Modification of the structure of keratin in an elongated hair before its breaks.
While it is not obvious when handling a single hair, you only need to try to break a small lock to be convinced: hair is extremely strong.
The organisation of keratin within its cortex allows it to resist a strain of up to about a hundred grams. A lock of 100 hairs can thus withstand a weight of 10 kilograms. As to the average head of hair, it could withstand 12 tons, if the scalp were strong enough!
Before breaking, a hair undergoes changes. For example, by delicately handling a reasonably long hair it can easily be shown that it behaves like a piece of elastic; after extending slightly, it returns to its original length.
Thus, for lengthening of up to 5%, hair is elastic. This is due to the structure of the keratin molecule. Called keratin a in its natural state, stretching arranges it into keratin b. When the stretching stops it returns to its initial form like a spring. Then the hair enters a condition known as flowing where, almost without effort, it can elongate by 25%: keratin a unwinds as keratin b.
Beyond that, keratin b begins to resist. However, in this phase before breaking, the hair can still be elongated and it often breaks only after its length has actually doubled!
Beyond the elastic phase, the hair has another property: at least for a while, it keeps the shape it has been given. Thus, if a hair is wound around a pen and after several hours the pen is removed, the hair retains its curled shape. This is known as the plasticity of hair. In combination with water and heat, this property allows temporary modification of the hair’s shaper by using, for example, the technique of blow-drying.
Breaking under a strain of 60 grams after an elongation of 40%, African hair seems to be the most fragile. At the other end of the scale, Asian hair is the strongest, withstanding a weight of 100 grams and an elongation of 55%. For either of these features Caucasian hair occupies an intermediate position.
However, by extrapolating these various measurements to equal shapes and surface areas, scientists can show that hair from these three ethnic groups behaves in an intrinsically comparable fashion and can thus confirm a common structure for all the world’s hair.
Another factor influences the properties of hair, namely water.