The method of obtaining it is as follows:

You can first collect soil that is often exposed to excrement, then mix it with plant ashes in daily life and soak it in water. After a period of time (half an hour to be on the safe side) It's better), then light the fire and boil it, and during the cooking process, take a little bit of liquid every once in a while and put it into other containers. If the liquid released once becomes solid, there is no need to boil it. After filtering By cooling down, you can get saltpeter.