This Rock Art in China Has Remained Bright Red for Nearly 2,000 Years. How? The Secret Ingredient Might Be Blood

For nearly 2,000 years, vivid red rock art has adorned the karst limestone cliffs of southern China, depicting drums, bells, weapons, animals and human figures. Despite being regularly blasted by wind, rain, heat and humidity, many of the ruby murals have survived largely intact for centuries—a feat that has long puzzled scientists. Now, a team of researchers says it may have uncovered the secret behind the rock art’s durability.

A study that will be published in the September issue of the Journal of Archaeological Science suggests blood—possibly including blood collected during human childbirth—was a key ingredient in the paint used to create the enduring images. The paintings, known collectively as the Zuojiang Huashan Rock Art Cultural Landscape, are in China’s Zuojiang River Valley, near the country’s border with Vietnam. They’re spread across more than 80 sites spanning roughly 160 miles.

Archaeologists suspect the art was created between the fifth century B.C.E. and the second century C.E. by the Luoyue people, a women-led fisher-hunter-gatherer society that practiced matrilineality, a system in which descent and property are traced through the mother’s side of the family. To figure out how the art gallery has survived so long in such harsh environmental conditions, researchers collected and analyzed five fallen fragments of the paintings from three sites. The team used several techniques to analyze the samples, including how they interact with infrared light, their appearance under a high-powered microscope and their protein makeup.

Analyses hinted that the Luoyue artists created their long-lasting rock art in two steps. First, they gathered animal bones or bird droppings, as well as limestone or shells. They heated these materials to produce a mixture of calcium phosphate and quicklime, then streamed in enough water and blood to create a “milky emulsion of mortar,” the researchers write in the paper.

They used this mortar emulsion as a primer of sorts, painting a base layer directly onto the limestone cliffs. Once this layer had partially dried, they painted over it with a pigment solution made from water and local red clay rich in iron oxide. As the slaked lime reacted with carbon dioxide in the air, the blood proteins helped form calcium carbonate, securing the pigment to the rock.

The Luoyue’s mortar created such a strong bond with the limestone that, hundreds of years later, researchers had to use an electric drill to separate the paint layers from the base rock, they write in the paper. By contrast, the rock’s natural sediments, without any paint, were only weakly bonded to the surface. A closer look at the blood revealed that around 97 percent of the detected proteins in samples from one site came from humans.