A 2000-year-old recipe for hair dye shows Ancient Greeks and Romans used nanotechnology to permanently colour grey hair black, say experts.

Dr Philippe Walter of the French state museum agency's Centre for Research and Restoration and colleagues report their findings online in the journal Nano Letters.

The researchers made up a batch of dye according to a recipe used since Greco-Roman times, which includes a mixture of lead oxide and slaked lime.

They soaked 50 milligrams of blond human hair in the dye for three days, then studied the hair closely.

The hair turned progressively black and when the researchers took cross-sections of hair and studied it under the microscope they found nanocrystals of lead sulfide inside the hair shaft.

The lead in the lead oxide had reacted with sulfur from the amino acids found in hair keratins, the scientists say, giving the black colour.

They say the 5 nanometre lead sulfide crystals look very much like lead sulfide quantum dots that are made today by advanced materials science methods.

The researchers say their discovery might help develop new mineral-based nanomaterials.

Dr Ivan Kempson, a materials scientist and research fellow at the University of South Australia, is impressed with the research.

"It's the highest resolution and most detailed study of the incorporation of a metal like lead into hair," he says.

Kempson says the findings are interesting for his own work, which looks at how hair takes up metals from the environment.

But he says it is not yet clear how the sulfur in the keratin is made available to interact with the lead in the hair.

Kempson says the research is also of interest to the cosmetics industry in developing hair dyes.

"If you know how they penetrate the hair and how they react within the hair then you can develop better cosmetic products," he says.

The research team includes a member from L'Oréal cosmetics company.

Source: News in Science