Tooth blackening, historically practiced across parts of Asia and the Pacific, was often both aesthetic and cultural rather than medicinal. Chemically, the process typically involves applying iron or plant-based compounds that react with the enamel surface, creating a durable dark coating. Beyond appearance, it may have offered incidental protection against decay, as the acidic or metallic compounds could inhibit bacterial growth, though this was likely a secondary effect. The practice also carried strong social and symbolic significance, signaling age, marital status, or status within a community. Modern analysis of ancient remains, like the 2,000-year-old skull in the study, helps us understand how these cultural rituals intersected with early forms of oral care and material science, showing that even cosmetic practices often had practical dimensions in historical contexts.
Tooth-blackening (often called "ohaguro" in later Japanese contexts, with related practices across parts of Southeast Asia) typically works by creating a stable, dark metal-organic film on the tooth surface rather than "staining" enamel like food pigments. A common mechanism described in the broader literature is the reaction of iron salts with plant-derived tannins/polyphenols, forming iron-tannate complexes that appear black. Those complexes can also bind to the acquired pellicle and plaque matrix and, over time, can become fairly tenacious as they interact with calcium and phosphate at the enamel surface. From an oral-chemistry perspective, what's most interesting is why this may have been protective in some populations: tannins can have antimicrobial and protein-precipitating effects, and iron-polyphenol films can behave a bit like a primitive sealant by reducing diffusion at the surface and altering the local chemistry that drives demineralization. It's not the same as fluoride-driven remineralization, and it wouldn't prevent all disease, but it's plausible that repeated applications produced a durable barrier that changed bacterial adhesion and acid attack. When I read studies like this, I look for supporting signals such as reduced caries prevalence relative to controls, evidence of a distinct surface layer under microscopy (SEM/EDS), and confirmation of iron-rich deposits via spectroscopy (e.g., Raman/FTIR/XRF), because those triangulate "what it is" with "what it likely did."
Hi Liz, My name is Dr. Afsoon Fazeli, a pediatric dentist at Discovery Children's Dentistry & Orthodontics. I thought I might be able to offer a useful perspective on the chemistry of tooth-blackening. In our clinic, we use a treatment called silver diamine fluoride (SDF), which intentionally darkens decayed areas of a tooth to stop a cavity from getting worse. Simply put, the silver in the SDF reacts directly with the damaged part of the tooth, which is what causes the color change. This is why the blackening is contained to the cavity and doesn't spread across healthy enamel. If you're looking for some background reading, searching for "silver diamine fluoride mechanism of action" in peer-reviewed journals is a great place to start. Major dental organizations also have summaries that explain the science for a general audience. I'd be happy to share how we explain this process to families in plain language, if that would be helpful for your piece. Afsoon Fazeli, DDS
Tooth blackening was not an accident or a sign of poor health. In many parts of Southeast Asia and in Japan, people darkened their teeth on purpose because it was seen as beautiful and a sign of adulthood. In Japan this practice was known as Ohaguro, where a liquid made from iron and plant extracts was brushed onto the teeth. The iron reacted with natural compounds and formed a dark coating that stuck to the enamel. Outside researchers have pointed out that this coating could also help protect teeth from decay, almost like a basic seal. So when scientists find blackened teeth on a two thousand year old skull, it is likely not random staining. It was probably a cultural choice, similar to how people today dye their hair or get cosmetic treatments to express identity and fit social ideals.
Founder & Medical Director at New York Cosmetic Skin & Laser Surgery Center
Answered 2 months ago
I am a board certified dermatologist and laser surgeon in New York, and I spend a lot of time thinking about how chemistry changes the way tissues look. In that Archaeological and Anthropological Sciences paper, the team found a pigment layer on enamel with a diagnostic signal rich in iron and sulfur. They tie the black color to iron salts likely mixed with tannin rich plant material, backed by lab replication and non destructive testing on the ancient teeth. Here is the simple chemistry in plain English. Iron ions bind to tannins and form an iron tannate complex. It darkens further as the iron oxidizes, and the final pigment is far less water soluble, so it can stay put on the surface.