Learning to Make Soap: Part 2

Mar 25, 2021

Remembering I am a scientist

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Having successfully managed to make a couple of different recipes from the wonderful Lovely Greens, I wanted to start thinking about designing my own recipes for Small Kindness. But where to start? What oils and butters should I use? In what amounts?

Ideally I would have more experience with making soaps and using different types of ingredients, to fully understand the different properties they give to the soap. But time isn’t really on my side (or money) to try loads of different ingredients and recipes. So this is going to be the start of a journey, where the Small Kindness soaps will evolve as the business evolves. What I need to start with is a good bar of soap that I can build on and tweak as my knowledge and understanding expands.

Ok, where to begin? I broadly speaking understand how to design a soap recipe, have a very basic knowledge of the different oils and the properties they bring to the soap (although this area was still fairly overwhelming) and the saponification values for determining how much Sodium Hydroxide you need. What I was really struggling to get my head around was how much water I needed to add!

There really didn’t appear to be any clear explanation as to why people were using the amount of water they were in their recipes. Was there actually a correct amount to use? Then I stumbled upon this wonderful article about the ghost swirl on Auntie Clara’s Blog. 

The effect is breath taking. It is a soap with a swirled design that is achieved, not by the use of dyes or colours, but by using different amounts of water in the 3 soap batches to create the swirl. This happens because the amount of water used in each of the batches effects the saponification rate and the temperature that soap batch goes through gel phase. The soaping temperature used for this recipe (experiment really) was not high enough to push the low water soap through full gel phase, but was high enough for the high water soap – hence the difference in colour. I loved it – science and art working beautifully together. As I constantly like to point out to my two step kids ‘isn’t science great!’ And if this blog post hadn’t been joyous enough, she mentions a book by Kevin Dunn – Scientific Soapmaking: The chemistry of cold process. I felt that this book could be what I was looking for to help me distil the vast amount of information on soap making down to something clear – the science.


I wasn’t wrong, it is a great book, a must for all soap makers if you want to understand the Chemistry behind this wonderful craft. It is well pitched for non-scientists and for scientists who, like me, may have lost their way, gently reminding and guiding us back through the relevant basic chemistry. Although I am a PhD scientist who worked for 10 years at the bench, I left just over 9 years ago to work in Scientific Communications so I wasn’t thinking enough like a bench scientist whilst trying to design my soaps.

As well as a trip down Chemistry memory lane it is also describes a number of experiments that he and his researchers had performed to understand the effect of water content and temperature on the saponification process. Prof Dunn investigated the effect of water in the soap making process by preparing soaps with varying amounts (low water: 50% lye concentration, medium: 33.3% lye concentration and high: 25% lye concentration) and measuring the effect on bar moisture, hardness and alkalinity.

What it told me was that although ultimately over time the hardness of the soap will end up similar regardless of how much water was included, a low water bar will initially result in a harder bar (good for unmoulding) and a high water bar will be softer (good for cutting). It also highlighted that you can actually add too much water which could lead to separation of the mix and this is particularly relevant for soaps containing olive oil. I finally had the data to help me answer the question of how much water I need to include in my recipes.

As well as investigating the effect of water content, they also look at the effect of both temperature and time on the saponification reaction. Saponification is the chemical reaction between triglyceride and a strong alkali (for solid soaps this is NaOH) to produce glycerol and fatty acid salts (in other words soap). It is an exothermic reaction (releases heat) and some of this is absorbed by the soap, raising it’s temperature. They initially looked at the effect of water content on temperature and determined that a low water soap will saponify more quickly than a high water soap. They then looked at the effect of the starting temperature on saponification. They looked at soaping at a cold temperature: 40°C (104°F), warm: 58°C (136°F) and hot: 65°C (150°F). What their data reveals, probably unsurprisingly, is soaping at a warmer temperature results in faster saponification than soaping at a cold temperature.

Combined with the data regarding the water content we can see that low water, high temperature soaps saponify more quickly than high water, cold soaps. But when it comes to soaping there is an additional element to consider – that of gel phase. Whilst investigating the effect of water content they showed that the melting temperature of a low water soap was considerably higher than that of a high water soap. Put simply if you want to ensure your soap goes through gel phase, increase the soaping temperature and the water content. Conversely if you have a soap that has only partially gone into gel phase (resulting in a bar of soap with a darker centre then the outside) lowering the starting temperature and the water content should prevent it from going into gel phase.

All of this data really helped me to decide on these details for my recipes. I have so far been using a 33.3% Lye concentration (which he classes as a medium water content) and looking at the data this feels like a good place to start. And for the temperature I think that 40°C seems right. It means that saponification will be slow, but the combination of medium water content and 40°C soaping temp should mean that the soap won’t go through gel phase. 

But as Prof Dunn points out, different combination of oils may affect all of these things too so I will have to experiment to decipher the best oil/water/temp combination for my soaps. But this now feels easier to decide as I can plan it out as a science experiment, using his data as a guide, rather than blindly trying out combinations of oils, temp and water content hoping for the best. Don’t get me wrong, that process works too but for me and my scientific brain having a framework and data to work around feels much more comfortable and manageable and really helped me overcome my overwhelm and move forward with designing my soap recipes.

Ok I am off to plan my soap experiments – I will be back soon with a report on how that has been going.

. . .

I am Kelly Townsend and this is the Small Kindness Blog. I am a scientist, a bee lover, a rewilding obsessive, and I want to spread Small Kindnesses through the medium of soap. Follow me on Facebook, Instagram and Twitter for your daily dose of kindness (as well as to see how the soap making is going!)