Every fat requires a certain amount of lye to turn it into soap. For example, 500g of olive oil requires 67.7g of lye, whereas 500g of lard requires 70.5g of lye.
When you are making soap the idea is to always use more fat than the lye can convert into soap. This prevents the presence of any excess lye in the finished bar, which means it will be a nice, gentle bar, and will not be irritating.
A 5% superfat is fairly standard (you don’t want to go much higher than 10% or the bar will be very soft and can go rancid). 5% superfat is not an ingredient, it’s part of how you calculate the recipe—the lye calculator you use will have an area asking what superfat percentage you want to calculate at. Put the number “5” in that input box for a 5% superfat.
With a 5% superfat, you would only use 64.3g of lye for 500g of olive oil instead of 67.7g. That means 5% of that 500g of olive oil will not be turned into soap, giving you a bar of soap that is 475g of saponified olive oil with 25g of leftover olive oil mixed into the bar to moisturize your skin and work as a buffer against any errors in measuring the lye.
Posted in: Soap