A standard perm uses ammonium thioglycolate to break the natural disulfide bonds that create the natural texture of hair while it's in curlers. Then, a neutralizing agent stops the bond-breaking process and allows the hair to form new bonds that lock it into the shape of the curlers.
Hairdressers often use hydrogen peroxide as a neutralizer in a traditional cold perm. When applied, hydrogen peroxide enables the keratin molecules in the hair to form new bonds. This process happens while the perm sets over time.
Beauty professionals can also choose other perm styles. The acid perm uses heat and an alternative reducing agent that damages the hair less and produces better curls.