Alum crystal, used by ancient civilizations, has come back into fashion with the rise of natural cosmetics. It has many properties, including as a body deodorant. How does it benefit the skin? Is the aluminium it contains hazardous?

Alum crystal is a naturally occurring sweet-tasting mineral that looks like translucent glass. It is usually composed of an aluminium sulfate and a sulfate from another metal. The most commercially exploited alum is the hydrated form of potassium aluminium sulfate (potassium alum), which comes from a volcanic igneous rock called aluminiferous trachyte; it can also be manufactured industrially, however.
It is not known precisely when humanity began exploiting the properties of alum, but Pliny spoke glowingly of it in his Natural History. In fact, the word “alum” comes from the Latin ad lumine; meaning “give light”, this refers to one of the traditional uses of this mineral, which was to fix the dye in fabrics.

Over time, alum crystal has found many applications due to its antibacterial, healing, firming, antiseptic and deodorant properties. While its use in the cosmetics and personal hygiene industry is relatively recent, partly driven by the growing demand for “natural” products, alum cystal was, in fact, used in ancient times to control body odour.

How does it work?
Sweating is a mechanism that helps regulate body temperature by eliminating water via the skin’s pores. Sweat in itself is not odorous. Rather, the bacteria in the skin that live off perspiration produce volatile substances that smell.

Deodorants and anti-perspirants prevent unpleasant body odours. Their formulae usually combine a number of ingredients with different functions, chosen from among the dozens allowed by law. Certain ingredients are bactericidal (alcohol, for example), others are mere fragrances and other block pores to stop natural sweating.

How exactly does alum crystal work? When dampened and applied directly to the skin, it creates an invisible barrier of salt microcrystals that inhibit excessive proliferation of the bacteria that break down sweat; it does this, moreover, without clogging the pores and allowing natural perspiration. This means that it works on the basis of its potent bactericidal activity.

Alum crystal does not contain alcohol or any other substance, it is odourless, it does not stain and it is not sticky. Its effect is similar to other deodorants, so application once a day is usually sufficient.
Just as some people are allergic to cats or pollen, a small percentage of alum block users may experience an allergic reaction. Symptoms, usually shortlived, usually appear as redness in the armpits. When we first use alum (or any other cosmetic), it is recommended to check after a few hours to see if there has been any reaction in the application area. If there are any signs of a reaction, it is recommended to discontinue use and consult a dermatologist.

Aluminium: questioned
Although the possible harmful effects of aluminium for our bodies are openly debated, scientific studies are not conclusive. There are some erroneous claims that aluminium in alum crystal, given its molecular structure, is not absorbed by our skin. In fact, to be used as a deodorant, the crystal has to be wetted with water, which releases aluminium ions that can be absorbed through the skin’s pores.

Note, however, a study by the Purdue and Penn State Universities (USA) indicates that the amount of aluminium absorbed by the body through deodorants applied to the armpits is equivalent to a mere 2.5% of total daily aluminium intake through water and food.

By Elisabet Salmerón, science journalist
Sources: Food and Chemical Toxicology and Natural Cosmetic News