Algin

What it is

Algin is the sodium salt of alginic acid, extracted from brown seaweeds such as kelp. In the presence of calcium salts it sets rapidly into a soft rubber, which is the basis of the powder-and-water "modelling" or "rubber" masks used in salons. In ordinary leave-on products it simply acts as a thickener and helps a formula stay where you put it.

What it does well

• Forms a physical gel that holds water and other ingredients against the skin

  The gelling behaviour is well-characterised chemistry used across food, wound dressings and cosmetics. It slows evaporation during a mask session, which is why skin feels more hydrated afterwards; the alginate itself is not absorbed.

  Moderate evidence

Worth knowing

• Does nothing active on its own

  Alginate is a texture ingredient. Any benefit from a rubber mask comes from occlusion during wear and from whatever active ingredients are mixed in with it.

  Matters most toAnyone paying salon prices for a modelling mask on the strength of the alginate itself.

Where the EU stands