Soap is probably the oldest skin cleanser. For thousands of years, this product has been obtained from the saponification of oils and fats by alkali.

As is true for most human discoveries, the first chemical saponification was found by chance. According to a Roman legend, SOAP was discovered accidentally near Mount Sapo, an ancient location for animal sacrifices not far from Rome.

The animal fat, mixed with wood ashes (the ancient source of alkali) and rainwater, created an extraordinary soap mixture. Roman housewives noticed that the strange yellow mixture of
the Tiber’s waters made their clothes cleaner and brighter than ordinary water.

It is surprising that in spite of technical achievements and scientific developments, classic soap is still based on the same chemistry as that of ancient soaps.

Soap is chemically defined as the alkali salt of fatty acids. In general parlance, the term “soap” has taken on a more functional definition, by which any cleansing agent, regardless of its chemistry, is considered a Soap.

source : science direct