What butters are
In soap making, a butter is a plant fat that is solid or semi-solid at room temperature. Shea butter, which comes from the nuts of the shea tree, is the best known. Cocoa butter, mango butter, kokum butter and illipe butter are other common examples. Like oils, butters are made of fatty acids and react with lye to form soap. They bring a different set of properties, mostly to do with feel and firmness.
What shea butter adds
A creamy, cushioned lather. Shea tends to make the foam feel softer and denser. The effect is subtle, but many makers notice it in side-by-side comparisons.
A conditioning feel on the skin. After rinsing, bars with a share of shea are often described as feeling smoother or less tight. This is a perception that varies from person to person, and soap is rinsed away quickly, so it is best not to expect the lasting moisturizing effect of a lotion. For that kind of care, a leave-on product such as whipped body butter or lotion bars serves a different purpose.
Some added firmness. Shea is a fairly soft butter, but it helps a bar feel less slippery and a bit more solid. Harder butters like cocoa add more firmness.
A portion that stays unsaponified. Shea contains a relatively large fraction of components that do not turn into soap during saponification. These unsaponifiables are part of why shea is described as skin-friendly. How much of that survives into a finished bar, and whether it makes a measurable difference, is less clear, so treat marketing claims with caution.
Why moderation is the rule
If shea is so pleasant, why not use a lot of it? Most recipes keep butters, together, to a modest part of the total oils, for several reasons.
- Waxy or greasy feel. Too much butter can make a bar feel waxy, and the lather may become weaker.
- Crumbly or brittle texture. Bars with a large share of hard butters can crumble when cut or crack in use.
- Cost. Butters usually cost more per pound than common oils such as olive or coconut.
- Shelf life. Some butters can go rancid, and the effect of age can show as orange spots or an off smell in the bar.
- Lather balance. Butters do not contribute much in the way of bubbles, so a recipe heavy in butter still needs cleansing, bubbly oils like coconut or a lather helper like castor oil.
A frequent approach is to use butters in the range of a modest slice of the oils and let softer, liquid oils make up most of the rest. The right figure depends on the whole recipe, so lean on a tested formula and a soap calculator instead of rules of thumb.
Working with butters in the batch
Butters must be fully melted with the oils before mixing. Hard butters especially can leave tiny grainy bits if they are not melted completely. Some makers heat the oils until all butter is clear and then let them cool slightly. Shea can occasionally form small grainy crystals if cooled unevenly, and if you notice this in a batch it can show up as a slightly gritty texture in the bar.
Butters also influence trace. Most do not speed it up dramatically, though recipes with a large share of hard fats can thicken more quickly than those with soft oils. Keep an eye on the batter and take notes.
Refined or unrefined
Unrefined shea butter is off-white to yellowish and has a nutty, smoky scent. Refined shea is paler with little smell. Unrefined versions may keep more of the natural components but can leave a distinctive scent in the bar, though it usually fades. Which one suits you is a matter of preference. Whatever you choose, buy a fresh butter from a source that stores it well, and store it somewhere cool and dark. The general question of how long oils and butters stay good is dealt with in the topic of oil shelf life.
A note on allergies
Shea comes from a tree nut, though it is generally regarded as low-risk for people with nut allergies because the proteins are largely removed in processing. Even so, someone with a tree nut allergy should check with a health professional before using it. A patch test of a new bar is a sensible habit for anyone.
Safety reminder
As with all cold-process soap, the batter contains caustic lye until it has fully reacted. Protective gear, ventilation, a workspace kept clear of children and pets and a tested recipe from a reliable source are the baseline. Do not add or swap butters without recalculating the lye.