Curious Questions: How does soap work?

We've been using soap for thousands of years, as Martin Fone finds out. But how does it actually work?

Blessed, or cursed, with an enquiring mind, I find these days I have even more time on my hands to ponder those everyday things that, somehow, I have gone through life taking for granted.

Take soap. Following the right scientific advice at the right time, I have been washing my hands even more assiduously than usual. This means taking hold of a bar of soap that has been lying by the side of the sink, wetting it until a decent lather has been generated and then applying it vigorously to my skin. But how does it work?

I suppose I have always contented myself with the knowledge that it did the trick. After all, it has a long legacy. The first written record of soap, used to cleanse wool rather than for personal hygiene, appears on a Sumerian clay tablet from around 2500 BCE. Another tablet from 2200 BCE details a recipe for making soap, using water, alkali and cassia oil, a process not dissimilar to that used today. An almost complete soap factory was found in the ruins of Pompeii, destroyed in the volcanic eruption of 79CE, and the medical writer, Galen, a century later prescribed washing with soap as a way of removing impurities from the body as well as clothing.

The Romans were past masters at assimilating the best ideas of their subjects and, if Pliny the Elder is to be believed (Naturalis Historia (28:51)), they acquired the habit of using soap from the Gauls, who had something called sapo, made from goat’s tallow and beechwood ash. It was used to make their hair shiny, more favoured, he noted, by the men than the womenfolk. Our word for soap almost certainly originates from the Gaulish sapo or the Germanic saipa.

“The ready availability of alkali which was both cheap and consistent in quality allowed soap manufacturers to clean up”

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In Britain soap was initially made from rendered animal fat but by the middle ages more flavoursome ingredients such as oils from palm, coconut, olive, and linseed were used. As they were not indigenous to our green and pleasant land and had to be imported, soap production was concentrated around the major ports. It can be traced in Bristol back to at least the 12th century; records of the Bristol Company of Soap Makers from the mid-16th century detail the names of 180 individuals engaged in the trade. They produced a soft, black soap known as ‘Bristol soap’ and a harder version, ‘Bristol grey soap’.

London’s soapmaking industry was certainly in full swing during the 16th century and by 1700, there were 63 soap companies in operation. However, their products were phenomenally expensive, due in part to the reliance on costly imported alkali but also to a heavy excise duty imposed on the product in 1688. At one time the pans used to boil up the soap had to be fitted by law with a padlock. When it was time to make another batch, soap makers had to summon the local taxman, who would unlock the pan as he alone possessed the key. They also needed to give him a minimum of 12 hours’ notice. Red tape is no modern phenomenon. The soap tax was only abolished in 1852 by the then Chancellor, William Gladstone, at a reported cost to the Exchequer of £1.126m.

Nicholas Leblanc’s discovery of how to create a form of alkali known as lye from common salt in 1791 liberated soap makers from their dependence upon imports. Ernest Solvay simplified the process in 1861, manufacturing soda ash from brine and limestone. Improved upon and patented by him in 1872, it spurred the establishment in Britain and elsewhere of factories using the so-called Solway process to produce alkali. The ready availability of alkali which was both cheap and consistent in quality allowed soap manufacturers to clean up: the market boomed as they were able to offer their product to a wider audience.

One of the most well-known British soap manufacturers was A&F Pears Ltd, whose success in part was due to Thomas Barratt’s adoption of what could be termed as modern advertising techniques. Promoted as an aid to health and beauty, endorsed by the likes of Lillie Langtry, and using the painting, Bubbles, by Sir John Everett Millais on its posters, Pears Transparent Soap became a market leader, both at home and abroad.

In 1897 they introduced the Pears’ Shilling Cyclopaedia, the last edition of which appeared in 2017, to inform as well as to introduce their products subtly to the impressionable minds of children. I remember as a child spending many a contented hour browsing through its pages.

But how does soap work? The French chemist, Michel Eugene Chevreul, showed in 1811 the relationship between fats, glycerine and fatty acids and revealed that making soap was a chemical process that split fat and lye into soap and glycerine, what he called ‘the sweet principle of fat’.

“As the chemical bonds that hold the virus together are not very strong, the soap molecules pull it apart”

One end of the molecules in a bar of soap is attracted to water and repelled by fats and proteins, while the reverse is true of the other end. In simple terms, the soap is attracted to grease through its fat-loving side, but then surrounds the particles, pulling them apart with its water-loving side, such that they are moved away from each other, weakened and, eventually, washed away. Simple.

Of course, in these difficult times, do you, when picking up a bar of soap, unwittingly transfer germs that are present on the bar to your face and hands? It seems that these fears are ill-founded. Rather like the way it reacts to oil or grease, the side of soap’s molecule attracted to fat buries its way into the fat and protein shell of the virus. As the chemical bonds that hold the virus together are not very strong, the soap molecules pull it apart, making it soluble in water. Washing your hands to produce a good lather — twenty seconds should do it — is enough to destroy any germs or viruses lurking around. As an extra precaution, perhaps rinse the bar before you use it and store it on a dish.

As I have been pounding the keyboard for a while, I had better wash my hands. With soap, of course.