Translated from the introduction to Fargard 8 of the Vendidad in Charles-Joseph de Harlez de Deulin’s French translation of the Avesta.
“The passage concerning the fire deserves some clarifications. To better understand it, we must remember certain principles of the Zoroastrian system:
1. The fire is a sacred element, a direct representative of divine power and holiness. Every fire lit is an entity worthy of respect, which would be criminal to profane or destroy. If some blameworthy usage comes to soil the fire, it must be purified with care.
2. As a rule, the fire must only burn on altars in honor of God Supreme; yet as a concession for the needs of man, Ahura Mazda permits for it to be used for profane purposes. Nevertheless, this concession is not indefinite. After a certain amount of time, the fire that heats industrial furnaces, and even that which gleams in the domestic hearth, must be liberated from this servitude and returned to the sacred place, to take on again the only function suited to it. Great are the merits of the one who performs this prescription.”