What is superfatting in soap making?

Super fatting is such a random word - I don't know about you but to me it sounds like it should be associated with greasy burgers and the like.
But in soap making, super fatting is good - great even - it is the difference between a bar of soap that dries your skin out and one that is beautifully luxurious and moisturising and who doesn't like the sound of that!
To understand how it works we do need to learn a little chemistry but we can keep it simple.
SOAP is the end result of a chemical reaction between Sodium Hydroxide aka lye, water and the butters and oils you have chosen to make your soap.
Each type of butter and oil needs a specific amount of lye gram for gram to turn it in to soap. We can work out that figure quite easily using a lye calculator ( you can see how that works in my demo here
Now for the clever bit - once we know how much lye we need to turn a given amount of butters and oils in to soap we can add extra oils to our recipe and we will know those will remain free floating in our soap, as there is not enough lye to turn them in to soap as well. This is called superfatting.
Generally we would super fat to around 5 - 8% which means that every lovely bar of soap will have 5-8% of excess oils that will help moisturise and leave you soft as well as clean.

In cold process soap making as all our oils are added in at the start of the process it is not possible to choose which are used to super fat - it is likely that a combination will remain but with the hot process method we can be really selective and it is possible to make our recipe up and with hold the final 5% of oils until after our recipe has been turned in to soap and then add them - this means that we could choose to use our expensive, skin loving oils as our superfat making our finished bar of soap even more luxurious or personalised to our individual needs. Clever stuff!