Chemistry colour conundrum: Choosing a hair dye

Catherine Chan
11 February 2015

Above: Image © istockphoto.com/kraftmen

Did you know? Gradual hair colouring can only darken hair to a brown or black colour.The metal particles in the dye interact with the hair shaft to form metal sulfide, which accumulate over a period of weeks. Gradual? Semi-permanent? Natural? Have you ever stood in front of a shelf of hair products trying to figure out which dye option is best. It’s important to understand the chemistry of hair colouring because different products will colour your hair differently. In general, hair dyes are classified into four categories according to how long the colour remains in your hair:

Gradual colouring, normally used to colour grey hair, uses metallic dyes such as salts of lead, bismuth, or silver. These dyes cause a chemical reaction on the cuticle (the outermost layer of the hair) and then build up on the hair shaft. Particles from the dye often cause hair to becomes stiff, dull, and brittle. Using gradual colouring, it usually takes a few weeks for hair to reach the desired colour. Temporary colouring uses water soluble dyes (dyes that dissolve in water) made up of large molecules that normally can’t penetrate the hair shaft. Instead, they are temporarily deposited on the surface of your hair and eventually wash off with shampoo and water. Usually, this happens after just one shampoo. But , unless your hair has previously been chemically treated, which can cause the dye to penetrate beyond the surface of the hair shaft. Semi-permanent colouring uses mostly synthetic dyes that have a lower molecular weight (the mass of a singular molecule) than temporary dyes. This allows the colour to remain on the hair shaft longer. Semi-permanent colouring usually washes off after 6-8 shampoos. The smaller-sized particles in these dyes can penetrate freely into the hair shaft, increasing the likelihood of breakage. They are a good option if you’re looking for a temporary change, or you just can’t commit to a colour! Permanent colouring uses synthetic dyes and accounts for the majority of hair dye sales. The colour results from a chemical reaction that occurs between the hair shaft and the synthetic chemicals in the dye. An ammonia and hydrogen peroxide solution is often used to help the hair dye molecules penetrate the hair shaft by causing the shaft to swell up. However, the solution has a very strong odour and can irritate the scalp.

Natural versus synthetic hair dyes

Did you know? If you have a light hair colour, you can use lily stamens, belladonna flower, turmeric, or even beet juice to give your hair bright red, violet or gold tints.These natural dyes coat the hair shaft rather than penetrating it.There are both natural and synthetic colouring agents available. Henna is the most common natural dye and colours hair an orange-reddish shade. Vegetable-based dyes like Brazilwood can also be used to blacken greying hair. Natural colouring has a limited colour range and it’s difficult to predict what colour intensity you’ll end up with because the biochemistry of these natural substances is not easily altered.

Synthetic dyes therefore provides greater colour variety and predictability because their chemical compounds can be more easily manipulated. This is why semi-permanent and permanent hair dyes are almost always made from synthetic chemicals.

Whether you can’t make up your mind and change your hair colour every month or you just want to cover some light patches, there is a hair dye option for you. With all of the options on the market today, making an educated decision about what products to use on your hair can be daunting. Understanding the chemistry behind how they work can help you make a choice.

Research shows that hair dyes are generally safe to use. But they still might damage your hair by causing dryness, brittleness, or breakage. So consider your options and select the product that is the best fit for you!

Learn more!

Resources freely available online

A Brief Guide To Hair Dye: Semi-Permanent vs. Permanent vs. Bleach (2013)

Dana Oliver, Huffington Post

Report on the differences between different types of hair dye.

Does eco-friendly hair dye exist? (2012)

Lindsay Coulter, David Suzuki Foundation

Blog entry on environmental considerations related to hair dyes.

Taking on Hair Color’s Bad Guy (2010)
Catherine Saint Louis, The New York Times

Report on the use of ammonia and ammonia alternatives in hair dyes.

Other resources

C. Bolduc & J. Shapiro. 2001. Hair care products: waving, straightening, conditioning, and coloring. Clinics in Dermatology 19(4), 431–436.

Overview of hair structure and different hair treatments, including colouring. A one-page preview is available on the publisher’s website. A subscription is required to view the full text.

Catherine Chan

An aspiring science writer with a master's in materials science engineering from the University of Toronto.


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