Boriyth refers to a cleansing or purifying substance — lye, soap, or potash used to wash garments and refine metals. It appears in Malachi 3:2 as one of the two agents by which the Lord's refining work will be accomplished at His coming. The word derives from a root meaning to be clean or pure, and carries the powerful imagery of divine purification.
The theological weight of boriyth rests in its eschatological use. When Malachi asks, 'Who can endure the day of his coming?' he answers with the image of a refiner's fire and boriyth — a fuller's soap. God does not merely punish; He cleanses. He removes what is impure so what remains is fit for His presence. This is sanctifying grace — the uncomfortable but redemptive process of divine holiness at work in human lives.