If you're just getting started as a soap maker you may have seen or heard the term “superfatting” come up in your research.
What exactly is super fatting? Why do soap makers do this and how much should you super fat your soap?
Superfatting, also called a lye discount, is when you either add extra oil to your recipe keeping the amount of lye the same, or use less lye/liquid and keep the oil amount the same in order to have oil “left over” after saponification.
If you were to perfectly balance your oils with the exact amount of lye needed to make an exact amount of soap, you would have no oil leftover and your superfat would be 0%.
Most soap makers intentionally use more oil than necessary in order to have a certain percentage leftover that has not been used up in the saponification process.
Why would you want to do this? Having that small amount of extra oil in your bar adds to the moisturizing ability and emollient properties of the soap.
It also provides a safety margin to insure that there is never any unreacted lye in your finished bar of soap.
How much should you superfat? Superfatting is typical done at 5-20%. I personally, prefer to superfat at 5% because I feel that it makes for a soothing bar without it being too soft or inhibiting lather. However, it does depend on your own personal preference.
How do you calculate superfat? That depends on which oils you are using in your recipe because each oil has its own “SAP Value”. The SAP value, or saponification value, is the amount of lye that it takes to turn 1 gram of oil into 1 gram of soap. SAP value charts are widely available online.
As you can imagine, this value is extremely important when it comes to formulating soap recipes. This is the reason why you can’t just substitute oils for one another without recalculating the recipe.
The equation looks like this:
(oil amount) x (SAP Value) = amount of lye needed (0% superfat)
So how does that calculation change if you want to superfat by 5%?
(lye required) x (1-.05) = New lye amount (5% superfat)
Here is an example:
I’m using 8oz of cocoa butter. The SAP value of cocoa butter is 0.137
(8oz cocoa butter) x (0.137) = 1.096oz lye (0% superfat)
1.096oz lye) x (1-.05) = 1.04oz lye (5% superfat)
Fortunately, there are several really great online soap calculators (like this one) that you can use. All you need to do is enter the amount of oils you’re using and the desired superfat percentage, and it will calculate the amount of lye needed for you.