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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.
The 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.
Different Types of Hair Dye
There are a few different types of hair dye, and there’s generally an inverse relationship between damage and how long the dye lasts (the longer-lasting it is, the more damaging it’ll be):
- Permanent: causes some lightening of hair, contains 3-6% hydrogen peroxide, contains oxidative dyes that form during application, does not wash out significantly but may fade
- Demi-permanent: causes mild lightening of hair, contains 1-2% hydrogen peroxide, contains oxidative dyes that form during application, lasts around 6 weeks
- Temporary or semi-permanent dye (direct dye): no lightening of hair, does not contain peroxide, contain dyes that are already formed before application, washes out in a few washes
These terms are sometimes mixed up – you might see semi-permanent hair dyes that contain hydrogen peroxide, for example – so it’s always best to check the ingredients.
Permanent and Demi-Permanent Hair Dyes
The main difference between permanent/demi-permanent hair dyes, and temporary/semi-permanent dyes is the type of dye they contain. Permanent and demi-permanent hair dyes use oxidative dyes, which form in the hair when the dye is applied.
There are a number of steps involved in the use of oxidative dyes. 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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1. Opening up the cuticle – The overlapping cuticle scales need to be lifted so the chemicals in the dye can get to the cortex. This is done using an alkaline chemical, usually ammonia (the mixed hair dye is usually at a pH of around 10).
2. Bleaching the hair – To help the new colour show up more prominently, hydrogen peroxide is used to bleach the hair, making it lighter. Hydrogen peroxide reacts with the coloured melanin chemicals in the hair and turns them into colourless chemicals. If more lightening is required (e.g. for dark hair), this can be done as a completely separate step, with the addition of persulfate salts.
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.
In an oxidative dye, the ammonia and dye precursors are usually in one container, while the peroxide is stored in a separate container. These are mixed just before applying it to hair. That’s why the dye usually looks nothing like the colour you want at the start (it’s usually white), then the colour slowly appears as you use it – the dye precursors need to react before turning into the final colour.
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 🙂