Aluminum Starch Octenylsuccinate

How it works

Starch on its own is a poor cosmetic material. It swells, it goes sticky in water and it will not disperse in an oil phase. This ingredient is starch modified to fix all three problems at once. Octenyl succinic anhydride attaches a short greasy tail to some of the glucose units, which makes the granule at home in oil as well as in water, and an aluminium salt caps the acid groups left behind.

What comes out is a fine, free-running powder that takes up oil and water, disperses easily and leaves a dry, velvety finish where a rich formula would otherwise feel tacky. Its granules scatter light too, so it contributes to a soft-focus look, though less than a powder made for the job.

The evidence

The jobs are physical and immediate: it absorbs, which is what takes shine down and stops a heavy cream feeling greasy, and it thickens an emulsion a little. There is nothing to wait for and nothing that builds up.

The aluminium in the name causes more alarm than the material warrants, and the reason is a collision of words. The aluminium salts argued about in antiperspirants are different compounds doing a different job, and the arguments about aluminium in food and in medicines concern other compounds again. This is an aluminium salt of a modified plant starch. That does not make the question of what crosses skin meaningless, but it does mean conclusions from those other debates do not transfer — and where regulators have looked at aluminium in cosmetics the opinions have been revised more than once, so the current rule in your own market is the thing to read.

Suitability

• Oily and combination skin get the most from it, because absorbing oil is the point of it
• No skin type has a reason to avoid it, though on very dry skin it can make a moisturiser feel less comforting than it otherwise would
• Common in mattifying moisturisers, primers, sunscreens and dry-touch body products
• Plant-derived and chemically modified: which of those two words a brand prints tells you about the brand

Concentration

No figure. The level follows the finish — enough to take the tack out of a formula, not so much that it turns powdery or pills as it dries. Nothing about skin sets a threshold, and there is no dose at which it becomes an active.

Conflicts & combinations

Nothing conflicts with it. Two practical notes. It is a starch, and starch feeds micro-organisms, so a water-containing product carrying it leans on its preservative system exactly as any other does. And absorbency has a ceiling: once the powder holds as much oil as it will hold, shine returns, which is why mattifying products are reapplied rather than applied once.