In Plain English: Superfatting soap. What it is and why we care about it.

Because soap's main job is to wash, but it doesn't have to strip the skin of everything it can lay its hands on.

Soap works thanks to molecules that have one part attracted to water and the other to fats. That's how it can loosen grease and dirt from the skin and let them be rinsed away with water. That's its job, and it does it very well.

When washing, though, it can't tell the difference between the grease we want to remove from our hands and the lipids on the surface of the skin that we'd quite like to keep there.

That's exactly why we don't aim for maximum cleansing power in a recipe. We look for a balance: the soap should wash well, lather nicely, have the right hardness and at the same time not strip the skin more than it needs to.

And one of the tools for influencing that balance is superfatting.

Because oils aren't just different names on bottles.

Every fat has its own characteristic make-up of fatty acids, and these have a big influence on the properties of the finished soap. Coconut oil makes a different soap from olive oil, shea butter behaves differently from castor oil, and a recipe built on one type of fat needs a different balance from a recipe built on another.

That's why, when developing a recipe, we don't just look at the resulting superfat percentage. We look at the whole blend of oils and butters, their saponification values, their fatty acid profile and what properties we expect from that particular soap.

A gentle body soap has different requirements from a soap meant mainly for hands, and a household cleaning soap, for example, follows a completely different logic.

A soap calculator is very good at calculating, but it doesn't know what soap we want to make. That's up to us.

If you dig a little deeper into recipes for making soap at home, you'll probably come across the term lye discount – reducing the amount of hydroxide.

In practice, superfatting and lye discount are often used almost as two sides of the same coin. Instead of saying we added more fat, we can say we used less hydroxide than would theoretically be needed to saponify all the fats.

The second way is more precise in production, because it describes what actually happens in the calculation. If the calculation says we need a certain amount of NaOH to fully saponify the recipe and we set a 5% lye discount, we use roughly 5% less hydroxide. That leaves a reserve of the fatty component in the recipe that won't saponify.

It sounds less romantic than “added nourishing oils”, but chemistry occasionally has the annoying habit of spoiling a nice bit of marketing.

This is where we get to the part that often surprises even people who make soap at home. In the classic cold process, it isn't right to imagine a recipe in which the coconut oil obediently saponifies, the olive oil waits its turn and the expensive shea butter stays untouched because we picked it as our “superfat”.

Saponification is a chemical reaction that takes place throughout the whole mixture. The resulting unsaponified fat fraction therefore doesn't necessarily correspond exactly to the one particular oil we marked on paper as the superfatting oil.

That's also why we're careful with claims like “the soap contains 5% unsaponified shea butter”, unless we can back them up analytically or the production method genuinely allows that kind of control.

We can know what we put into the recipe, but chemistry decides what exactly happens during the reaction. And chemistry doesn't read marketing copy.

These two things sometimes get mixed up when it comes to handmade soap, but they're two different concepts.

Glycerine forms naturally during saponification. In traditional soap making it stays in the soap mass and is part of the finished product, as we wrote in the article What is soap and what is it made from?

Superfat is the fatty component that wasn't turned into soap during saponification.

So both can be present in handmade soap at the same time, but each got there in a completely different way.

Superfatting has one more practical purpose. In a well-built recipe, it creates a buffer against excess hydroxide.

In production, however, it's certainly no excuse for imprecise weighing.

The amount of hydroxide is calculated for the specific oils and their saponification values, and we weigh the ingredients precisely. Superfat is a part of the recipe designed in advance, not a safety net of the “a bit here, a bit there and it'll work out somehow” kind. With hydroxide, “somehow” is a unit we don't use in our soap workshop.

Exactly as much as makes sense for its recipe, and it's a different number for every recipe. That may be the most important thing to take away about superfatting. Good soap doesn't come from maximising one value. It's the result of a balance between the oils and butters used, the amount of hydroxide, water, hardness, lather, cleansing power, superfat and, finally, the time we give it to cure.

Superfat is just a small number in a recipe. Yet it plays a bigger part in how the finished soap feels than those few per cent might suggest.

And that's exactly why we don't guess it by eye but calculate it deliberately for every recipe. Because with soap, what matters isn't only what we put into it, but also how much of it we let become soap.

To lather is human. Doing the maths is our job.

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