Soap

Soap is made by boiling or mixing fat with a strong alkali, which splits the fat into glycerol and the salts of fatty acids that lift grease and dirt into water. Recipes for soap-like cleaners go back to Mesopotamia in the third millennium BCE; cheap industrial alkali made soap an everyday product in the nineteenth century.

What it is

Soap is a cleaning substance made from fats or oils and an alkali. Each soap molecule has one end that holds on to water and a long tail that holds on to oil, so it can carry grease, dirt and many microbes off the skin and into the rinsing water. Soap made with sodium hydroxide sets into hard bars; potassium soaps stay soft or liquid. The lye that farmers leached from wood ash held potassium carbonate rather than a modern hydroxide, and gave a soft soap.

The plate shows soapmaking as it was done on farms in Europe and North America until the nineteenth century: water poured through wood ash to make lye, a crock to catch it, rendered tallow, a kettle boiling over the fire, and a mould with cut bars drying.

How it works

Fats and oils are triglycerides: three fatty acid chains joined to one molecule of glycerol. A strong alkali breaks those joints. Each fatty acid takes the sodium or potassium of the alkali and becomes a soap molecule, and the glycerol is set free. Chemists call the reaction saponification.

A micelle: the soap’s oily tails hold the grease inside, and their water-loving heads let the rinse carry it away.

In water, soap molecules gather into tiny spheres called micelles, with their oily tails inside and their water-loving heads outside. A drop of grease is drawn into the core and rinsed away with it. The same tails can wedge into the fatty envelope of some bacteria and viruses, including coronaviruses, and pry it apart. That is why washing with soap and water removes far more germs than water alone, although microbes with tougher coats resist it better.

On a farm, the alkali came from ashes. Water poured through a barrel of hardwood ash dissolved potassium salts, mainly potassium carbonate, and ran out as a brown lye. Boiled with tallow for six to eight hours, it gave a soft, jelly-like soap that was kept in a barrel and ladled out as needed. Common salt stirred in at the end of the boil made a firmer soap rise as a cake that could be cut into bars, but salt was dear and many farm families did without. Truly hard soda soap needed soda, which came from the ashes of seaweed and coastal plants until Nicolas Leblanc found how to make it from common salt.

How to build it

Small batches of soap are made today by the cold process, with sodium hydroxide bought for soapmaking and a soap calculator. Sodium hydroxide is corrosive and can blind in seconds, so it is a job for adults only, in a calm, uncluttered room.

Gather the kit. You need safety goggles, chemical-resistant gloves and long sleeves, a digital scale reading to the gram, containers of stainless steel or of polypropylene sold as safe for hot lye (never aluminium, which reacts with it), a stick blender, a thermometer, a silicone or lined mould, and pure olive oil or a tested mix of oils.

Calculate. Enter the exact weight of each oil into a saponification calculator with a fat margin of about 5%. For 500 g of olive oil it gives about 64 g of sodium hydroxide and about 170 g of water. Check the result twice: too much alkali makes a soap that burns the skin.

Make the lye. Outdoors or by an open window, weigh the water first, then pour the sodium hydroxide slowly into the water while stirring, never the water onto the alkali. The solution heats up quickly and gives off fumes for a moment; do not breathe them. Let it cool to about 40 °C.

Mix and pour. Warm the oil to a similar temperature, pour in the lye and blend in short bursts until the mixture thickens like thin custard. Pour it into the mould, cover it and leave it for one to two days.

Cure. Unmould after one to two days and cut into bars. A correctly measured batch has finished reacting by then; four to six weeks of curing on edge in a dry, airy place, longer for pure olive soap, let water evaporate so the bars harden and last. Curing cannot rescue a batch made with too much alkali: if a cured bar still stings the skin, throw the batch away.

Safety. Always wear goggles and gloves while handling lye or fresh soap, and keep children and pets out of the room. If lye touches the skin, rinse at once with plenty of running water; if it reaches the eyes, rinse with running water for at least 30 minutes while someone calls emergency services. Label every container, store sodium hydroxide locked away in its original packaging, and never use kitchen utensils again for food once they have held lye. Handle fresh soap with gloves for the first days.

History and evolution

Before soap was common, soapwort was crushed in water to give a mild lather for washing wool and linen.

Soap-like cleaners are old, but the early evidence is thin. Babylonian sources of about 2800 BCE are often cited as the first record, and later accounts describe Mesopotamian recipes of the third millennium BCE that heated oil with wood ash to wash wool, although the tablets themselves are rarely identified. The Egyptian Ebers papyrus, around 1550 BCE, prescribes animal fats or vegetable oils mixed with an alkaline salt as a medicine.

Roman writers treated soap as a foreign product. Pliny the Elder, in the first century CE, credited it to the Gauls, who made it from tallow and ashes and used it to tint their hair; his is the first known use of the word sapo. A century later the physician Galen described soap made with lye and advised washing with it.

Aleppo soap, made with olive and laurel oil, is cut into cubes and stacked in airy towers to cure for months.

Hard soaps of olive oil came from the eastern Mediterranean. The Persian physician al-Razi wrote down soap recipes around 900, and by the thirteenth century soapmaking was a major trade in the cities of Syria and Palestine. In Europe, workshops in Castile and Marseille were turning out olive-oil soap on a large scale by the fifteenth century, while northern soapmakers boiled tallow with ash lye.

Soap became cheap only when its alkali did. In 1791 Nicolas Leblanc patented a process that made soda from salt, sulphuric acid, coal and limestone, so soapmakers no longer depended on plant ash. In 1823 Michel-Eugène Chevreul showed that fats are compounds of glycerol and fatty acids, which explained what happens in the kettle. Factories such as Lever Brothers, founded in 1885 at Warrington, sold soap in wrapped, branded bars. Hygiene followed slowly: in 1847 Ignaz Semmelweis made the staff of his Vienna maternity clinic disinfect their hands with chlorinated lime, and deaths from childbed fever there fell from about 18% to about 1% the next year.