Ancient Egyptian hair dye - technology finally catches up

Ever wondered why the people in Egyptian tomb paintings all seem to have such lovely lustrous black hair? Apart from the fact that the well-off wore wigs, of course. It turns out that the ancient world used a hair dye that - apart from the unfortunate drawback of being toxic - was several thousand years ahead of its time. I’m indebted to Philip Ball of Homunculus for an explanation of this ancient technology. Annoyingly, the American Chemical Society website with the original research report isn’t open-access and I haven’t got a subscription, but there’s enough information in the Homunculus post to work out the basics.

First, a little background on hair structure. Hair is made up mostly of a protein called keratin, which is the same material that makes up hooves, claws and fingernails. Each keratin molecule is a long slender thread coiled to form a cylindrical helix, exactly like a spring. This is why hair is flexible and will stretch before it breaks - the keratin molecules can uncoil and stretch out before springing back into their original shape, like a spring. Four of these keratin springs are twisted together to form a rope, called a protofibril, and eleven of these protofibrils are twisted together to form a thicker rope, called a microfibril. Microfibrils are packed together in long thin bundles called macrofibrils, the macrofibrils pack together to form long thin cortical cells, and the cortical cells pack together to form a single hair. There is a neat little animation here showing how all this works. The ropes are held together by chemical bridges between sulphur atoms in adjacent keratin molecules. You need a lot of these sulphur bridges to hold the many millions of keratin molecules together in a single hair, so hair contains a lot of sulphur. This high sulphur content is why hair and feathers smell so disgusting when burned - many sulphur compounds smell foul (hydrogen sulphide, the characteristic stink of rotten eggs, is a good example). The more sulphur bridges, the stronger the structure - so fingernails have more sulphur bridges than hair and are therefore stronger, even though they are made of the same keratin protein. Permanent wave and hair straightening products work by breaking these sulphur bridges and then reforming them while the hair is pulled into a different shape.

With me so far? Good. The ancient hair dye recipe is as follows: Mix lead oxide with slaked lime (a strong alkali) and water to make a paste. Rub this paste into the hair for three days. After this time, the hair will be dyed black throughout, and the dye won’t fade or wash out.

The alkali component of the dye breaks some of the sulphur bridges holding the keratin molecules together, and the sulphur then reacts with the lead oxide to form lead sulphide. Lead sulphide is a dense black substance that’s insoluble in water - it’s similar to the black corrosion that forms on silver jewellery. The clever part of the dye recipe is that the lead sulphide forms tiny crystals that attach to individual keratin microfibrils, deep within the structure of the hair. This makes the dye deep, long-lasting and permanent. It’s also something that materials science has only recently learnt how to do.

The drawback is that lead is poisonous. Homunculus comments that this rarely bothered people in the ancient world, since lead compounds were used in cosmetics (and paint, until very recently). This may be because life expectancy was shorter then, as he points out. Lead is a slow cumulative poison and it would probably have taken years to accumulate a lethal dose from face paint or eyeshadow, quite possibly more years than the average lifespan. It is also often very difficult to recognise the connection if the effects are delayed, as would be the case with chronic lead poisoning. If you have been using lead cosmetics for 40 years before you fall ill, your face paint or hair dye isn’t the first place you look for the cause of illness. It took Richard Doll years of rigorous epidemiological research to demonstrate a connection between smoking and lung cancer, and I don’t suppose he had a counterpart in Ancient Egypt. Moreover, the ancient Egyptians were great users of wigs, and some examples have even survived to this day - the British Museum has some in its collection (the link is to a picture gallery, click on page 6). A wig would only need to be dyed once as the dye would not grow out, so exposure to lead from a hair dye used on wigs would be much lower than the same dye used on natural hair. It seems quite possible to me that the lead hair dye might not be sufficient to cause poisonous effects if it was applied to wigs rather than to hair (unless there was some poor slave whose task it was to dye wigs all day, which would surely be a candidate for Worst Job in History).