The backup vessels of the Beit HaMikdash did not include a backup mizbeach (altar). But they assumed everything was impure because of the kohanim who were amei ha-aretz, which means that the mizbeach didn't need purification. The question is what mechanism kept it pure? Also, the fire of Gehenna has no power over Torah scholars, as can be inferred from the case of the fire salamander. Nor even over the sinners of Israel, as can be inferred from the golden altar. HADRAN ALAKH Chagigah and Seder Moed