Nitre

Nitre (Hebrew nether) in the biblical context refers not to saltpeter (potassium nitrate) but to natron — sodium carbonate — a naturally occurring alkali salt mined from dried lake beds in Egypt and used in ancient times for washing, cleaning, and embalming. Proverbs 25:20 compares one who sings songs to a heavy heart to vinegar poured on nitre, where the fizzing and dissolution of the natron illustrates how cheer poured on sorrow produces the opposite of comfort. Jeremiah 2:22 uses nitre along with soap (borith) to declare that even the strongest cleansing agents cannot remove Israel's stain of iniquity before God — only genuine repentance and divine forgiveness can accomplish that.