Answer:
Heat opens up the cuticle, much the same way ammonia does chemically. This way some dyes will suggest covering your head or applying heat while the color sets. The extra heat, whether from your own head or an external source like a blow dryer or steamer, optimizes the dye's processing for stronger results.
Explanation:
- The cortex is the main body of the hair shaft - it contains natural moisture and color pigment, which is the translucent cuticle.
- The outer layer of the hair shaft, its cuticle, must be opened before permanent color can be deposited into the hair. Once the cuticle is open, the dye reacts with the inner portion of the hair, the cortex, to deposit or remove the color.
