Soap might seem like a modern invention but it can actually be traced to ancient Babylon 4,800 years ago. Back then, people made soap by boiling animal fats and plant ashes. While the scale and technology involved in soap making have evolved, the main ingredients and methods have not.
Known as saponification, the process of making soap always involves heating fats or oil and an alkaline substance such as sodium hydroxide or lye. The reaction causes the ingredients to hydrolyse into glycerol and carboxylate salts (soap). A fragrance, either from essential oils or synthetic sources, is also added.
Continue reading to learn more about the chemistry of soap making and the basics of saponification. We also include step-by-step instructions on how to make your own soap at home.
In this post:
Introduction to soap making
Humans have been making soap for thousands of years using the same basic chemical reactions. Essentially it involves boiling oil/fat and an alkaline substance together. Modern soaps may also contain fragrances, and fruit and medicinal plant extracts. The mixture is stirred continuously until a gel-like consistency is achieved. It’s then allowed to cool and set in moulds to take its final form.
Soap making in England began around the twelfth century, with Bristol at the centre of soap production during that era. Bristol soap makers were using olive oil imported from Spain by around the mid-1500s, and this became a common ingredient in soaps of the time.
Soap making continued to evolve during the seventeenth and eighteenth centuries, with the Industrial Revolution helping to advance production and improve economies of scale. The industry flourished further when Gladstone repealed the British tax on soap in 1853.
Hygiene wasn’t great during the Middle Ages. It only became fashionable in the 1700s and, even then, soap was mainly used by the wealthy class who could afford it. However, by the end of the nineteenth century, bathing had become much more widespread as people began to realise the importance of personal hygiene. Today, soap is not only for cleaning but also for therapeutic purposes.
Basics of saponification
In chemistry, saponification is the process of converting esters into soaps using an alkaline substance such as sodium hydroxide. The chemical reaction between the fat and the alkali forms glycerol and a fatty acid salt (soap).
However, saponification doesn’t just happen when making soaps; it can also occur naturally, such as in the case of Mrs Ellenbogen. Known as the Soap Lady, Mrs Ellenbogen’s dead body was famously preserved when the saponification process naturally turned her fatty tissue into adipocere, an organic material with a soapy, wax-like texture.
Soap molecules have both hydrophilic and hydrophobic components. The ionic metal atoms of sodium or potassium serve as the hydrophilic head of a soap molecule. They can remove polar molecules of dirt on a surface like our skin. Meanwhile, the oily component of soap molecules forms a nonpolar glyceride tail. This is the hydrophobic component, which is capable of removing nonpolar molecules of dirt or stains.
How is soap made?
Today, soaps can either be made locally on a small artisan scale or mass-produced industrially. In either case, the basic principles remain the same. Industrially manufactured soaps are usually oval or rectangular, whereas artisan soaps typically come in more creative and complex shapes.
Step 1- Measure the main ingredients
Start by carefully measuring your chosen ingredients. Many recipes use 40% lye (sodium hydroxide) concentration by volume dissolved in water, although feel free to experiment. The proportion of oil with the lye solution may vary depending on the type of oil you’re using. For example, coconut oil can be as high as 33% of the lye solution-oil mixture, whereas just 5% grapeseed oil is recommended.
Step 2 – Cook the mixture
After measuring and mixing the lye solution and oil, boil the mixture and continue stirring until it reaches a certain consistency. This is the most crucial and dangerous step. Lye is a corrosive substance that can cause serious injuries if it comes into contact with the skin, so be sure to take the necessary safety precautions.
Step 3 – Remove excess lye
It may take a few hours or as much as two days of boiling and stirring before the alkali molecularly binds with the oil. Even if you have very precise measurements using molar computations, there’s likely to be some excess alkali that doesn’t bind with the oil. Once you reach the saturation point, you’ll need to remove any excess alkali to ensure the soap doesn’t irritate or dry the skin.
Step 4 – Add other ingredients and stir
Once the mixture reaches the desired saponification level, you can then add any optional ingredients. These might include essential oils, fragrances, colours, or small abrasive particles. Continue stirring the mixture until it is homogeneous.
Step 5 – Pour into moulds
You can begin pouring the mixture into moulds while it’s still hot. You can use any shape of mould you want. These can either be elaborate silicone moulds or even a simple wooden box (if you decide to use the latter, make sure to line it with parchment paper!).
Step 6 – Allow the mixture to cool
Allow the mixture to cool and set for a few hours. You may need to wait a bit longer if you’re using silicone moulds because the air won’t come into contact with the soap mixture.
Step 7 – Remove soap and trim excess parts
Remove the soaps from the moulds and trim any excess or deformed parts to enhance the appearance. Your soaps are now ready to be used.
Step 8 – Pack the soaps
This step is optional if your soaps are for personal use. However, if you’re intending to sell the soap, you’ll need to package it. Not only will this make it look more presentable but it also ensures the soaps are protected during transit.
Chemicals in soap
Chemically speaking, soaps are classified as salts of fatty acids because of the presence of an ionic, or polar, head, and a nonpolar glyceride tail. All soaps contain two main chemical ingredients – oil and alkali – although the exact type will vary depending on the product.
Some soaps are made using palm oil, coconut oil or olive oil, whereas others contain tallows from beef or mutton fats. You can also use potassium hydroxide as an alkali instead of sodium hydroxide. Potassium soaps have a putty-like, almost liquid texture, which is perfect for making whipped soap.
Other chemicals such as fragrances, essential oils, colouring agents, abrasive particles, enzymes, and moisturising cream can be added too. Some artisan soap makers even include fruit and vegetable extracts in their products.
Liquid soaps contain slightly different ingredients because they need to maintain their liquid state at room temperature. Some of the common ingredients in liquid soaps include:
- Sodium laureth sulphate
- Sodium benzoate and benzoic acid
- Cocamidopropyl betaine
- Methylisothiazolinone and methylchloroisothiazolinone
- pH adjusters
- Fragrance, either from essential oils of flowers or from synthetic sources
Harmful chemicals in soapSodium hydroxide – the alkaline substance used to make most soaps – is corrosive and toxic in its pure form. However, it’s neutralised during the soap making process and then becomes safe to use. That being said, some soaps can still irritate people with sensitive skin.
The suds and surfactants that are produced when soap is dissolved in water can also be harmful to the environment. These chemicals can change the pH of the soil and potentially kill essential microorganisms.
Here in the UK, the Health and Safety Executive is the main government agency that regulates and enforces standards on handling and storing hazardous chemicals. For instance, UK manufacturers are required by law to store corrosive substances on separate shelves to prevent them from reacting with other chemicals.
Refer to our article ‘How to manage chemical hazards’ for more information.
Sustainability and eco-friendly practices in soap making
For thousands of years, soap makers have used alkaline substances derived from wood ashes. Although these ingredients are still found in some artisan soaps, large-scale soap manufacturers now use sodium hydroxide or potassium hydroxide. Unfortunately, both of these ingredients can have a detrimental impact on the environment. For example, burning wood to create wood ashes contributes to greenhouse gas emissions. On the other hand, if you use synthetic ingredients, the overall proportional carbon footprint is lower but it would be nominally larger because of mass production.
Soap packaging also contributes to solid waste and other types of pollution. This is a particular problem when it comes to liquid soaps, which are usually sold in plastic bottles. In an effort to become more environmentally friendly, some manufacturers now sell eco refills so you can pour your liquid soap into the existing container. Many artisan soap makers also package their products in eco-friendly kraft boxes or biodegradable paper wraps.
In addition to packaging changes, technological innovations such as carbon capture technology are being introduced to help reduce the environmental impact of soap manufacturing. However, it’s still early days and industrial soap making remains a significant source of pollution.
Soap making is almost as old as civilization itself, and while technology has evolved, the process still follows the same basic principles. To make soap, you need to heat fats or oil and an alkaline substance such as sodium hydroxide. The chemical reaction causes the fat and alkali to hydrolyse into glycerol and carboxylate salts (soap). The chemical ingredients and reactions necessary for soap making contribute to pollution, although efforts are now being made to find more eco-friendly practices.