Soap: a substance used with water for washing and cleaning, made of a compound of natural oils or fats with sodium hydroxide or another strong alkali, and typically having perfume and colouring added.


Sounds like a simple combination. And it is. Especially with all the free online recipes to get you enthused over it.

So here goes for the lowdown on soap-making.

Now I’m no expert, I have just made my first batch of soap, but I searched the web for easy-to-follow instructions and recipes, and what came up as my favourite was AromaZone, which is a French site. If you’re fluent in French then check it out for yourself here, if not, I’ve taken the most pertinent bits and translated them below.

There are a few different methods of soap-making but the cold method (or more accurately room temperature) is the simplest way of making home-made soaps.

As I said above, soap is simply a mixture of oils and caustic soda (= sodium hydroxide, or lye). You need to precisely calculate the quantity of caustic soda (lye) to set off the saponification process. Oils and lye solution are then mixed at room temperature.

Mixing oil and caustic soda

Mixing oil and caustic soda

What is saponification? It is a chemical reaction between oil or fats and sodium hydroxide (lye or caustic soda). The result of this reaction is soap.

Saponification is a total reaction: it carries on until one of the reactives (oil or caustic soda) is exhausted. In order to be sure that no caustic soda is left in your soap at the end of the process, there needs to be an excess of oils or not quite enough caustic soda to transform all the oil into soap. The saponification process stops when all the caustic soda has been exhausted and so some non-saponified oil will be left in the soap. The soap is then said to have a “superfat” content which makes it gentle and gives it its nourishing and soothing properties.

Saponification is quite a slow process at room temperature, so when using the cold process, your soap has to “cure”  (= dry, basically) to give the saponification process time to finish. This also allows the soap to dry and harden.

The superfat content of a soap is the percentage of the initial weight of the oils that remains in the finished soap. There are two methods of creating a superfat content:

  1. reducing the quantity of caustic soda in the recipe (=discount%)
  2. increasing the fat/oil content when you have reached the “trace” stage of soap-making (=superfat%)

1. Reducing caustic soda

Basically this just means using less caustic soda than is theoretically necessary to saponify all the oils in your soap recipe. As a result all the caustic soda will be used up by the saponification process and there will be an excess of non-saponified oils in the finished soap.

Reducing the caustiic soda content is an essential safety margin and the saponification indexes used to calculate the quantity of caustic soda to be added to your oils are average amounts and may vary. It is recommended to choose a discount/reduction of at least 5% for safety.

A reduction higher than 5% will give a superfat soap which will be much gentler. Your soap will have remains of each oil or butter of your mix which, since they have not been transformed into soap, will each bring their own particuliar properties to the soap. For this reason the caustic soda reduction/discount is often referred to as the superfat content.

2. Superfat content added at trace

Another way of producing a superfat content is by adding oil at the trace stage of soap-making (just before pouring into moulds). This final oil will, for the most part, not be saponified i.e. turned into soap. This is an interesting method when using expensive oils which are sensitive to temperature, or oils and butters with specific properties.

Adding oil at this stage is also interesting for dispersing other ingredients you wish to add at this stage (colours, essential oils). Adding them to the oil helps even distribution in the final soap.

3. What superfat content do I need?

Total superfat content (%) = Caustic soda discount (%)+ Additional oils (%)

To obtain a mild soap, AromaZone recommends a total superfat content of between 8% and 10%. You can go up to 12 or even 15% but a high superfat content will give a soft soap. It may also go off quickly (oils, like butter, can go rancid!), especially if the oils used for the superfat content oxydise easily. In certain cases, an overly “fat” soap may “sweat” oil. It is highly recommended to do do both a reduction in caustic soda of at least 5% AND a superfat content at trace.

For example:

A safety margin of 5% caustic soda reduction and additional oils at trace of 3 to 7%

6 to 10% caustic soda reduction and additional oils at trace of 2 to 3%

5 to 12% caustic soda reduction for a simple superfat content without adding oils at the trace stage (this is what I do for my basic soap recipe in Part 2)

4. In practice

Use a lye calculator. There are several on the internet. This one here is from AromaZone, a French site, so it’s in French, but it’s the one I use. For an English language one, try soapcalc.

It is used to determine the quantity of caustic soda or lye you need. You choose the percentage reduction or discount (keeping in mind what we’ve said above) and the calculator gives you the corresponding weight.

How to use the AromaZone lye calculator:

  1. Double click on the oil(s) you are using (left hand box). The oil will appear in a table on the right. Click under “Quantité( g)” and enter the weight of oil using (it’s in your recipe).  Repeat for every oil in your recipe (BUT NOT THOSE TO BE ADDED AT TRACE). For example, double click on olive oil (huile d’olive) and type in 255 g. Now double-click on Babassu oil (huile de babassu) and type in 45 g. The final line will show the  percentages of each oil (85% and 15%).
  2. Scroll down to choose the type of caustic soda you’re using (Sous quelle forme se présente la soude que vous utilisez ?). Click on liquid form (solution commerciale), then type in the % concentration marked on the bottle e.g. 30.
  3. If you’re adding oils at trace, type in on the next line (Ajout d’huile). NOTE: this is a percentage of the TOTAL oil content (should be in your recipe). If you’re not adding any, don’t type anything.
  4. Click on Result (voir les résultats) and you’ll get a table showing different percentages of caustic soda discount and the necessary amount in grams of caustic soda solution. The pink area is to be avoided, the yellow is OK but will give a non-too- mild soap, green is recommended, blue will probably be too soft.

That’s enough for today, don’t want to overload you, my next post will be about essential equipment then the all important FIRST basic recipe and then there’ll be another post again on the whys and wherefores of different oils for soap-making .

Basic equipment

Basic equipment