Lightening and bleaching hair
There are two main forms of bleaches - emulsion and powder - and both types are mixed with hydrogen peroxide.
Emulsion bleaches have the advantage of being particularly suitable of full head bleaches because they have a cooling agent added to them. They also have an excellent consistency that prevents them from dripping and drying out. However, they have a disadvantage of expanding whilst the bleach is workig which can cause the product to seep onto other areas of the hair.
Powder bleach is particularly suitable for partial colouring techniques, as it does not expand quite as much as emulsion bleach. If used for full head bleach it could cause scalp irritation.
Bleaching is the process of making the hair colour lighter. Bleaches are alkaline and so have the effect of opening the hair cuticles on the outside of the hair shaft, enabling the bleach to reach the cortex where the natural colour pigment is found. The colour pigment eumelanin is the first to be acted on. This affects the black and brown colouring. The more difficult to alter is pheomelanin, which gives red and yellow colouring. As bleaching proceeds, the hair becomes lighter and lighter and the hair changes colour in this order black - brown - red brown - orange - light yellow - very light yellow.
Owing to the alkalinity and strength of bleaching products, oxidation damage to the hair is inevitable. The cuticle will become more porous and the internal structure of the hair will become weaker and lessen the hair's elasticity and strength. Over-bleaching can result in such weakening of the hair that it will actually break off.