Does Kaolin Clay Really Anchor Scent in Soap?

Short answer: there's no good evidence that kaolin clay holds scent in cold process soap. In a blind test, soapers couldn't tell bars made with kaolin from bars made without it. What reliably makes scent last is simpler: use enough fragrance, choose fragrances that hold up in soap, build your scent on base notes, and cure and store your bars well.

Where the idea comes from

Kaolin is a fine, absorbent clay. The idea is that it soaks up fragrance oil and holds on to it while water leaves the soap during cure. It's been passed around soapmaking groups for years, and we've suggested it too. When we looked for real tests, though, the evidence wasn't there.

What the testing shows

• A blind side-by-side test. One soapmaker made the same lemon essential oil soap four ways: no clay, kaolin, bentonite, and kaolin soaked in the essential oil first. Volunteers who didn't know which bar was which ranked them by scent every month for six months. They couldn't tell the difference. It was a small test with one essential oil, but it's the only side-by-side test we found. You can read it on Miles Away Farm.
• Experienced soapers. On the Soapmaking Forum, soapers who tried clay report little or no difference. Many skip it and simply stop using fragrances that fade.

What about zeolite?

Zeolite is a mineral full of tiny pores that can hold perfume and release it when it gets wet. Lab studies and laundry detergents use it that way. We haven't tested it in soap yet, but based on the research, it's much more likely to anchor scent than kaolin clay.

What does make scent last

1. Use enough fragrance. For cold process, around 5% of your oil weight (up to 6% for a strong scent), within the fragrance's IFRA limit. A light rate smells nice in the pot and faint after cure. See How Much Fragrance Oil Should I Use in Soap?
2. Choose fragrances that hold up. Read the maker's soap testing notes and reviews before you buy. If a fragrance fades in your soap even at a strong rate, it's the fragrance, so try a different one.
3. Build on base notes. Deep notes like patchouli, cedarwood, sandalwood and amber last longest. Pair a bright citrus or a light floral with one of them. Benzoin is a classic fixative in perfume, and our article Benzoin Resin... The Other Scent Fixative shows how one soapmaker uses it.
4. Cure and store well. Cure bars somewhere cool, dry and out of direct sun. Once they're cured, wrap or box them.
5. Remember the surface. The outside of a bar can smell faint while the inside is still strong. Lather it up before you decide the scent is gone.

Still want to try clay?

Go ahead. Kaolin clay is gentle and adds a nice slip to soap, so it's a good additive for its own sake. If you want to know whether it helps your scent, test it: split one batch in two, add clay to half, label the bars, and have someone else hand them to you to smell each month without telling you which is which.

Want us to take a look?

Thinking about a new fragrance? Fragrance Check looks up how it holds up in soap. Already made the soap and the scent faded? Tell us about it in Fragrance Rescue. More on fading in Why Did My Soap Lose Its Scent?