The basic idea
When oils react with lye, they turn into soap. In theory, there is an exact amount of lye that would use up every drop of a given oil. Soap makers deliberately use a little less lye than that. The oil that is left over, unreacted, stays in the finished bar. That leftover oil is the superfat.
It is usually expressed as a percentage in recipes and calculators. A setting of, say, 5 percent (a hypothetical example) means the recipe is designed so that roughly that share of the oils is left unconverted. The mechanics of the reaction are explained in the article on saponification, and a good soap calculator will take the setting as an input.
Why makers do it
A safety margin. Weighing, measuring and oil quality are never perfect. If you use slightly too much lye, a bar with no superfat would be left with excess alkali, which can irritate skin. A bit of built-in margin reduces that risk. It is not a substitute for accurate measurement or testing, and a bar that is lye heavy can still result from errors.
Mildness. The free oil can make a bar feel gentler and less stripping on skin. Many people find a bar with more conditioning oils left in it feels more moisturizing. Experiences vary, and the bar’s feel depends on the whole recipe and the person.
Lather and feel. The free oils change how the lather feels, usually making it creamier and less harsh. Too much, however, can dampen lather.
What goes wrong with too much
More is not always better. A high superfat can cause several problems.
- Softer bars. Unreacted oil does not harden, so the bar may be softer and wear away faster in the shower. The effect is related to soap texture generally.
- Weaker lather. Excess free oil can suppress bubbles, so the bar may seem flat.
- Greasy feel. Some people dislike the slick feeling on the skin or in the shower.
- Shorter shelf life. Free oils can go off over time. Orange spots and a stale smell are signs of soap rancidity. Oils that are more prone to spoiling make the problem worse when left unreacted, as discussed in the article on oil shelf life.
- Mushy or sweaty bars. Combined with other factors, excess oil can contribute to a bar that feels mushy soap.
What goes wrong with too little
A very low superfat, or zero, makes the bar harder and more cleansing, and it may lather well, but it can be harsh on skin and leaves no margin for error. Laundry-style bars, discussed in the article on laundry soap, sometimes use little or none because mildness is not the goal. For body soap, that approach is generally less forgiving.
Where the oils go
The leftover oil in a bar is not uniform. It depends on which oils were chosen. Some makers add a special oil, such as a luxury oil, late in the process, a technique sometimes called superfatting at trace, hoping it stays unreacted. Others simply apply the percentage across the whole recipe. Both are used, and neither guarantees that a particular oil ends up unreacted, since the lye does not discriminate perfectly.
Typical ranges and caution
Many body soap recipes use a modest superfat, and it is common to see figures in the low-to-mid single digits discussed for beginner recipes, with higher figures for some special-purpose bars. Treat any figure as a starting point, not a rule. The best number for a given recipe depends on its oils, the intended use and your preferences. A tested recipe from a reliable source already has a considered setting, so change it only with understanding, and recalculate with a calculator if you do.
Superfat also depends on your cure and storage. A bar with more free oil and a poorly ventilated storage place is more likely to go off.
Using superfat for specific needs
People with dry skin often favor higher superfat, while those who want a stronger cleanse choose lower. The article on soap for skin types looks at matching bars to needs, and sensitive skin soap raises the same point. Remember that superfat alone does not make a soap suitable for eczema prone skin or other skin problems. A health professional is the right person to advise on a skin condition.
A habit worth keeping
Change superfat in small steps, one batch at a time, and record the result. Your notes, together with careful testing, teach you more about your own preferences than any chart.