Incombustible, hence (according to the theory of phlogiston
of the 17th and 18th centuries) pure and irreducible.
Phlogiston theory was the dominant chemical theory of the 18th century. It was promoted by Georg Ernst Stahl (1660–1734), and finally disproven and replaced by Antoine Lavoisier (1743-1794). Phlogiston was thought to be a universally occurring element, not unlike Aristotle’s ether, which formed compounds with many other substances. The process that we know as combustion was considered to be the liberation of this phlogiston, and the "dephlogisticated" ash left behind was the true substance of the burned matter. Much chemical research centered on identifying the "dephlogisticated" materials produced by combustion, and there was a running debate between Lavoisier's Antiphlogistians and defenders of phlogiston theory such as the eminent Joseph Priestley (1733-1804).
Apart from its historical interest, I think dephlogisticated is an attractive word that might be put to occasional use in the sense of "pure and irreducible".