Boyd’s Bible Dictionary:
(tramper on). Fuller’s art used for cleaning clothes. They were placed in vessels of water impregnated with natron or soap and trodden with the feet (Prov. 25:20; Jer. 2:22; Mal. 3:2). Chalk and fuller’s earth used for bleaching (2 Kings 18:17; Isa. 7:3; 36:2).
Concise Bible Dictionary:
The word kabas simply implies “to wash,” as it is often translated, and would include “bleaching.” The coming of the Lord is compared to a “refiner's fire, and like fullers' soap,” when the dross and dirt will be cleared away (Mal. 3:2). At the transfiguration the clothing of the Lord became so white that it exceeded the whiteness produced by any fuller on earth (Mark 9:3). It was a reflection of heavenly glory.
Strong’s Dictionary of Hebrew Words:
Meaning:
a primitive root; to trample; hence, to wash (properly, by stamping with the feet), whether literal (including the fulling process) or figurative
KJV Usage:
fuller, wash(-ing)
From Manners and Customs of the Bible:
Malachi 3:2. He is like a refiner’s fire, and like fullers’ soap.
The precise character of all the articles used by the ancient fullers is unknown. They had mineral alkali in niter, to which reference is made in Proverbs 25:20 and Jeremiah 2:22. They obtained vegetable alkali, as the Arabs do at the present time, from the ashes of some plants and from the juices of others.
They likewise used, for cleansing their cloth, urine and chalk, and bean-meal mixed with water.
The cloths are thought to have been first trodden by the feet. They were also rubbed with the knuckles. A subsequent operation probably consisted in rubbing the cloth on an inclined plane, after the manner still followed in the East, and one which was common among the ancient Egyptians.