In Malachi 3:2 God is compared to a refiner’s fire and to fullers’ soap. In the NT the word is used only once (Mark 9:3), where the garments of Jesus at the time of His transfiguration are described as being whiter than any fuller on earth could whiten them.
R. J. Forbes, Studies in Ancient Technology (1956), IV, 81-89.
International Standard Bible Encyclopedia (1915)
The fuller was usually the dyer, since, before the woven cloth could be properly dyed, it must be freed from the oily and gummy substances naturally found on the raw fiber. Many different substances were in ancient times used for cleansing. Among them were white clay, putrid urine, and the ashes of certain desert plants (Arabic qali, Biblical "soap"; Mal 3:2). The fuller’s shop was usually outside the city (2Ki 18:17; Isa 7:3; 36:2), first, that he might have sufficient room to spread out his cloth for drying and sunning, and second, because of the offensive odors sometimes produced by his processes. The Syrian indigo dyer still uses a cleaning process closely allied to that pictured on the Egyptian monuments. The unbleached cotton is soaked in water and then sprinkled with the powdered ashes of the ishnan, locally called qali, and then beaten in heaps on a flat stone either with another stone or with a large wooden paddle. The cloth is washed free from the alkali by small boys treading on it in a running stream or in many changes of clean water (compare En-rogel, literally, "foot fountain," but translated also "fuller’s fountain" because of the fullers’ method of washing their cloth). Mark describes Jesus’ garments at the time of His transfiguration as being whiter than any fuller on earth could whiten them (Mr 9:3).
James A. Patch