Boyd’s Bible Dictionary:
The saltpeter of commerce
Evidently natron or washing soda is meant (Prov. 25:20; Jer. 2:22).
Concise Bible Dictionary:
The natron of the moderns, not what is now called nitre, which is saltpeter. As vinegar upon natron or alkali (which would effervesce and evaporate) so is the unsuitableness of singing mirthful songs to a heavy heart (Prov. 25:20). It is a mineral alkali, and with oil is made into soap (Jer. 2:22).
Strong’s Dictionary of Hebrew Words:
Meaning:
from 5425; mineral potash (so called from effervescing with acid)