soap
(Jer. 2:22; Mal. 3:2; Hebrew: borith)
properly a vegetable alkali, obtained from the ashes of certain plants, particularly the salsola kali (saltwort), which abounds on the shores of the Dead Sea and of the Mediterranean
It does not appear that the Hebrews were acquainted with what is now called “soap,” which is a compound of alkaline carbonates with oleaginous matter.
The word “purely” in Isa. 1:25 (Revised Version, “throughly;” marginal note, “as with lye”) is literally “as with bor.” This word means “clearness,” and hence, also, that which makes clear, or pure, alkali.
“The ancients made use of alkali mingled with oil, instead of soap (Job 9:30), and also in smelting metals, to make them melt and flow more readily and purely” (Heinrich Friedrich Wilhelm Gesenius).
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