Some 4000 years ago, the ancient Egyptians used lead to make eyeliners with medical properties. To better understand this practice, chemists from CNRS, UPMC and ENS Paris (1), working with the C2RMF (2), attempted to evaluate the impact of very small amounts of lead on skin cells.
Results show that, at low dosages, lead does not kill skin cells. Instead, it leads to the production of a molecule, nitrogen monoxide, which is known to activate the immune system. Applying lead-based eyeliners can therefore set off a defense mechanism which, for eye infections, limits bacterial growth. This phenomenon provides a partial explanation for why Egyptian eyeliners were used therapeutically. This study was published online in Analytical Chemistry.
Previous research has shown the complex nature of cosmetics used by Egyptians some 4000 years ago (3). Most-often lead-based, Egyptian eyeliners were made of a mixture of black galena (a lead sulfate) and of white substances, either natural or synthesized from lead salts. In their written works, Greek and Roman physicians made much of the role of these substances in eye care. Now that lead is more often known for its potential toxicity, this therapeutic usage may seem surprising.
What was the function of the lead salts? To answer this question, the researchers looked into laurionite, a lead chlorate among the salts synthesized by the ancient Egyptians, and at its action on an isolated skin cell. Laurionite can trigger the presence of Pb2+ lead ions in the eye or on the skin, at infinitesimally small (sub-micromolar) concentrations. Its activity on the cells of the epidermis (keratinocytes) was studied thanks to a modern electrochemical tool, the ultramicroelectrode.
This miniature device has a remarkable capacity to analyze the very weak signals produced by single cells. After depositing very small quantities of laurionite (4) solution on a keratinocyte, the scientists were able to observe the overproduction of tens of thousands of NO° nitrogen monoxide molecules.This radical (5) appears as a messenger from the immune system, playing a major role in the regulation of blood pressure. It stimulates the arrival of macrophages, cells which ingest bacteria, and favors their passing through capillary and blood vessel walls.
Conclusion: an Egyptian with an eye painted with black eyeliner had his/her tears enriched in Pb2+ ions, after the eyeliner dissolved slightly, which stimulated the production of macrophages. The latter created an inhospitable environment for any bacteria that may appear there accidentally. This explains the therapeutic properties of ancient Egyptian eyeliners, and makes it easier to understand why ancient Egyptians believed eyeliners to be emanations from the eyes of the protective gods Horus and Ra.