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 to be made, it ſhould be examined; for if it is not well cleanſed, and of a good ſort, your works will not have their proper effect; […]
1776, Robert Jones, John Muller, “Sect. I. Saltpetre.”, in Artificial Fireworks, Improved to the Modern Practice, from the Minutest to the Highest Branches; [...] Also, Mr. Muller's Fireworks, for Sea and Land Service, [...], 2nd corr. edition, London: Printed for J. Millan, near Whitehall, page 1