Boron Nitride

How it works

Boron nitride is a manufactured ceramic — boron and nitrogen in equal number — and in the form used in cosmetics those atoms sit in flat hexagonal sheets stacked loosely one on another. The arrangement is the same idea as graphite's, which is where the nickname white graphite comes from, and it behaves the same way: the sheets slide over each other, so the powder feels slippery rather than gritty and cleaves into very thin plates.

Those plates are a compromise between letting light through and throwing it sideways. Some passes, some scatters, and the result on skin is soft-focus behaviour — a surface that reads smoother without reading covered. The plates also lie flat and cling, which is why a powder carrying it spreads evenly and stays where it was put.

The evidence

What it does is physics and is not in dispute: it spreads, it adheres, it slips, it diffuses light. Those effects are also the whole of what it does, which is the useful thing to know about it. It is not an active, it takes no part in skin chemistry and it is not absorbed. Neither effect takes time: both are there on the first application, and neither accumulates.

One claim is worth flagging. Boron nitride conducts heat unusually well for a white powder, and that property is sometimes sold as a cooling effect. The conductivity is real and it is a supplier's figure for the raw material. Whether the amount of it in a finished cream produces a sensation anybody would notice is a different question, and not one the raw-material figure answers.

Suitability

• Not an active; nothing about a skin type argues for it or against it
• Chosen for feel and for optics — slip, adhesion, and a diffused finish
• Common in pressed and loose powders, primers, and mineral sunscreens, where it cuts the chalky drag of the filters
• Inert and insoluble: it sits on the surface and leaves with your cleanser

Concentration

No figure. The level follows the finish a formulator is after, and the cost — it is expensive next to talc or starch. There is no threshold at which it starts working and none at which it does something else. In a pressed powder it is often present for cohesion as much as for optics — enough of it and the cake binds rather than crumbling, which is a texture decision as much as a cost one.

Conflicts & combinations

Nothing conflicts with it. The trade-off it sits inside is worth knowing: put it with mineral UV filters and it improves how they spread and how they look, but every extra white powder adds to the cast, and a sunscreen's protection is measured on the finished product rather than assembled from its parts.