soap, n.1  A substance formed by the combination of certain oils and fats with alkaline bases, and used for washing or cleansing purposes. —Oxford English Dictionary.

Several articles in the press have noted that when the coronavirus is outside the human body, it is killed most effectively by soap, one of the oldest of human discoveries.  To explain why, however, demands extreme skill on the part of the writer. (see below)

There are fanciful explanations of how soap was discovered. But it seems to have happened in prehistory. Someone noticed how animal fat, especially tallow, when mixed with ashes, can produce in water a foam that has powerful cleansing properties—and, crucially, can be used on the human body. Shreds of evidence suggest that this discovery may have been made in more than one place—in Babylon, and perhaps in Egypt, and in Gaul according to Pliny the Elder.

The Romans and Greeks were unfamiliar with soap. They knew, and used, the discovery of pearl ash, which is really potassium carbonate, and lye—mostly sodium carbonate—from ashes, and they used them to alkalise water for the washing of clothes. Aristophanes uses the generic word κονία for such cleaning liquids. But soap—σάπων—does not seem to appear in classical Greek. The Latin sapo is first known to us in Martial (XIV.27) where he refers to it as  a hair dye for older women, imported from Germany.

What he describes is neatly confirmed by Pliny the Elder in his Natural History, Book XXVIII Chapter 51. After talking about possible cures for scrofula, he goes on to say:

prodest et sapo, Galliarum hoc inventum rutilandis capillis. fit ex sebo et cinere, optimus fagino et caprino, duobus modis, spissus ac liquidus, uterque apud Germanos maiore in usu viris quam feminis.

¶  Soap, too, is very useful for this purpose, an invention of the Gauls for giving a reddish tint to the hair. This substance is prepared from tallow and ashes, the best ashes for the purpose being those of the beech and hornbeam: there are two kinds of it, the hard soap and the liquid, both of them much used by the people of Germany, the men, in particular, more than the women. (Translation: John Bostock & H.T.Riley, Taylor & Francis 1855)

It seems that, like the skill of refining sugar, the making of soap for personal washing came to Europe from the Arab countries, and the knowledge of its virtues seems to have come from the golden age of Islamic science.

One of our students was asked at his successful University interview, “Did Shakespeare use soap?”—to which his answer was “Possibly not.”  Soap was certainly then to be found in this country, not least Castile soap, made in Spain from olive oil rather than animal fat; but it was a luxury, expensive, and the privilege of rich women. It was regarded as cosmetic until the mid 19th century, when Mr Gladstone removed the 50% tax on it.  It seems that in the wake of Louis Pasteur’s discoveries, (confirming the suspicions of the hapless Dr Ignaz Semmelweis, ridiculed and hounded for noticing that women in Hungary were dying in childbirth because doctors were not washing their hands) the realisation was dawning that personal hygiene was a necessity and not a luxury.

The history of soap production in this country is well documented. To study it is to wander into areas of social history, science, medicine, economics, human psychology, and of course the industrial revolution, among whose luminaries were the soap makers William Gossage  and Andrew Pears.

An article that appeared in The New York Times in March was written by someone who has mastered the art of how to explain a process of microbiology to a lay person: