Hair Care and Hairdressing: Structure, Services, and Chemical Safety
Hair structure and growth
The hair shaft has three layers: the cuticle (overlapping protective scales), the cortex (the bulk of the fiber, holding pigment and tensile strength), and the medulla (a central core that may be absent in fine hair). Growth cycles are anagen (active, roughly 2–6 years), catagen (transition, about 2–3 weeks), and telogen (resting/shedding, about 3–4 months). Persistent shedding or scalp disease is outside your scope: refer to a physician rather than guessing.
Service fundamentals
Drape every client with a cape and towel, and keep chemical products off the skin. Shampoo with cool-to-warm water; hot water accelerates color fading and irritates the scalp. Section hair cleanly for cutting, use tension suited to the hair type, and keep shears sharp. For thermal styling, use lower heat on fine or damaged hair and always apply a heat protectant.
Chemical services
Patch-test oxidative color and lighteners 24–48 hours before the service. Strand-test for processing time, porosity, and elasticity. Relaxers go on new growth only, never overlap, and must be thoroughly rinsed and neutralized to restore pH. For color, match developer volume to the goal: 10 volume deposits, 20 volume lifts about 1–2 levels, and 30 volume lifts about 2–3 levels.
Worked example
A client with fine, porous hair requests a two-level lift and mentions itching after a previous color. Reasoning: perform a patch test now and postpone if positive; choose 20-volume rather than 30-volume; strand-test; process by visual check, not the clock alone.
Traps that cost marks
Confusing cortex with cuticle; skipping patch or strand tests; overprocessing fine hair; failing to neutralize a relaxer fully; using hot water immediately after color.