Bentonite is a clay formed from weathered volcanic ash, composed mainly of montmorillonite. Unlike kaolin, it swells substantially when wet, absorbing several times its own weight in water.
How it works
Two properties distinguish bentonite from milder clays.
Swelling and absorption. The layered mineral structure expands as water enters between the layers, which is why bentonite forms a thick gel rather than a simple paste. That expanded structure absorbs considerably more oil and fluid than kaolin.
Negative surface charge. Bentonite particles carry a net negative charge. This gives the clay ion-exchange capacity, meaning it can bind positively charged substances at its surface.
That charge is the basis of most claims about bentonite drawing out impurities. There is real chemistry behind it, but it operates on material at the skin surface, not on anything deeper. Claims about pulling toxins from within the body are not supported.
Stronger than kaolin, and correspondingly less forgiving
Bentonite is genuinely more absorbent, which makes it more effective for oily skin and more drying for everyone else. Formulations often combine both clays, as this one does, to get the absorbency of bentonite moderated by the mildness of kaolin.
The metal spoon question
A persistent claim holds that bentonite must never touch metal, as this deactivates it. The reasoning invokes the ion exchange described above.
Brief contact with a stainless steel spoon does not meaningfully affect a clay masque. Long-term storage in reactive metal containers is a genuine consideration in industrial contexts. The kitchen version of this advice is considerably overstated.
Sourcing and purity
Bentonite deposits vary in composition, and natural clays can contain heavy metals including lead. Cosmetic-grade material is purified and tested against limits for these. This is the meaningful difference between cosmetic bentonite and clay bought as a raw commodity.
