Perming and Chemical Straightening
Core concepts
Understand the chemistry of a perm: a reducing agent (often ammonium thioglycolate) breaks disulphide bonds in the cortex; the hair is then shaped on rods; an oxidising neutraliser (often hydrogen peroxide or sodium bromate) rebuilds the bonds in the new shape. Know that the same chemistry underlies chemical straightening, where the hair is smoothed rather than wound.
Know rod size and winding tension effects: smaller rods give tighter curls; larger rods give looser waves. Understand processing time depends on hair porosity, elasticity, previous chemical history, and product strength. Know test curls and strand tests before processing.
Know contraindications: recent lightening or bleach, metallic salts, damaged or over-porous hair, scalp lesions, and very recent chemical straightening. Know aftercare: no shampooing or heat styling for the manufacturer's advised period, use of sulphate-free and pH-balanced products, and warning about mechanical stress.
Traps
Questions often confuse the reducing and oxidising steps, or claim a perm can be done safely on freshly bleached hair. Another trap is ignoring the role of elasticity in processing.
Rule of thumb
Reduce, shape, neutralise: break the bonds, wind to shape, then rebuild them. Anything that compromises bond integrity (bleach, damage) is a contraindication.