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.
First you must have some filtring bags, hung on a rack, with glazed earthen pans under them, in the same manner as those for refining saltpetre: then take any quantity of damaged powder, and put it into a copper, with as much clean water as will just cover it; and when it begins to boil take off the scum, and after it has boiled a few minutes, stir it up; then take it out of the copper with a small hand kettle for that purpose, and put some into each bag, beginning at one end of the rack, so that by the time you have got to the last bag, the first will be ready for more; continue thus, till all the bags are full; then take the liquor out of the pans, which boil and filter, as before, two or three times, till the water runs quite clear, which you must let stand in the pans for some time, and the saltpetre will appear at top. Now to get all the saltpetre entirely out of the powder, take the water from the saltpetre already extracted, to which add some fresh water and the dregs of the powder that remain in the bags, and put them together in a vessel, to stand as long as you please, and when you want to extract the nitre, you must proceed with this mixture as with the powder at first, by which means you will draw out all the saltpetre; but this process must be boiled longer than the first.
Of Sulphur, or Brimstone.
Sulphur is by nature the food of fire, and one of the principal ingredients in gunpowder, and almost in all compositions of fireworks; therefore great care ought to be taken, of its being good and brought to the highest perfection. Now to know when the sulphur is good, you are to observe that it be of a high yellow, and if, when held in one’s hand, it crackles and bounces, it is a sign that it is fresh and good: but as the method of reducing brimstone to a powder, is very troublesome to do, it is better to buy the flower ready made, which is done in large quantities, and in great perfection: but when a grand collection of fireworks are to be made, the strongest and best sulphur to use, is the lump brimstone ground in the same manner as gun powder, which we shall treat of in the following part of the treatise.