How Potassium Alum Works in Our Crystal Deodorant Classic

If you've started reading deodorant labels, you've probably spotted "potassium alum" on the ingredients list of our Crystal Deodorant Classic. It's a simple ingredient, but it does a clever job. Here's what it is, how it works, and how it differs from aluminium chlorohydrate, the ingredient found in products designed to block sweat.

What Is Potassium Alum?

Potassium alum is a naturally occurring mineral salt [1]. Its full chemical name is potassium aluminium sulfate, sometimes called alum stone [1]. People have used potassium alum (and a close relative, ammonium alum) as an underarm deodorant for a long time, across many different cultures, including in Europe, Mexico, Thailand and the Philippines [5]. It isn't a new idea. Salt of the Earth has used potassium alum since we launched our original crystal deodorant back in 1993 [10]. It's the ingredient our Crystal Deodorant Classic is built around, and it hasn't changed since.

How Does Potassium Alum Stop Odour?

Body odour comes from bacteria on your skin [4]. When you apply potassium alum, it sits on the surface of your skin as a thin layer [2][3]. From there, it works against the bacteria that cause underarm odour. A lab study testing potassium alum against these odour-causing bacteria found it stopped their growth, working from a concentration of 7.5mg per millilitre upward [4]. That's the whole job: fewer active bacteria on your skin means less odour.

Why the Crystal Format Matters

The Crystal Deodorant Classic is a solid block. You dampen it with water and rub it gently onto your skin. This is how the thin surface layer forms in the first place [2][3]. There's no spray, no stick residue, just a light, even layer of potassium alum sitting where it needs to be: on the surface of your skin.

Potassium Alum vs Aluminium Chlorohydrate: What's the Difference?

This is the question most people want answered. Potassium alum is made of bigger particles that can't get through skin [2]. Aluminium chlorohydrate, the ingredient used in many products designed to block sweat, works differently. It's made of tiny particles, small enough to get into your sweat ducts [6][7]. Once inside, these tiny particles get into the duct and block it up, near the surface of your skin [6]. Only particles this small can do that [7]. Potassium alum simply isn't built that way, so it can't block anything, it stays on top of your skin instead [2][3][7].

Does Potassium Alum Get Into Your Body?

No. Potassium alum is made of particles too large to pass through your skin [2][3]. It stays on the surface, doing its job against odour-causing bacteria there, rather than being absorbed into your body [3].

Deodorant, Not a Sweat-Blocking Product: Why That Matters

The Crystal Deodorant Classic is a deodorant. It doesn't block sweat [11]. That distinction matters. Deodorants tackle odour. Products designed to block sweat work by physically blocking your sweat glands, to reduce how much you sweat. Our Crystal Deodorant Classic doesn't block your pores, and it doesn't stop or reduce sweating [11]. It works on odour, and that's all it claims to do.

Is a Potassium Alum Deodorant Right for You?

If you like to check what's in your products before you buy, potassium alum is a straightforward, single ingredient with a clear job. Our products, including the Crystal Deodorant Classic, are COSMOS Natural certified by the Soil Association [9]. This certification requires a minimum of 95% natural-origin ingredients, checked independently rather than just taken on our word [9].

If your skin is sensitive, it's worth thinking a bit further. We'd suggest trying our Unscented range, which has fewer ingredients simply because it leaves out fragrance [12]. Fragrance is the most common trigger of cosmetic sensitivity [12], so removing it altogether can help. Whichever version you choose, we'd always recommend a patch test first: a small amount on the inside of your wrist or elbow, left for 24 hours. Everyone's skin is different, and we can't promise any product will suit everyone. A patch test is the honest way to find out before you commit to full use.

How to Use the Crystal Deodorant Classic

Apply your Crystal Deodorant Classic to clean, slightly damp skin. Dampening the crystal itself, then rubbing it gently under each arm, is enough to leave a thin, even layer behind. That thin layer sits on your skin, working against odour-causing bacteria [2][3]. It's a simple routine, and it's stayed the same since we first made it back in 1993. Try it for yourself with our Crystal Deodorant Classic.

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Sources

1. Wikipedia; PubChem — potassium alum chemical name and formula
2. PubChem/DrugBank — potassium alum particle size and skin penetration
3. PubChem/DrugBank — potassium alum not absorbed through skin when used topically
4. Peer-reviewed study, jchs-medicine.uitm.edu.my — potassium alum stops growth of odour-causing bacteria from 7.5mg/mL
5. Wikipedia — long, traditional use of potassium/ammonium alum as a deodorant across cultures
6. Nature/PMC studies — how aluminium chlorohydrate's small particles enter and block sweat ducts
7. Nature/PMC study — only small particles can enter and block a sweat duct
8. Salt of the Earth brand data / COSMOS Standard v3.1 — COSMOS Natural certification, Soil Association, minimum 95% natural-origin threshold
9. Salt of the Earth company history — natural deodorants since the 1990s, original crystal deodorant launched 1993, UK-based
10. Salt of the Earth product classification — Crystal Deodorant Classic is a deodorant that does not block sweat
11. EU SCCS opinion on fragrance allergens / COSMOS formulation records (Salt of the Earth internal) — Unscented variant has fewer ingredients, fragrance is the most common trigger of cosmetic sensitivity