get the lead in

Cpp_avatarI recall reading an article asserting that women living in the same house or dorm would end up with synchronized menstrual cycles.  I'm thinking that women who blog together must have some universal rhythm too, since I started working on a "lead post" a week ago – well before Lee posted hers yesterday.  Luckily, lead is a dense enough topic that the only overlap between our posts is the Romans.

My inspiration wasn't a half-nekkid HughJackman (see Lee's post), although that is an inspirational vision.  I was trying to find out whether L'Oreal sponsored programming on PBS, as I'm looking for a funding source for a program I'd like to do on cosmetics.  What came up on was a paper from Nanoletters by a group of researchers from L'Oreal R&D (in France) about nanoparticles of lead sulfide (PbS) used by the ancient Greeks and Romans for dying hair black.

Lead has been a cosmetics component for a very long time.  Lead white (lead carbonate or 2PbCO3·Pb(OH)2 + PbCO3) was used for foundation by the Hellenes, which is pretty impressive because it takes a lot of chemical synthesis.  Pliny the Elder described how to prepare it from metallic lead and vinegar in a paper in a very early edition of JACS.  The Volos museum in Greece has power compacts from the end of the 4th Century B.C.  Lead white also used to be common in paints, but (as Lee points out), it poses a risk of lead poisoning.  The Romans just used talc and gypsum – as did I the one time my husband and I tried drywalling. 

The natural look is in now, thankfully, but it surprised me that the many of the Greco-Roman recipes are still being used.  Mix lead oxide (PbO) with slaked lime (Ca (OH)2) and a small amount of water.  The resulting paste applied to gray or light-colored hair and, after 24-72 hours, turns the hair black.  Grecian formula is Keratin lead (II) acetate (Pb(CH3COO)2) and it works the same way as the ancient formula.  (Just for Men hair coloring does not use lead acetate – but it doesn't work as gradually as Grecian formula.  Or so I'm told.)

The protein alpha-keratin makes up most of your hair (as well as your fingernails).  (Beta keratin makes up harder things, like bird beaks, reptile claws and scales). Keratin
is a long coiled molecule that acts like a spring, as shown in yellow in the picture at left.  Four keratin springs twist together to form a protofibril.  Eleven protofibrils twist together to
form a microfibril, which is the largest structure shown in the diagram at left.  Keratins have lots of the sulfur-containing amino acid cysteine and that allows the formation of disulfide bridges
that hold together keratin molecules.  Think of a disulfide bridge
as two keratin molecules holding hands.  The bridge forms a much
stronger structure than the two individual molecules.  Sulfur bridges
do the same thing in vulcanized rubber. HairStructure

Microfibrils pack
together in long thin bundles called macrofibrils.  Macrofibrils
pack together to form long thin cortical cells, and cortical cells
pack together to form a hair. Human hair is about 14%
cysteine and most of it is in the grey area in the figure that
separates macrofibrils.  There is a fair amount of sulfur in hair,
which is why burning hair is one of the absolute worst smells in the
world – even when it is someone else's hair.  Permanent curling and
straightening products break sulfur bridges, reshape the hair, and
reform the bridges so that the hair adapts the new shape.To give you an idea of size, the average hair is about 70,000 nanometers.  A macrofibril is roughly 7 nanometers in diameter. L'Oreal has a great animation
showing the composition of the hair. The picture at right (from the L'Oreal animation) shows the macrofibrils (the smallest structures visible in that shot) and the cortical cells that make up a hair.

The little black things you see within each macrofibril is melanin, which is responsible for giving you your hair color.  Large star-shaped cells called melanocytes reside at the bottom of the hair follicle and manufacture the melanin, which is incorporated into the hair structure as the hair is formed. Only about 1% of the hair is melanin, so it doesn't take much to give it its color.

There are only two types of melanin.  Eumelanin is rice-shaped and comes in brown and black varieties. Phaeomelanin is irregularly shaped and imparts a pink to red hue. Japanese hair contains mostly eumelanin and red hair is rich is phaeomelanin.  Black eumelanin is in mostly non-Europeans, while brown eumelanin is in mostly young Europeans.  A small
amount of brown eumelanin in the absence of other pigments makes hair blond.  A small amount of black
eumelanin without other pigments causes grey hair. With no melanin, hair is white, although we don't know yet whether that is because your body stops producing melanin, or if it just isn't incorporated into the hair.

When the coloring formula interacts with the hair, the lead in the colorant combines with the sulfur in the hair and forms nanoparticles of lead sulfide (PbS) with diameters between 4 nanometers and 15 nanometers.  In contrast, natural melanin that produces black hair is about 300 nanometers in diameter.  The longer you leave the formula on the hair, the more nanocrystals you form and the blacker the hair looks.  This is why coloring products like Grecian formula slowly change the hair color and can get rid of grey gradually, unlike permanent color.  The PbS nanocrystals are very small and it doesn't take a lot of them to change the apparent color of the hair, so the mechanical properties of the hair aren't really affected.

Why do they form nanocrystals and not microcrystals?  One theory is that peptides – polymers that surround the organized keratin proteins – form nanoreactors that limit the size of the PbS nanoparticles.  The nanoparticles accumulate preferentially at the boundaries between the microfibrils, whereas melanin colorants are randomly distributed throughout the hair.

L'Oreal presents the L'Oreal-UNESCO awards each year to outstanding women scientists across the world:  one in African/Arab countries, one in Europe, one in North America, one in Asia, and one in Latin America.  The awards recognize the important role science plays in their industry and in the rest of the world.  They also offer a variety of awards and fellowships for women at other stages in their careers.. because we're worth it.

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