Permanent hair colour
Permanent hair colour is, as it says, PERMANENT. It can be good for correcting unwanted colour (covering white hair), altering the tone and depth of natural colour, for fashion or alternative colour. As it is a permanent hair colour, the colour will not fade, but regrowth will need to be recoloured.
Permanent colour or tints are mixed with the active ingredient hydrogen peroxide to create what are called oxidation dyes.
Permanent colour molecules are much smaller than semi-permanent colour molecules. Being alkaline, permanent colour opens up the cuticle scales allowing the very small colour molecules to enter into the cortex. The small molecules then attach themselves to hydrogen peroxide to create much larger molecules. These larger molecules become trapped inside the hair shaft and cannot escape no matter how many times the hair is washed. This process is called oxidation and is the basis of all permanent colouring or tinting. (see diagram)
Heat will effect the processing time. Cold temperatures will slow down the processing time and hot temperatures will speed up processing time. If atmospheric conditions appear to be cold then additional heat may be required. If conditions are hot then colour will take quicker. The method of application maybe effected by hot temperatures too. i.e. If temperature appears to be hot then application may need to start from mid-lengths and ends (as in virgin hair) due to the additional heat given off by the clients scalp.
If the hair is porous the processing time may be shortened due to hair damage. i.e. if ends are porous delay colour application until the last 15-30 minutes.