Hairdressing and Hair Care
Hair structure and chemistry
The hair shaft has three layers: the cuticle (overlapping protective scales), the cortex (keratin bundles and melanin), and the medulla (central core, often absent in fine hair). Hair is acidic at about pH 4.5–5.5; alkaline products raise the cuticle and increase porosity. Porosity and elasticity tests guide service selection: high-porosity hair absorbs and loses moisture quickly, while normal wet hair stretches about 50% and returns without breaking.
Colour and chemical services
Perform a patch test 48 hours before oxidative dye and a strand test for development timing. Developer volume controls lift: 10 vol deposits colour, 20 vol lifts 1–2 levels, 30 vol lifts 2–3, and 40 vol lifts 3–4. Lye relaxers use sodium hydroxide at pH ~12–13; no-lye relaxers use calcium hydroxide/guanidine at pH ~9–10. Neutralizing shampoo lowers pH and stops processing. Never relax over previously relaxed hair without assessing overlap and strength.
Worked example
A client with 60% resistant grey hair requests a level 5 ash brown. Reasoning: resistant grey needs a grey-coverage formula; choose a natural/gold base to avoid an ash-green result, use 20 vol rather than 40 vol on the scalp, pre-soften if directed, and strand-test for timing. Apply to regrowth first, then pull through for the last 5–10 minutes.
Traps that cost marks
- Skipping the 48-hour patch test or strand test.
- Overlapping relaxer onto previously relaxed hair, causing breakage.
- Using 40 vol on the scalp for a deposit-only result.
- Misjudging porosity: high-porosity hair processes faster, so check early.