Soap

(Jer. 2:22; Mal. 3:2; Heb. 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 (R.V., "throughly;" marg., "as with
lye") is lit. "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" (Gesenius).