Friday, August 6, 1999 Published at 15:58 GMT 16:58 UK
Clean and easy
We take soap for granted
By Pauline Newman of BBC Science
You take the greasy animal fat from the bottom of a cooking pan. You add the grimy wood ash from the fire below. You mix them together and use the resulting goo to clean yourself.
This is the essence of soap. OK, so modern soaps are a little more sophisticated, but this is essentially how our forebears would have made it. If you boil and cool the mixture, successively, over several days, it will eventually go quite hard.
Chemist John Emsley from Cambridge University says it is quite easy to understand how soap might have been invented.
"You can see how if someone was cooking something on a wood fire and it had fat in it and it boiled over and the fat dropped onto the wood-ash then the following morning, when they went to clean up the mess, they would find that when they put water on it that it would lather and it would clean very well. That was just a simple type of soap."
The Pharaohs used soap nearly 3,000 years ago, and it worked for them in exactly the same way as it works for us today. The secret of its success lies in what are called amphiphilic molecules.
"Soap molecules have a head and a tail," says John Emsley. "The head likes to attach itself to water; the tail likes to attach itself to grease. So when soap is put into water, it will find the grease, it will attach itself to the grease, and it will pull it into the water and then it can be washed away."
But somehow, this miraculous property was forgotten, at least in some parts of the world. Instead, medical men from Ancient Greece and Arab countries used it as a skin balm, as did the Romans.
"We know that the ancient Romans used soap because it's mentioned in several of their books," says Nina Hall. "But they didn't use it to wash their bodies, but instead used it as an ointment to treat skin diseases. And in fact, a soap making factory was discovered in the ruins of Ancient Roman Pompeii, which was destroyed by a volcano in AD 79."
"There were great pots at the street corners where people could add their own contributions to this," says John Emsley. "The urine was left for several weeks until it actually decomposed and gave ammonia, and ammonia is a very good cleaning agent. It will remove grease and dirt from fabrics."
After the Romans, soap making, as an industry, almost ceased to exist for hundreds of years. But there was an alternative to boiling up smelly fats and ashes.
"One of the simplest ways that people around the world have found to get clean is to wash using a plant," says Pat Griggs of the Royal Botanic Gardens at Kew, near London. "Plants are absolutely amazing for this and there are many species around the world that contain a substance called a saponin which works in a similar way to a soap."
Over 100 different types of plants make saponin and sometimes its found in the leaves of a plant, sometimes in the stem or roots and sometimes in the berries or other fruits.
"What's happening when we use one of these plants is that the saponins make a foam when mixed with water, and this lifts off dirt and grease," says Griggs.
"It works just like an ordinary soap and it's often very good for washing your hair."
In North America, native people sometimes used the root of the Yucca plant. In India, a shrub called the soap nut bush became very popular. The nuts are crushed and mixed with water to make the soap.
The bars of soap we buy today contain lots of plant material, plant oil to make the soap itself and plant fragrance to scent it.
We are not sure when commercial soap making began again after Roman times, but by 1200 AD, soap was being made in Bristol in England, and 200 years later the finest soap was being made and exported from Spain. This soap was based on olive oil.
"All kinds of oils and fats can be used to make soap," says John Emsley. "Mutton fat, tallow from cattle, palm kernel oil, coconut oil, olive oil, palm oil - any fish oil can be used to make soap.
"If you want the soap to last a long time then you want to use a saturated fat. If you use an unsaturated fat then the soap will tend to go rancid after a certain length of time. Soaps made from saturated fats will last for years, they don't go bad."
Today's chemists boil the fat and oil, not with wood ash, but with sodium hydroxide, sometimes called caustic soda.
"The general principle is to boil up fats with caustic soda," says Nina Hall. "Water is added and they're boiled for hours and hours until we get the soap forming. And as well as soap forming, you get glycerine and the two have to be separated. The soap curdles, a bit like milk curdling, and it gathers at the top of the big pans in which it's made and then the glycerine and salts and water settle at the bottom."
Over eight million tonnes of soap are produced each year worldwide and the demand for soap is rising. 50 years ago, it would have all have been made from animal and plant fats, but in recent years, more and more synthetic soaps have been coming on the market, particularly for washing clothes.
These are made from petrochemicals. Manufacturers like the synthetic soaps because the raw materials, the petrochemicals, are of consistent quality. Consumers like them because they work more efficiently and do not produce the scum sometimes seen in hard water.