Chemical Services (Permanent Wave, Relaxing, Color)
Chemical services all come back to hair structure and pH. The cortex holds the disulfide bonds that give hair its shape; the cuticle is the protective outer layer. Any service that changes shape or color must lift the cuticle, alter the cortex, then restore the cuticle.
Permanent waving uses an alkaline reducing agent, usually ammonium thioglycolate, to break disulfide bonds; the hair is wrapped on rods wet, processed, rinsed, then neutralized with a hydrogen peroxide or sodium bromate product that rebuilds the bonds in the new shape. Neutralizer is not shampoo.
Relaxers are also reducing/alkaline products. Sodium hydroxide (lye) relaxers are high pH (roughly 12-14); no-lye guanidine or calcium hydroxide relaxers run lower; ammonium thioglycolate "thio" relaxers are milder and need no neutralization step. Always do a strand test, protect the scalp with base, never relax over abrasions or a previously relaxed scalp irritation, and use a neutralizing shampoo. Do not relax and color on the same day without specific product guidance.
Color categories: temporary (large molecules coating the cuticle), semi-permanent (direct dyes, deposit only, no lift), demi-permanent (low-volume developer, deposit with slight tone change), and permanent (oxidative dyes plus developer, lifting and depositing). Know the color wheel - blue neutralizes orange, violet neutralizes yellow, green neutralizes red - and levels 1 (black) through 10 (lightest blonde). Developer volumes: 10 (3%), 20 (6%), 30 (9%), 40 (12%). Higher volume means more lift, not more color deposit. Patch test 24-48 hours ahead and strand test every time.
Trap: exam items often claim semi-permanent color lifts or that volume equals color intensity.