Why do metals and minerals in hard water cause brassiness and dullness?

Unfiltered tap water is full of hard water minerals like magnesium and calcium, as well as metals like iron and copper. These metals (iron and copper) and minerals (calcium and magnesium) cling to your hair, especially if it’s bleached or damaged, creating a dull, invisible build-up. Iron and copper are the colour‑shifters. They are especially attracted to damaged blonde hair and can create undesired brassy tones. 

When the iron and copper react with sunlight or heat, they oxidise, which intensifies yellow or orange tones and makes hair appear even more brassy. Iron and copper also often interfere with salon chemical services because they react with peroxide during coloring or bleaching, creating free radicals that cause uneven lift, breakage, and unwanted tones like green or orange.

Calcium and magnesium from unfiltered tap water build up as mineral deposits on the hair shaft, creating uneven application of dyes or treatments in the salon and leaving hair dull. These don’t change hair color directly, but they leave a build-up that may contribute to hair looking dull and feeling dry.