Hair color changes caused by dyeing and thermal treatments

The aim of this study was to show the effect of heat exposure, dyeing and shampooing on hair color as measured by diffuse reflectance spectrophotometry. Successive dyeing of virgin hair with six permanent commercial formulations showed that color saturation was obtained after the first dyeing cycle. An unexpectedly high difference in hair color saturation, measured as DE* values, was obtained for virgin hair samples that differed only in cleansing history. After six sequential washings of the dyed hair samples, no difference was observed in color durability, indicating that the adhesion strength is similar to long-lasting and tone-up dyeing formulations. Exposure to a hot plate at 172 °C showed a significant darkening of the virgin hair samples after 2 min. On the other hand, virgin hair samples exposed to the gentler heat of a hand dryer (~60 °C) showed partial disappearance of the hair medulla after 60 min. However, values of total color difference were near the error limit.