Thanks! I think you got it. The handwritting was hard to read - but I think Salnitraro or Salnitraio makes sense now that I re-look at the records. Furthermore, I was looking at some other records and found another relative with the same occupation!
I never heard of someone who "makes" saltpeter. I looked it up on web. Doesn't sound like a very nice job. But I guess it was a way to make a living......
Historically, nitre-beds were prepared by mixing manure with either mortar or wood ashes, common earth and organic materials such as straw to give porosity to a compost pile typically 1.5 meters high by 2 meters wide by 5 metres long. The heap was usually under a cover from the rain, kept moist with urine, turned often to accelerate the decomposition and leached with water after approximately one year. The liquid containing various nitrates was then converted with wood ashes to potassium nitrates, crystallized and refined for use in gunpowder.
In more rural times, urine was collected and used in the manufacture of gunpowder. Stale urine was filtered through a barrel full of straw and allowed to continue to sour for a year or more. After this period of time, water was used to wash the resulting chemical salts from the straw. This slurry was filtered through wood ashes and allowed to dry in the sun. Saltpeter crystals were then collected and added to brimstone and charcoal to create black powder.
Potassium nitrate could also be harvested from accumulations of bat guano in caves.
Are the birth records you found of relatives? I only ask because one of my ancestors kept popping up as a witness to births and deaths of non-relatives in the same era as yours, and his occupation was hard to read, as old records can be, and it ended up being "sacerdote", which means priest. This was in the province of Potenza, so some of the language would be similar to that of Puglia.
Yes, the records are from relatives. But, I'm pretty sure the occupation in question is Salnitraro or Salnitraio rather than Sacerdote. I couldn't find anything similar to Salnitraro in any of the lists of occupations that I found in books or on line.