How Soap is Made

Soap comes in all shapes, sizes and smells but its fundamental purpose remains the same today as it did 5,000 years ago - to clean! We all use it constantly, but when you’re washing your hands, have you ever stopped to think ‘how do they make soap?’ Read on to find out the answer to two of the most common questions of all - ‘how is soap manufactured’ and ‘how was soap made in the old days’.

Engineering How It’s Made
29 June 2022

Soap is one of the most ubiquitous items in the world. When we hear the word ‘soap’ we usually think of the liquid soap or bar soap sitting next to a sink, but soaps are also used in types of grease and thickening agents as well as detergents and even in the production of oil paint.

Soap is everywhere, however in this article we’ll be concentrating on domestic soap. So if you’ve ever asked ‘how is bar soap made’ you’re about to find out.

Coming Clean - A Short History of Soap

How was soap made in the old days? Soap was actually used for cleaning clothes as early as 2800 BC. Virtually all civilisations across the Middle East, the ancient Romans and Chinese through to medieval Europe, have made soap in broadly the same way, that is mixing fats and oils with alkali-based substances such as ash with the eventual addition of aromatic herbs and fragrances.

The ancient Babylonians (in modern-day Iraq) used a mixture of oil and wood ash – believed to be the world’s earliest chemical reaction – and in ancient Egypt, they used a combination of animal fat or vegetable oil with soda ash. The ancient Romans were famous for their soap. They used a variety of ingredients to make it, including animal fat, wood ash, and plant oils. The soap was usually made in the autumn, when there was an abundance of fat from slaughtered animals. The ruins of a soap factory were even found in the remains of Roman Pompeii.

Through the centuries the recipes and manufacturing techniques for producing soap continued to evolve and eventually became industrialised. Today, the biggest soap manufacturers make tens of millions of bars every year.

The Process of Making Soap

Bubbles on a bar of soap (Photo: Image Source via Getty Images)

Of course the process of how soap is made today has changed dramatically from the methods of the past. Like making chocolate or bubblegum, the exact recipes used by manufacturers of soap are closely-guarded trade secrets, and the shapes and sizes of bars of soap differ greatly – but here, we’ll tell you the general process of making soap.

First, olive oil (the same type of olive oil you use to cook with) is added to a giant steel drum known as a kettle. Next, a hydrogenated fat – known as vegetable shortening – is added to the kettle, and lastly lye is added. Each batch uses very precise quantities of each substance. Lye is a strong alkali solution called sodium hydroxide (more commonly known as caustic soda) and the process of how soap is made couldn’t happen without it.

The oils and lye are heated to boiling point and mixed together, and it’s here that the chemical reactions take place known as saponification. The mixture thickens and, as the fat reacts with the alkali, it produces two products – soap and glycerin. The soap rises to the top of the kettle and the glycerin sinks to the bottom, making them easy to separate.

Next, the liquid soap is poured into large block moulds to cool and set. We’ve almost answered the question ‘how is soap manufactured’ but there’s still a little more involved in the soap manufacturing process before it becomes the product you’d actually buy.

Going Through The Mill

A stack of soap (Photo: Luis Diaz Devesa via Getty Images)

Most soaps used at home undergo a process known as milling. Once the soap has cooled into solid blocks, it’s passed through heavy rollers that knead it – similar to the process of kneading dough – and it renders it very smooth. It’s at this point that fragrances can be added because volatile (and often very expensive) oils won’t evaporate in cold soap as they do in hot liquid soap.

Milled soap has a much finer consistency and produces more lather when it comes into contact with water than non-milled soap. ‘ this is it.

Once the milling is complete, and if we’re looking specifically at how bar soap is made, then the soap is pushed through shaping machines to be cut to size, stamped with the brand name and wrapped ready for the shelves.

How Liquid Soap and Shower Gel are Manufactured

Hand washing (Photo: Solskin via Getty Images)

Of course, nowadays soap takes many forms, including liquid hand soaps and shower gels. While the process that produces these products is broadly similar, there are differences from the manufacturing process used to make bar soap.

Liquid soap is made by combining an alkali with a fatty acid. The alkali can be either sodium hydroxide or potassium hydroxide, while the fatty acid can be derived from vegetable or animal oils. In the first step, the oil is mixed with the alkali and heated until it reacts to form a soap molecule. In the second step, water is added to the soap molecule and it is mixed with an emulsifier. This produces liquid soap.

Shower gel is another popular soap product that’s widely used by millions of people every day all over the world. It’s manufactured using a variety of different ingredients, depending on the brand and the desired properties of the end product. The main ingredients in shower gel are water, surfactants, and emulsifiers. The surfactants are responsible for the lather of the shower gel, while the emulsifiers help to keep the oil and water from separating. Fragrance and colour are also commonly added to shower gel.

So from bar soap, to shower gel and beyond, that’s your quick fire answer to the question ‘how is soap made?’


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