The Chemistry of Soap Making

Following on from the success of last week's blog, on the history of soap making, we thought we'd follow-up with an overview of the chemistry involved in the soap making process. 

You can read last weeks blog about the history of soap making here.

We'd love for you to share this little lesson with your children, grandchildren, any scientists of tomorrow - or anyone else who might be interested in learning something new, or finding a little escape during lock-down. 

So, just how is soap formed?

The Chemistry of Soap Making | Cosy Cottage Soap

Our founder and chief soap maker, Clara, is a fully trained chemist from Imperial College, London and an Associate of the Royal College of Science. She most definitely knows her stuff where the science of soap-making is concerned! All this experience has been invaluable in setting up Cosy Cottage Soap.

Creating an ethical skincare range and developing new products from scratch requires a lot more scientific knowledge than may first come to mind!  Using this knowledge and experience, Clara experiments to develop brand new products such as our best selling natural deodorant and solid shampoo

The chemistry of soap making | Cosy Cottage Soap

Those who have been to our soap making afternoons and demo days have all been amazed by the complexity involved in the process of making soaps from scratch.

Soap is the product of a chemical reaction between white, alkaline crystals called lye (also known by the chemical name sodium hydroxide) with triglycerides (the chemical name for oils, waxes and fats). The scientific word which describes this chemical reaction is saponification.

The oils, waxes and fats we use are almost identical to what you would find in your kitchen cupboard. Different combinations of oils, waxes and fats create different characteristics in the soap. We like to use coconut oil, extra virgin olive oil, castor oil and sometimes sweet almond oil, among others, in our products.  

The diagram below shows the process from start to finish. You can see on the right hand side, that in addition to soap, glycerin is formed as a by-product of saponification. This isn't a problem for us at all, as glycerin is a colourless, viscous substance, which stays within the soap and acts as a humectant (the scientific term for something which preserves moisture).  

The chemistry of soap making | Cosy Cottage Soap

So here's the method we use in the workshop to make saponification happen in real life. 

We begin by melting and mixing our oils, waxes and fats in exactly the right quantities. To make soap which is safe to use, different combinations of oils, waxes and fats need different quantities of lye. Very careful calculation of the right quantities of each needs to be done in advance.

We then very accurately weigh our lye (or sodium hydroxide) and mix it with just enough water to make it dissolve. As the lye dissolves, the sodium hydroxide molecules split apart (into sodium ions and hydroxide ions), energy is released and as a result, the mixture heats up. The scientific name for this type of heat generation is an exothermic reaction.

Lye is a very alkaline substance and both the solid and dissolved forms can cause very serious burns if they come into contact with skin.  We always make sure we are wearing, gloves, face masks and safety glasses throughout the whole process. 

We then slowly add the hot lye solution to our melted oils, waxes and fats, then mix thoroughly. After several minutes of vigorous mixing, the mixture becomes thicker and paler, as soap and the by-product glycerin, start to form. When the mixture reaches the right thickness (traditionally called the point of trace), we add essential oil fragrances and pour it into moulds to set. 

Soap Making Process

After 24 hours, the solidified soap in the moulds is ready to cut into bars. The soap isn't ready just yet, though as it's still very alkaline! If we were to use the soap at this stage we'd burn our skin very badly!

We need to leave it to cure, during which time it dries out and becomes less alkaline. This curing process takes around 2-3 weeks, by the end of which, the soap is a lot more skin-friendly. The finished soap remains very slightly alkaline which helps to discourage the growth of bacteria, fungi and other microorganisms. Great for hygiene, great for our skin and great chemistry!

You learn something every day, hey! 


1 comment


  • Lauren

    Thank you for sharing the chemistry of soap. I’m going to share it with my 4 children that are being home schooled at the moment. Two of whom are dyslexic, and struggle with English and literacy. But science is one of their strengths. I’m hoping seeing the practical application of science in this way will really inspire them. So thank you. X


Leave a comment

Please note, comments must be approved before they are published