Sure, saltpeter (salt with nitrates) were used in some areas, in fact (from my understanding) salt from certain salt mines / areas contained saltpeter naturally (Chile saltpetre) but the ancients did not know the difference and just assumed the salt from "this" area was better quality...
People have always smoked meat to preserve, example the natives / indians in north america to create a jerky. Smoking can create anaerobic environment in the "danger zone" for bacteria. They did not have fancy ovens, temperature gauge , timers or nitrates. How did they not get botulism? What...