Hair structure and hair types

Hair is not just something that grows out of your head – it is a material with a specific structure and that structure determines how hair reacts to cutting heat colour and treatments. As a hairdresser your knowledge of hair is what distinguishes a professional result from a random one.

A hair is made up of three layers: the outer layer (cuticle/scale layer) the middle layer (cortex) and the core (medulla). The outer layer consists of overlapping scales which like roof tiles protect the hair. When the scale layer lies flat the hair reflects light and appears shiny. When it is lifted or damaged the hair appears dull and dry.

• 01Cuticle – the outer protective scale layer that opens during coloring and perming.
• 02Cortex – the middle layer where the hair's color pigment and strength sit; most of the action during coloring and perming happens here.
• 03Medulla – the core of the hair, most visible in thick hair.

Hair is often divided by shape and structure: straight wavy curly and kinky/Afro hair. Shape is determined by the hair follicle angle and the cross-section of the hair strand. Curly and kinky hair is typically drier because sebum has difficulty travelling down the twisted strand — this places special demands on treatment and product selection.

Porosity describes how easily hair absorbs and releases fluid. High-porosity hair (often bleached or heat-treated) absorbs color and moisture quickly, but also releases it quickly again. Low-porosity hair rejects fluid and requires more time. Assessing porosity and condition before a treatment is part of your professional diagnosis.

The same colour or treatment can give very different results on two customers precisely because hair type and condition are different. When you understand the structure you can predict the reaction, choose the right method and explain to the customer what is realistic — and what will damage the hair.