This term designates the soap-like cleansing agent made from plant ash alkali, used for washing in the ancient world. Jeremiah uses it to illustrate the futility of trying to cleanse sin through external means—even abundant soap cannot remove the stain of iniquity. Malachi employs it in describing the coming Messenger who will purify the Levites like a refiner using strong alkali to remove impurities from metal. The substance was essential for both personal hygiene and ritual purification, making it a powerful metaphor for spiritual cleansing.