Natron is a naturally occurring mineral alkali, essentially carbonate of soda, used in the ancient world for washing and cleaning. The word appears in two contexts: Jeremiah 2:22 says even washing with natron cannot cleanse Judah's guilt (paired with 'soap'), and Proverbs 25:20 uses the chemical reaction of vinegar on natron as an image of incongruity (like singing to a heavy heart). The term is related to Greek 'nitron' and Latin 'nitrum.' Natron deposits were mined in Egypt and elsewhere and were prized for their cleansing properties. The biblical writers use it both literally (for washing) and metaphorically (illustrating moral stain and incompatibility).