When formulating soap recipes you want to make sure that the soap is mild enough for the skin. There are two terms used in determining how to make sure there is excess oils in the final soap bar so that the soap will not strip that skin after cleansing; superfatting and lye discount.
Superfatting and lye discounting actually mean the same thing. Superfat means an excess of fats left in the soap after the lye has reacted with the fatty acids. The superfat is determined by using a lye discount when formulating. This is how the excess fats are calculated. By utilizing less the amount of sodium hydroxide needed to completely saponify with the fats the soap will be gentle and mild to the end user while still providing all the qualities of a great bar of soap.
The amount of the superfat is where it can become tricky. You don’t want your bar of soap to have too much excess oils as it may never become a hard bar of soap and also it has a higher potential to develop the dreaded orange spots much quicker. You also don’t want the bar of soap to be completely saponified since a moisturizing bar of soap is the ultimate goal.
A range of zero to ten percent is the scale for determining the superfat/lye discount. Zero to four percent the soap is not as mild and is more suited for a laundry soap. Five to eight percent is ideal for soap formulated for the body. More closely around the five percent mark but it does depend on what you are doing and the oils you are using to formulate with. The higher end can be used for specialty soaps such as baby and facial soap where a milder soap bar is desired. The nine to ten percent range is not recommended as the soap bar may have too much excess oil and never become a hard bar of soap. It would be recommended to create a test batch and see the results you get when attempting the higher lye discount formulations. No sense in wasting valuable oils and the soap doesn’t harden.
When superfatting you don’t have to wait until the soap traces to add the excess oils to the soap batch. You can save time by adding all your oils to the initial batch. Soaping oils saponify at different rates. It all depends on the fatty acid and how they react with the lye. Some react quicker with the lye than others. The percentage of excess oils are generally the same regardless whether you add the oils at trace or in the oil phase.
No longer confuse yourself with superfatting or lye discount. When you hear those terms remember they mean the same thing and used synonymous.