Humans have used soap since time immemorial. Yet bathing was not always a high priority, not even at the elegant court of Louis XIV, where noblemen relied largely on using perfume. “I am coming home, do not wash,” wrote Napoleon to his wife Josephine, concerned that she would wash away her pheromones. John Wesley preached to no avail that cleanliness was next to godliness, and Ignác Semmelweis was given a bad time for wanting obstetricians to wash their hands between deliveries.

Yet already the ancient Indians used soap. As early as 2800 BC the Sumerians and Babylonians made soap by mixing animal fat with wood ash and water. According to the Ebers Papyrus, the Egyptians bathed regularly using a combination of animal oil and ashes. The Phoenicians made soap from tree ash and animal fat by 600 BC, and Roman women washed their clothes with it. Pliny the Elder described in his Naturalis Historia of AD 77 that the Gauls made soap from tallow and ashes to give their hair a reddish tint; Galen (AD 130–200) in Rome recommended it for personal hygiene.

Bathing habits declined after the fall of the Roman Empire, but soap-making continued in the Middle East, where the Arabs produced harder and more fragrant soaps by using olive oil along with alkali derived from plant ashes. The knowledge of these advances made its way back to Europe during the Crusades, and by the eleventh century, soap making revived as an art in Castile, Italy, and France, olive oil being used in particular to make pure white soap. In Britain, soap making began around the 13th century and became popular, especially among royal houses, leading to the destruction of large areas of British woodlands. As soap became a luxury, it was heavily taxed, but in Britain it became affordable when William Gladstone abolished the tax on it.

Soap is made by a process called saponification, in which an alkaline substance is used to treat fats or oils. The reaction breaks down the triglycerides into glycerol and soap molecules (salts of fatty acids). Sodium hydroxide is used for making solid soap, potassium hydroxide for liquid soap, and a combination of both for certain cream soaps. Saponification disperses one liquid into another with which it normally does not mix, thus surrounding the fat molecules with water, making it easier to rinse them away and making soap able to remove pathogens from the skin and other surfaces.

At first, soap was expected to provide only cleansing benefits, especially when it was a merely a foul-smelling crude product. In time it went through an evolutionary change, with various ingredients added to make it competitive, cosmetically acceptable, and even supposedly to confer health. Some soaps incorporate milder surfactants to reduce irritation and emollients such as oils, glycerin, and petrolatum to increase moisturization. Soap may be harsh or soft and may have added to it perfumes, fruit flavoring, surfactants, or skin-conditioning agents. There are many kinds of soap, with manufacturers competing to offer a variety of different products.

P.S.: Not related to physical soap are SOAP notes, invented by Dr. Lawrence Weed in the early seventies and presented as an exciting new rational system for doctors to document and structure their findings. It proved to be impractical and never caught on, except for the enticing acronym SOAP, meaning Subjective, Objective, Assessment and Plan, a classic example of what looks good in theory but does not work in practice.