SOAP (Heb. bōrîth). Soap in a modern sense was unknown in OT times. Even until recent time it was not used in some parts of Syria. Clothes, cooking utensils, and even the body were cleansed with the ashes of certain plants containing alkali (e.g., soapwort, glasswort, and saltwort). This cleansing material is referred to in Jer.2.22 and Mal.3.2.
International Standard Bible Encyclopedia (1915)
Borith is a derivative of bor, "purity," hence, something which cleanses or makes pure. Soap in the modern sense, as referring to a salt of a fatty acid, for example, that produced by treating olive oil with caustic soda, was probably unknown in Jer 2:22 (compare Septuagint at the place, where borith is rendered by poia = "grass") and Mal 3:2 was probably the vegetable lye called in Arabic el qali (the origin of English alkali). This material, which is a mixture of crude sodium and potassium carbonates, is sold in the market in the form of grayish lumps. It is produced by burning the desert plants and adding enough water to the ashes to agglomerate them. Before the discovery of Leblanc’s process large quantities of qali were exported from Syria to Europe.
For washing clothes the women sprinkle the powdered qali over the wet garments and then place them on a flat stone and pound them with a wooden paddle. For washing the body, oil is first smeared over the skin and then qali rubbed on and the whole slimy mixture rinsed off with water. Qali was also used in ancient times as a flux in refining precious metals (compare Mal 3:2). At the present time many Syrian soap-makers prefer the qali to the imported caustic soda for soap-making.
In Susanna (verse 17) is a curious reference to "washing balls" (smegmata).
James A. Patch