Bleach:
Bleaching hair effectively strips away colour, and can be done for three reasons-Highlights- this is a cosmetic effect of lightening streaks, or sections of the hair to give a variation of colour over the whole head.
Blonding – usually achieved with a mixture of bleach and colouring agent, this process strips away the hairs colour and adding colour to the hair to achieve a blonde appearance. This combination of bleaching and colouring agents is also used in permanent hair colouring for darker, non blonde shades, and if the hair colour is to change by more than a few shades.
Pre colouring – as bleaching the hair gives a blank canvass any hair colouring applied afterward will effectively be brighter. This is particularly useful if a bright highly coloured effect is desired, or if semi permanent colouring is to be used. The bleaching process allows the semi permanent dyes to penetrate the hair more making the colour adhere better and last longer.
Pre colouring bleaching can be done using a peroxide rinse (a 1:1 mix of cream peroxide and normal shampoo). This minimises the lightening effects and the damage to the hair but gives a good base for further colouring.
How does bleach work?
Cosmetic bleaches use the oxidising effects of hydrogen peroxide (diagram on the right). A low volume of peroxide (5-30%) in a cream form, combined with powder bleach creates a paste (usually blue) which is applied to the hair and left until the desired amount of colour is stripped from the hair, then rinsed out.
Before the bleach can change the colour of the hair it must first be able to penetrate below the cuticle surface. This is usually done by mixing the bleaching component in an alkaline solution. This alkali (often ammonia (left))
swells the hair fibres casing the cuticles to separate and open allowing the bleach to penetrate the cortex of the hair. This cuticle opening effect is also important when colour is being added to or implanted into the hair.
The hydrogen peroxide and bleach reacts with the melanin within the hair. In an irreversible chemical reaction the peroxide oxidises the melanin rendering it colourless. Complete bleaching tends to leave hair a pale yellow colour rather than pure white. The yellow colour is of the natural colour of the keratin that makes up the hair fibre. The degree of colouring left after bleaching depends upon the natural pigmentation of the hair. Bleach reacts more readily with the dark eumelanin pigments than the lighter phaeomelanin pigment within the hair so some residual yellow and red tones may remain, depending upon the original colour of the hair.
If hair has been previously coloured with permanent (and to some extent semi permanent) dye the bleaching process may have varying degrees of effectiveness. Coloured hair will react differently to natural hair, resulting in a variation of unpredictable shades after bleaching. This is especially noticeable if your hair has roots showing (natural hair regrowth after colouring), as the sections of your natural hair will be lightened to a greater extent that the coloured hair, which in its self has implications in the results of any further colouring.
Unfortunately bleaching hair is very damaging to its structure. It is also often a very stinky process! Hydrogen peroxide breaks chemical bond in the hair releasing large amounts of sulphur, causing hardening, loss of mass and distinctive smell. This effect is minimised by the use of low levels of peroxide in cosmetic bleaches.
The oxidative process of bleaching causes further problems other that hair damage. The use of metal implements to apply bleach should be avoided as they can be oxidised, causing reactions in the bleach which can result in green hair.