* Underlying Pigment: Hair naturally contains warm undertones, primarily red and orange pigments (pheomelanin). When you lighten hair, you're essentially lifting (removing) the existing color. If the lightening process isn't strong enough or isn't left on long enough, it might not fully eliminate these warm undertones. This leaves the hair with a brassy or orange/ginger hue.
* Incomplete Lightening: To achieve a true blonde, the hair needs to be lifted to a pale yellow stage. If the hair is only lifted to a dark yellow or orange stage, the resulting blonde will have a brassy or ginger cast. This can happen if the developer volume is too low, the bleach is not potent enough, or the hair is resistant to lightening.
* Oxidation: After bleaching, the exposed pigments can react with oxygen in the air and water, leading to further oxidation and brassiness. This is why using a toner after bleaching is crucial to neutralize unwanted warm tones.
* Water and Product Buildup: Minerals in hard water (like iron or copper) and certain hair products can deposit on the hair shaft and contribute to brassiness over time.
* Sun Exposure: Prolonged exposure to sunlight can also oxidize the hair and bring out warm tones.
In summary, the "ginger" or brassy tone is a result of the incomplete removal of the natural warm pigments during the lightening process, compounded by environmental factors or product buildup. Using the right lightening products, proper techniques, toner and color-safe products can help to achieve and maintain a true blonde color.