The Greek noun gnapheus refers to a fuller — a craftsman who cleaned, thickened, and finished woolen cloth. Fullers worked with harsh alkalis (such as urine or fuller's earth) to remove oils and impurities from raw wool, and they would tread or beat the cloth and bleach it. It was dirty but essential work.
The fuller appears in the New Testament at the Transfiguration: Jesus' garments became 'whiter than anyone in the world could bleach them' (Mark 9:3). The comparison is deliberately humble — the most extreme whiteness human artisans could achieve fell short of the radiant glory of the transfigured Christ. In the Old Testament, Malachi 3:2 asks, 'Who can endure the day of his coming? For he will be like a refiner's fire or a launderer's soap.' The fuller's bleaching is a picture of purification through intense process — a purification only God can accomplish completely.