Chemistry: keratin, bonds, pigment

Bonds

One strong bond, two weak ones

Neighbouring keratin chains are tied together in three ways. Sulphur bridges, or disulphide bonds, are covalent and strong. Hydrogen bonds and salt links are weak.⁴⁷

Water breaks most of the weak ones. Wet scalp hair needed about four times less force to start stretching, and broke at about a third less. Heat helps the weak bonds move too: dry hair softens at 144 degrees, but at ordinary room humidity a change shows near 55.⁷⁸⁹

This is why a heated curler or a wet lash only changes shape for a while. The weak bonds settle in the new position and let go again the next time the lash is wet.⁴

In the imaging study, women who wore mascara more often had more cracking in the outer layer of their lashes. Use of a mechanical curler showed no measurable effect. It was a correlation in 36 women, not a test.¹⁸

Strength

How much a hair can take

Pulled until it breaks, a hair holds 150 to 270 million pascals, depending on humidity and speed. Measured on fibres from 13 donors, it behaves like a spring up to about 3 percent stretch, then yields and flows, then stiffens again, and stretched as much as 50 percent before breaking.¹⁰¹¹

The long flat part is thought to be the spirals of keratin unwinding. A wet hair can be stretched about 30 percent without lasting harm.¹⁰⁵

No one has measured an eyelash this way. Every figure here comes from scalp hair, and the same is true of almost every claim made about what treatments do to lashes.