Superfat

Superfat is the portion of oil in a recipe that is deliberately left without enough lye to react with it. Instead of calculating a recipe so every bit of oil saponifies, soap makers reduce the lye slightly, or add extra oil after trace, so a small percentage of oil remains free in the finished bar.

This leftover oil makes a bar feel milder and more moisturizing, and it also acts as a buffer against small measuring errors, since a slightly high lye amount is less likely to leave the soap harsh. Most cold process recipes use a superfat between about 3 and 8 percent, adjusted with a lye calculator.

When choosing a superfat level, higher percentages can shorten shelf life and make a bar softer or more prone to orange spots, while very low superfat leaves less margin for error in measuring. A common mistake is manually adding extra oil after trace without recalculating the lye amount, which can push the actual superfat much higher than intended.