Dyeing your hair is one of the easiest ways to change up your appearance. It’s quick and low maintenance, but the science behind it is actually very interesting. Here’s how they get the colour inside your hair so it doesn’t wash out!
Structure of hair
To understand what happens, first we need know what exactly a strand of hair contains. Each strand of hair is made up of three layers:
Medulla – This is the innermost core, and frankly, not very interesting.
Cortex – This surrounds the medulla, and contains most of the pigment (melanin) that gives hair its colour.
Cuticle – This is the outer layer, and is made of lots of overlapping dead cells (like roof shingles) which protect the inner layers of hair. When the cells lie flat, light bounces off and your hair looks shiny.
Steps in dyeing hair
There are a number of steps involved in dyeing hair permanently. These usually aren’t distinct steps, especially if you’re using a one step dye – they’re all happening around the same time.
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2. Bleaching the hair – To help the new colour show up more prominently, hydrogen peroxide is used to bleach the hair white. Hydrogen peroxide reacts with the coloured melanin chemicals in the hair and turns them into colourless chemicals. Sometimes this is done as a completely separate step for darker hair.
3. Adding dye precursors – Small precursor molecules, which later react to form the final dye, soak into the cortex through the gaps in the open cuticle.
4. Reacting precursors to form the dye – The precursor molecules react with each other, with the help of hydrogen peroxide and ammonia, to form the final, large dye molecules, which are too big to wash out easily.
5. Closing the cuticle – The cuticle is closed after dyeing, usually with an acidic conditioner. Some damage occurs during the opening/closing process – I’ve exaggerated it in the diagram above.
What about temporary/semi-permanent dye?
The difference between permanent and temporary dye is the all-important cuticle opening/closing process. With temporary dyes, the cuticle isn’t opened – the dye molecules are attached to the outside of the hair shaft, which means the colour washes off easily.
Semi-permanent dyes open the cuticle slightly – some dye sticks to the outside, while some of it gets inside to the cortex, so it takes quite a few washes for the colour to fade. As you’d expect, the less cuticle opening and closing you do, the less damaging the dyeing process will be – of course, many other things you do day-to-day will make a difference to how much damage your hair will experience overall.
And that’s how hair dye works! Hopefully it’ll give you something to think about next time you’re stuck in your hair colourist’s chair 🙂