FULLER’s SOAP, Heb. phrase, וּכְבֹרִ֖ית מְכַבְּסִֽים, consisting of a term בֹּרִית, H1383, “alkaline salt,” “natural lye” extracted from the Asiatic soap plants such as Mesembrianthemum cristallinum; Salicornia solacea; Salsala kali and the like (cf. I. Löw, Die Flora der Juden [1924-1934]) which are reduced by burning to produce a pasty mass used as a bleach, esp. in the presence of olive oil. The other term is the common Sem. term כָּבַס, H3891, “to tread,” “knead” and thus to wash in the Near Eastern fashion.
FULLER’S SOAP. An alkali prepared from the ashes of certain plants and used for cleansing and fulling new cloth. The word is used figuratively in