Malachi 3:2

a refiner's fire Comp. Matthew 3:12.

fullers" sope "The process of fulling or cleansing cloth, so far as it may be gathered from the practice of other nations, consisted in treading or stamping on the garments with the feet or with bats in tubs of water, in which some alkaline substance answering the purpose of soap had been dissolved. The substances used for this purpose which are mentioned in Scripture are nitre, Proverbs 25:20; Jeremiah 2:22, and soap, Malachi 3:2.… The juice also of some saponaceous plant, perhaps Gypsaphila struthium, or Saponaria officinalis, was sometimes mixed with the water for the like purpose, and may thus be regarded as representing the soap of Scripture." Dict. of Bible, Art. Fuller. But probably borax is meant.