2,000-year-old skulls reveal people in ancient Vietnam permanently blackened their teeth — a stylish practice that persists today

Gleaming black teeth have been considered a high standard of beauty in parts of Vietnam since at least the late 1800s. But now, archaeologists have traced this practice back 2,000 years, discovering that ancient people used their abundant iron resources to dye their pearly whites black.

In a study published Jan. 22 in the journal Archaeological and Anthropological Sciences, researchers investigated skeletons from Dong Xa, an archaeological site in the Red River delta of northern Vietnam. The settlement at Dong Xa was occupied during the Iron Age (550 B.C. to A.D. 50), and the cemetery held numerous skeletons with unique dental colors. To figure out how people discolored their teeth thousands of years ago, the researchers nondestructively analyzed the skeletons' enamel using a variety of techniques.

When they targeted the colored areas of dental enamel using X-ray fluorescence, which measures the X-rays emitted by a sample to characterize its chemical composition, they found a high concentration of iron oxide, the researchers wrote in the study. Then, they used scanning electron microscopy with energy dispersive spectrometry (SEM-EDS). This technique involves bombarding a sample with electrons, which produces X-rays characteristic of the chemical elements in the sample. The researchers found that the discolored ancient enamel samples from Dong Xa were positive for iron (Fe) and sulfur (S).

"We believe that the combined presence of Fe and S signals is a strong indicator of the involvement of iron salts," study lead author Yue Zhang, an archaeologist at the Australian National University, told Live Science in an email. Nowadays, botanical materials are also used as part of the process to blacken teeth, so it's likely that finding traces of these on ancient teeth may also signify the practice, Zhang added.

One modern method of blackening teeth involves the combination of an iron-based substance with a tannin-rich plant material, such as betel nuts (Areca catechu). Betel-nut chewing has been popular for thousands of years among peoples of the Pacific and Southeast Asia, and prolonged use of the natural stimulant can stain a person's teeth and gums red or reddish-brown. But when tannic acids and iron salts are combined and exposed to air, they create a dark-black color.

Based on information from modern populations that blacken their teeth, the researchers suspect that the ancient blackening process likely took several days or weeks of application of an iron-tannin mixture to achieve the intensely dark shade. But once the process was completed, the person's teeth remained black throughout their lifetime, with touch-ups needed every few years to preserve their luster.

"The practice is still observed today, not only in Vietnam, but also more widely across parts of Southeast Asia," Zhang said.

While the precise procedures for tooth blackening likely changed over time, the underlying mechanism responsible for the dark coloration — the interaction between tannic acid and iron salt — was likely the same, according to the researchers. This means the presence of iron salt and sulfur on ancient teeth can be considered a diagnostic marker of purposeful blackening, they wrote.

"To our knowledge, our research on the Dong Xa teeth is the first to connect archaeologically discovered blackened teeth with modern intentional tooth-blackening practices," Zhang said.

But there are still unsolved questions surrounding why the practice of tooth blackening arose.

One possibility is that blackening was developed as a less-extreme version of tooth ablation, a practice that involves removing otherwise-healthy teeth as a rite of passage or as a group identification marker, the researchers noted. Another possibility is that blackening was invented to enhance the visual impact of the staining that resulted from betel-nut chewing.

Regardless of the original purpose, the researchers wrote, "tooth blackening plausibly became widespread around the Iron Age, when iron utensils became more accessible for producing blackening dye paste."

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