With just three basic rules you can formulate soap recipes for your particular skincare needs. I have even included a link to a very good online SAP value chart that does all the calculations for you. So although it is good to understand the rules for saponification and to be able to do the calculations yourself, it is not absolutely necessary in order to formulate soap recipes. A very basic SAP value chart is included at the bottom of the right hand column of the blog...
Firstly, when formulating soap recipes you cannot substitute oils in a recipe without re-calculating how much sodium hydroxide to use. The reason for this is that each oil has it's own SAP value. SAP stands for saponification, and the 'value' is a number that indicates how much sodium hydroxide is required to turn that particular oil into soap. The SAP values have already been determined by chemists and they are fixed, however, because the oils and fats we are using are natural there will be variations between, for example, olive oils coming from different parts of the world or produced in different seasons (grown in different soil etc.... The SAP values therefore are only approximate, but since the variations between oils from different origins is quite small, we have a way of managing - more on this later.
Secondly, when formulating soap recipes the type of oil you use in your recipe will determine the qualities inherent in the finished soap. For example: coconut oil generally gives you a very hard bar with big bubbles whereas sunflower oil gives a much softer bar with light medium sized bubbles. The qualities are determined by the fatty acids or triglycerides in the oils and fats. The greater your knowledge of these, the easier it will be for you to manipulate your recipes. But don't be put off - you can still make tailored soap with only a very rough understanding of the fatty acids in oil.
Thirdly, when formulating soap recipes please work in grams. It is so much easier to do the calculations and since accuracy is really important to the successful outcome of your recipe,there is less likelihood of you making a mistake.
OK, so you understand that in order to formulate soap recipes you need to calculate how much sodium hydroxide to use so that you can turn your chosen oil into soap.
But surely that means the amount is fixed.. why would you want to vary the amount? If you totally saponify a batch of oil to turn it into soap (that is, if you use the full amount of sodium hydroxide required to turn ALL of the oil into soap), you will get a very 'cleansing' bar.
If you deducted a small percentage of sodium hydroxide from the recipe you will get a bar of soap that contains a small percentage of 'free oil'. This bar will still cleanse but because of the free oil it would be more suitable for drier skins, which might find a totally saponified bar of soap just a little bit too harsh. But of course if you are a teenager with acne, the totally saponified bar of soap may be just the ticket - bear in mind that even though it is totally saponified it is still a much more gentle choice for washing than a mass-produced-commercially made bar of soap.
HOW MUCH SHOULD I REDUCE THE SODIUM HYDROXIDE BY? It is generally accepted among hobby soap makers that when formulating soap recipes you should always reduce by 5% in order to allow for those little variations that nature has thrown up, but you can reduce by more if you wish.
I have personally reduced the sodium hydroxide by 15% and still had a decent bar of soap that lathered well. I would not suggest that you reduce the sodium hydroxide by more than this. Unless of course you are experimenting - in which case do keep careful notes and let me know the outcome! The benefits of having such a large reduction of sodium hydroxide (it is usually called a sodium hydorxide discount) would be in the large amount of free-oil in the bar of soap making it very suitable for someone with chronic dry skin. The only downside would be that large amounts of free-oil reduce the shelf life of the soap.
The shelf life of a totally saponified bar of soap is pretty much infinite (of course fragrance may disappear as it gets older and colours will fade) but the moment you have some unsaponified oil left in the bar the clock starts ticking. Some recipes suggest you add Grapefruit Seed Extract to the soap in order to prolong the shelf life. In my experience it made no difference whatsoever and since it is expensive I do not include it in my recipes. Of course you can try it if you wish. Go by the suppliers recommendations for quantity to use.
What does a soap past it's shelf life look like? You will spot it straight away. The smell is usually of rancid fat or oil and it has a tendency to go yellow. It may still lather but it isn't exactly pleasant to use.
Lets imagine that you are using a recipe that includes 100g of olive oil and 50g of castor oil and 100g of coconut oil.
You calculate each oil separately like this.
The amount of oil in the recipe multiplied by it's sap value.thus the olive oil calculation looks like this:
100(g) x 0.134 =13.4
This means that to totally saponify 100g of olive oil you would use 13.4g of Sodium Hydroxide.
Next you would calculate the castor oil:
50(g) x 0.128 = 6.25
This means that to totally saponify 50g of castor oil you would use 12.8 g of sodium hydroxide.
Next you would calculate the coconut oil:
100(g) x 0.190 = 19
Then you add all three answers together:
13.4 + 6.25 + 19 = 38.65grams of Sodium Hydroxide will totally saponify this particular combination of oils.
Do you remember we said that hobby soapmakers always reduce the sodium hydroxide by at least 5%? Well now is the time to do that.
Since most kitchen scales do not weigh down below 1gram I always round the total down to the next whole gram, therefore 38.65 becomes 38grams. Now we do the reduction by 5% = 36.1g of sodium hydroxide required. Of course I will again round it down to 36g. If you wish to increase the percentage then you can.
This process of reducing the sodium hydroxide and leaving a small percentage of free oil in the soap is called 'superfatting'.
When formulating soap recipes there is another way to superfat. The first, outlined above is all about reducing the sodium hydroxide (a caustic soda discount). The second way is to leave the sodium hydroxide at its full amount (i.e. the amount required to totally saponify the oils) and add a small percentage of extra oil or fat to the recipe. The reason why you might choose this method of superfatting is if you wish to use a small amount of a particular oil (perhaps a more expensive or more luxurious oil) in your soap. In this way you can incorporate particular qualities from a more unstable or more expensive vegetable oil without having to use large quantities of it which might upset the recipe or break the bank!