keep it in a dry place. How to extract SALTPETRE from damaged GUN-POWDER.