I’m not the world greatest expert in saponification, but as a chemist I always found this topic very fascinating.
I will try to give you some guidelines on what is behind the craft of soap making and how can we do it at home without making our house explode. Beware: this is NOT the most comprehensive guide about soap making. Read it to get an idea of what you should know and pay attention to, but before you start to do it, make sure that you are confident with the safety measures and that you know exactly what you’re doing!
- What is a soap?
- How is soap made?
- How is a soap recipe made?
- The practice
What is a soap?
In our every day life we tend to name soap literally anything that makes foam and that we use to clean something: hand soap, face soap, laundry soap, dish soap… But for a chemist, a soap is a particular type of molecule: it’s the salt of a fatty acid.
It looks like this:
As you see, it has a long fatty chain, that is big friend of fat stuff, and a hydrophilic head – actually, a salt – that prefers aqueous company. When we use soap to wash our hands, the fatty chains will interact with the dirt on our hands (which typically is fatty as well), while the hydrophilic head will carry the whole group away with water.
The soap molecule forms when a fatty acid (typically coming from triglycerides) reacts with a strong base, like sodium hydroxide (NaOH, or lye) or potassium hydroxide (KOH).
How is soap made?
To make the soap molecule, we have to make the fatty acids meet the strong base. There are several ways to do this, but the most often used by soap home-makers is the so-called cold process.
In this method, we have two phases: a fat one and an aqueous one containing NaOH. When the two phases are at the same temperature – between 35°C and 45°C, they are mixed together and NaOH will react with the fatty acids. The reaction will take a long time to reach completion, that’s why soaps made with this method will then require roughly a month of storage before they can be used.
How is a soap recipe made?
You can of course follow recipes made and tested by others – and you should, at the beginning! – but it won’t take long before you start thinking about replacing this or that ingredient with something else, therefore you should know how a recipe is calculated.
Making a soap is not like making a cream, but in the practice there are some points in common. We have a fat phase, an aqueous phase, and when they’re at the same temperature we mix them together and we get a sort of emulsion. When we make a cream, we add a surfactant or emulsifier to make the two phases stay together; when we make a soap, it’s as if we were making the surfactant.
NaOH calculation I: the dummy soap
A classical example that makes a good soap for dummies is the 100% olive oil soap.
Let’s assume we want to prepare a soap with 1 Kg of olive oil. How much NaOH do we need to transform it completely into soap molecules?
Each oil has its specific number, known as saponification value. It tells us how much NaOH we need to react completely with a certain amount of a specific oil. It is typically expressed as milligrams NaOH/grams oil or as grams NaOH/grams oil. Here is a chart where you can find a list of saponification values.
Olive oil has an average saponification value of 134 grams NaOH/kilogram oil. This means that in our dummy soap with 1 Kg of oil we will need 134 g NaOH.
Our NaOH must be dissolved in water, though. How much water?
Scientific soap makers suggest to make a 30% w/w solution of NaOH in water. We are scientist, therefore we do what scientist do.
To calculate the amount of water, we consider that we need a solution in which 30% is NaOH and 70% is water. We calculate how much water we need with the following proportion:
30 : 70 = 134 : x
And x = 134 * 70/30 = 313 g of water is all we need.
Cool, our recipe is done!
But you will be tired very soon of the dummy soap, and especially if you live in Germany like me, such a soap will cost you a fortune.
NaOH calculation II
There are many other oils and fats that make good soaps. We can count on oils that are rich in saturated fatty acids, like coconut oil, palm oil and animal fats, and on oils that are richer in insaturated fatty acids, like sunflower seed oil, rapseed oil and of course, olive oil.
Let’s make a step back and let’s go back to the long fat-friendly chain of the soap molecule.
Some fatty acids, like the one that forms the soap in the drawing, have double bonds in their fatty chain. These are insaturated fats and their conformation is such that their molecules cannot pack very well and they have a certain degree of movement and freedom. For this reason, the oils that contain them are typically liquid at room temperature.
Other fatty acids do not have double bonds and are called saturated. Their molecules can pack like Legos and they don’t move much when they’re together, therefore at room temperature they tend to be solid. Fats that are rich in these types of fatty acids are coconut oil, shea butter, cocoa butter, palm oil and animal fats, if you’re very vintage and you want to make a soap with… ehm… ox tallow.
A good soap contains both kinds of fats, with a preference towards the saturated ones because they’re less prone to rancidity processes.
There are even recipes that do not have insaturated oils at all. But we want to use them all, so let’s assume we want to make a soap with 50% insaturated-rich oils and 50% saturated-rich oils.
Our composition could look like this:
- Olive oil: 500 g
- Coconut oil: 350 g
- Shea butter: 100 g
- Cocoa butter: 50 g
How do we deal with NaOH? We have to look for the saponification values of each one of these oils and calculate how much NaOH we need to make the reaction with those quantities of fat.
Our calculation table will look like this:
|Saponification number (grams NaOH/grams oil)||Grams in the recipe||Necessary grams of NaOH|
Now, to obtain the total amount of NaOH that we need to react with all this stuff we have to add the numbers we obtained:
67 + 66,5 + 12,8 + 6,85 = 153,15 g NaOH
And in how much water do we dissolve this amount of NaOH? We want a 30% solution, therefore 153,15 * 70/30 = 357,35 g water.
The lye discount
The lye discount is not the place where you buy NaOH for a very cheap price. It’s something that soap-makers typically do: they calculate the amount of NaOH needed to react with all the fat of the recipe, but then they use a bit less than needed. How much less? That’s the lye discount and it typically ranges from 1 to 10%. This way, some fats will not react and should be still free in the final product, giving the perception of a milder soap.
How do we calculate it then? You will need to calculate the NaOH amount like shown earlier, then subtract a percentage to that amount.
For example, if I want to make a 5% lye discount in the last recipe, I will subtract to 153,15 g its 5%, that is, 7,66 g. Therefore I will use 145,47 g of NaOH. I will have to recalculate also the amount of water that i need to get the 30% solution.
OK, if you have come so far, it means you’re really interested in soaps. But don’t worry: you don’t always have to make astronomic calculations every time you make a soap. There are online tools and calculators for that, where you just have to type the amount of oils you want to use. Or you can do like I do and have your own Excel sheet.
I recommend learning how to calculate the recipe anyway, even if you’re going to use your own Excel sheet or an online calculator over time, and to always check the numbers of recipes that you find online.
Before we can talk about the actual practice of soap making, we must be sure that we can work in safe conditions. Making soap is not difficult, but as for all cosmetics, we must pay attention when we handle chemical reagents and we must be aware of what we’re doing.
Before you even think about making soap, please check that you have the following equipment:
For your own safety
- Protective eyewear (safety glasses/goggles): rule n.1 of the chemistry lab!)
For the survival of your table
- Something to protect the surface (I use old newspaper)
To scale the ingredients and carry the reaction
- A big pot (if you prepare 1 Kg of soap, for example, use a 2 L pot)
- A big beaker (500 ml) in pyrex glass or polypropylene for the lye solution
- A smaller beaker (250 ml) in pyrex glass or polypropylene to scale the lye pellets
- Lab spatulas or glass rod; alternatively, polypropylene spoons
- An immersion blender
- A digital scale
- A thermometer
To pack and form the soap
- Soap moulds: I recommend the ones in silicone. You can choose between the big one, that will yield a single big rectangular soap that you will then cut, or the ones with multiple little soap moulds with different shapes and incisions.
Ok, now that we are prepared and protected we can start.
Prepare everything you need: all the equipment we discussed above, and all the ingredients you will need. Wear the safety glasses and the gloves, then let’s start!
- Oil phase (preparation). Prepare the oil phase: scale and transfer all the oil components of the recipe in the big pot.
- Lye solution. Prepare the lye: wear the mask, too, then scale the (deionized) water in the big beaker. Put the beaker on one side. Now take the smaller beaker and scale the lye. Don’t be scared: it won’t kill you. Don’t touch it, ever (you’re still wearing the gloves, by the way?). Once you scaled it and you have it in the small beaker, go with your small and big beaker on the balcony or close to an open window. Now slowly pour the lye into the water (never the other way round!) while mixing with the glass rod. It will develop a lot of fumes, that you should not breathe. At first, the solution will be white, then it will become clear. When you’re done transferring the lye into the water, leave the lye solution on the balcony or close to the open window.
- Go back to the oil phase. While you wait for the lye solution to cool down, start melting the oil phase. You can do it in a water bath or directly on the cooker if you have the electric ones. Beware: you just have to melt the oils, not to cook them! Stop heating as soon as the components are melted.
- Wait. Now you have to wait for the two phases (oil and lye) to reach the same temperature – between 35°C and 45°C.
- Preparation of eventual extra ingredients. If your recipe includes extra-ingredients, like powders, clays, flowers, fragrances, etc., this is a good time to prepare them.
- Soap formation. When the two phases reach the same temperature (35°C-45°C) , you can start the soap formation. Slowly pour the lye solution into the oil phase while stirring with the glass rod or with a polypropylene spoon. Then start mixing with the immersion blender. Place the blender on the bottom of the pot before mixing: it will prevent the mixture to splash around and get to your face!
- The trace. After mixing for a while, you will see that the soap has become creamier and denser. If you let some soap mixture drop from your spoon or blender onto the surface, it will leave a trace. This means that the emulsion has been made and you’re kind of safe (safe from a separation of the two phases, not from the chemical hazard: keep wearing gloves and safety glasses and don’t touch the mixture with bare hands!)
- Extra ingredients. When you reach the trace moment, you can add eventual additional ingredients.
- Packing. Finally, you can transfer the soap into the mould.
- Wrap up the mould with some transparent film and then with a tissue. Place it in a dry place and let it rest.
- Now you can wash everything you used to make the soap. Keep wearing the gloves also while you wash the pot and the equipment!
- After 24 hours you can release the soap from the mould and – if you used the big one – you can cut it.
- Let the soap rest for at least 4 weeks before using it (the reaction must go to completion before you can use the soap safely!)
Antioxidants and chelating agents
Soaps do not need antibacterial preservatives per se, since their pH is so high that bacteria will not be so happy to grow in there. However, soaps can undergo rancidity processes. To avoid them, we can either add antioxidants or chelating agents to the recipe. The most widely used antioxidant for soaps is the rosemary resin that is added to the recipe in the amount of 1 g per 1000 g of oils. Chelating agents can chelate the ions that catalyse the rancidity reactions, and one of these is for example sodium gluconate (3 g per 1000 g of oil phase, it must be dissolved in water before adding the lye).
|Oil phase||% of oil phase||grams||saponification value (grams/grams)||required lye grams|
|Sunflower seed oil||14,9||149||0,134||19,966|
|Rosemary resin extract||0,1||1||0,135||0,135|
Required lye (with no discount) = 151,65 g
Lye with 5% discount = 151,65 – 7,58 = 144 g
Here are some useful links where you can get information on the theory of soap making.
[Italian] Il sapone scientifico – My favorite blog about the theory of soap making. It’s very complete and “chemical”, but it’s in Italian.
Soap queen : super useful not only for the theory but also for tutorials and tips for decoration and swirl techniques