Spring Cleaning: The History of Soap and the Leblanc Process

This ash, of course, was potash (literally meaning ashes from a pot), a mixture of potassium-containing salts, primarily potassium chloride. The historical prevalence of this substance is actually the reason why the element is called potassium, and even the source of the elemental symbol K, which references its Latin name kalium. This comes from the familiar term of alkali, which itself actually derives from the Arabic word القَلْيَه (al-qalyah), meaning “plant-ashes”. Yeah, it was that big of a deal.

This potash (or in general, any alkaline solution) reacts with fatty acids to form the amphiphilic molecules we know and love in a process known as saponification. For the ancient Sumerian priests, these came from the greasy fats on their skin themselves (which wasn’t horribly effective, you might imagine). For the modern soap maker, these amphiphiles are made in advance from animal or vegetable oils, and usually mixed with fragrance and colours to make the users of said soap to feel special.

Over time, the potash used in the production of soap became an increasingly valuable commodity. It was also used in several other industries, most notably the production of paper and many textiles. For a long time, the source of this was potash from trees (or, more specifically, the ashes of wood from trees), but strangely enough the excessive use of trees began to become a problem over the course of history, with deforestation running rampant from every industry under the sun chopping down trees like they were going out of style. Very soon trees were going out of style, becoming a resource to be rationed. Alternative sources of resources were needed.

By 1783, the current king of France, Louis the XVI, addressed the issue with a heavy reward for the person who could produce a suitable alternative for potash from readily available sodium chloride. The reward was 2400 french livres (like the book, only the currency was feminine and the book was masculine), approximately worth $630 000 today. Eight years later, in 1791, French chemist Nicholas Leblanc developed the Leblanc Process, and should have been a very rich man, were it not for the French Revolution causing more worries for France than the lack of soap.

The Leblanc process was incredibly successful, and variations of the process were used for the production of sodium carbonate for over eighty years, until Belgian chemist Ernest Solvay developed the (again eponymous) Solvay process in 1863. However, despite the process’ success, Leblanc himself fell upon incredibly hard times. During the French Revolution, he lost his soda ash production plant to the new government, went horridly in debt, and never received his promised reward. Even when he was given the plant back by Napoleon in 1802, he still was horridly in debt and prize-less, and unable to run it, causing him to fall into depression before eventually committing suicide in 1806.

The Leblanc process starts with the already known reaction for converting salt (and sulphuric acid) into sodium sulphate:

This sodium sulphate is then reduced via the oxidation of coal (carbon) into carbon dioxide, producing sodium sulphide:

This sulphide is then reacted with a calcium carbonate (usually limestone or chalk), resulting in a substitution of the salts’ anions to form the desired sodium carbonate, and the by-product of calcium sulphide.

The two are then separated via dissolution of the products into water, as calcium sulphide is far less water soluble than sodium carbonate, a process known as lixivation. Evaporating the product dissolved in the water will yield the desired sodium carbonate.