Soap has a fairly simple formula and a long history. But for centuries, water was the primary means of bathing. For example, in the Indus Valley Civilization—a culture that flourished in parts of what is now Pakistan, India, and Afghanistan from 2600 to 1900 BCE—the Great Bath at Mohenjo-Daro is thought to be one of the earliest public steam baths. But steam only lasts so long.
“Before soap became a common personal hygiene product, there were a lot of people who smelled really bad,” Judith Ridner, a historian at Mississippi State University who writes about material culture, told Live Science.
While modern soap formulas contain many additional ingredients, the base soap is a fairly simple mixture. It’s a fatty acid salt, which is a combination of lye — a water-soluble compound — and fat, Kristin Konkol, a chemist at the University of Albany, told Live Science. As a chemical compound, a soap molecule has a water-loving head and a fat- and oil-loving tail of a carbon chain that surrounds and lifts up dirt molecules, Konkol explained.
This basic formula was the key to pre-modern soaps in ancient civilizations. Plants, animal bile, oils, and exfoliants such as sand and wood ash were the main ingredients in these early cleansers.
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Ancient forms of soap are difficult for historians to track down because of one major barrier: “Soap degrades,” said Seth Rasmussen, a chemical historian at North Dakota State University. “We can use chemical dating and archaeology, but that requires samples to be preserved from the time they were first produced to the present.”
The earliest written records of soap-like substances date back to around 2500 BCE in Mesopotamia. Clay tablets indicate that the Sumerians used water and sodium carbonate — a powdered salt, such as from plant ashes — for cleansing, and beer and hot water to wash wounds.
A couple hundred years later, the Akkadian Empire in the Mesopotamian region used a mixture of plants like the date palm, pine cone, and a shrub called tamarisk. This mixture corresponds to the basic ingredients of modern soap: an alkali, like tamarisk; an oil, like date palm; and an abrasive, like pine cone, Konkol and Rasmussen explained in their article on soap in antiquity.
In fact, “there’s really not much difference between modern soap and old-fashioned soap,” Ridner said.
How is this possible if modern science didn’t exist in ancient times? People probably created soap unintentionally, Konkol said. Scrubbing a greasy pan with plant ash at high temperatures would produce soap, for example, as would simmering animal fats with wood ash. Historians have traced these methods back to Babylon and ancient Egypt. Natron (a type of salt), clay, and talc are other ingredients that have been documented in Egyptian remains, perhaps as part of their washings — or, more macabrely, as a chemical result of decomposing bodies, Rasmussen said.
Diagram showing how surfactants (blue structures) remove dirt (brown stains) from the skin’s surface. pOrbital.com
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The ancient Greeks and Romans took a slightly different approach to bathing. After rinsing with water, they lathered themselves with scented olive oils. Then they used a curved tool called a strigil to scrape away any remaining dirt. But this may have been less a cleaning technique than a disguise, Rasmussen said.
“Often the oils contained plant extracts that were fragrant,” he said. “So during those periods of time, the oils were used as perfumes.”
Most of these soap mixtures were used to clean textiles, not human bodies. “It was more of an industrial process than a matter of cleanliness,” Rasmussen said.
Historians aren’t sure exactly when bathing with soap became more common, but in the Western world it happened much later — most likely in the early to mid-1800s, Ridner estimates.
“It’s a confluence of factors that are causing this,” she said. First, inexpensive fats became more widely available, Ridner said. Then, the Industrial Revolution shifted soap production from households to factories. Urban engineers and reformers also turned to “cleaning up” immigrant communities, which also contributed to the shift. Plus, the Civil War and Crimean War put a greater emphasis on sterilization in hospitals and health care.
“All of these factors combined create a mass market for soap that companies like Procter & Gamble in the U.S. are beginning to exploit,” Ridner said.
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