Hair colouring products generally fall into four categories: temporary, semipermanent, deposit only/demi, and permanent. All these hair colour products, except for temporary colour, require a patch test before application to determine if the client is allergic to the product.

“Hair lightening,” often referred to as “bleaching” or “decolorizing,” is a chemical process involving the diffusion of the natural colour pigment or artificial colour from the hair. This process is central to both permanent hair colour and hair lighteners.

All permanent haircolour products and lighteners contain both a developer, or oxidizing agent, and an alkalizing ingredient as part of their ammonia or an ammonia substitute. The purpose of this is to: raise the cuticle of the hair fiber so the tint can penetrate,
facilitate the formation of tints within the hair fiber, bring about the lightening action of peroxide.

When the tint containing the alkalizing ingredient is combined with the developer (usually hydrogen peroxide), the peroxide becomes alkaline and diffuses through the hair fiber, entering the cortex, where the melanin is located. The lightening occurs when the alkaline peroxide breaks up the melanin and replaces it with new colour.

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