Blood-induced biomineralized calcium carbonate used for binding in the Huashan rock art of Guangxi, Southern China

Rock art is the physical evidence of early human symbolism, distributed across the globe as a consequence of human dispersal. In this study, we analyzed the Huashan rock art, which contains more than 80 sites and is in good condition. Created at least 2000 years ago, it is located in Guangxi, southern China, an extremely hot and humid subtropical monsoon zone. The process for producing the rock art and how they have survived the weathering remain mysteries. Rock art fragments from three sites in the Huashan rock art belt were collected and studied. They were analyzed using Raman spectroscopy, energy-dispersive spectroscopy (SEM-EDS), Fourier infrared spectroscopy (FTIR), and proteomics to determine their structure and mineral and organic composition. Our study indicates that red clay rich in nano-hematite was used as the pigment, and a composite mortar made from lime and blood served as the binder in the rock art. Biomineralized calcium carbonate induced by blood proteins tightly encapsulates nano-hematite and strongly bonds to the limestone. This stable organic-inorganic composite bonding system reveals the complex technological processes used by the creators and explains why the Huashan rock art has survived so well in such challenging conditions for over two millennia. The blood used may come from the females who gave birth, which could be linked to the worship of female fertility of the creators' community, a matriarchal fisher-hunter-gatherer society. These findings provide new insights into the creation of prehistoric rock art, showing that it was a technically intensive project rather than a labor-intensive one. Introduction
Rock art is a unique form of human cultural heritage with a history dating back to the Paleolithic era. It is a typical product of human symbolism: most of the known creators were Homo sapiens, though a small number may be Neanderthals, our close relatives (Hoffmann et al., 2018). As Homo sapiens migrated (Nielsen et al., 2017), the rock art spread throughout the areas they reached (Henshilwood et al., 2018; Oktaviana et al., 2026; Pike et al., 2012; Finch et al., 2020; Romero Villanueva et al., 2024 ; Jalandoni et al., 2019). These humans used natural materials to produce pigments and binders, and painted them on rock as documentation or ritual observance. These prehistoric rock artworks have survived for thousands of years despite being exposed to a variety of natural environments, ranging from deserts to rainforests. The durability of these ancient man-made materials is astonishing. However, the process for producing the prehistoric rock art and the reasons behind its resistance to natural weathering remain not entirely clear. Some modern experimental rock art suggests that it is the result of a combination of rock art techniques and environmental factors (Ozán et al., 2020, 2024).
A rock art belt runs along the valley of the Zuojiang River in the southwestern part of the Guangxi Zhuang Autonomous Region of China (Fig. 1a and b). This rock art belt features similar red figures and stretches for approximately 260 km, comprising over eighty sites with a total painting area of tens of thousands of square meters (Wang et al., 1988). Since 2016, it has been on the UNESCO World Heritage List as the Zuojiang Huashan Rock Art Cultural Landscape, commonly known as the Huashan rock art, because the Huashan site is the earliest and largest of the sites discovered (Wang et al., 1988).
The main patterns of the Huashan rock art are abstract representations of humans, animals, and artifacts that are similar to bronze objects from the Yangtze River Basin and Yunnan cultures of China, such as ring-handled daggers, bronze drums, and sheep-horn knob bells (Qin and Qin, 1985). Therefore, it is speculated that the Huashan rock art dates from the late Chinese Bronze Age, spanning the Spring and Autumn Period to the Han dynasty (770 BC- 220AD) (Guangxi Zhuang Autonomous Region Institute of Ethnic Studies, 1987). Samples of stalactites measuring a few millimeters in thickness were taken from above and below the rock art at the Huashan site (Fig. 1b) for radiocarbon dating. The results indicate that the rock art was created between 2370 and 2115 years before present (BP) (Yuan et al., 1986), which falls within the time range inferred from the figure patterns. U/Th dating techniques have also been used to date seven Huashan rock art sites. The rock art at the Huashan site was dated to between 1856 ± 16 and 1728 ± 41 years BP (Shao et al., 2017). However, the resulting date ranges for other sites are broader than those obtained from the aforementioned art-archaeological studies (Shao, 2016).
The Huashan rock art belt is located in a subtropical monsoon climate zone and karst landform area, where it has been subject to intense natural weathering processes. It is mainly located within Chongzuo City, Guangxi, China. Chongzuo City currently receives an average annual rainfall of 1261 mm, mainly concentrated in the summer months from April to September. The average annual temperature is 22.1 °C, with extremely high temperatures reaching over 40 °C in July, while January is the coldest month with extremely low temperatures dropping to 0 °C. On average, the city experiences two to three typhoons each year (Chongzuo Municipal People’s Government website, 2025). The rock art was painted on cliffs dozens of meters above the ground or the river (Fig. 1). Despite being left completely exposed to hot and humid conditions for thousands of years without protection or repair, most of the pigments have remained firmly attached to the base rock. This suggests that the binders are highly resistant to environmental erosion. For this study, fallen rock art fragments from these rock art sites were collected to analyze the pigments, binders, and techniques used in the creation of the Huashan rock art.
The Huashan rock art belt belongs to the Pearl River Basin, where there is abundant rainfall and rich flora and fauna resources. Human livelihood patterns in the Pearl River Basin can be divided into three distinct phases: a fishing-hunting-gathering age (7.0 - 2.9 cal ka BP), a transitional period when fishing-hunting-gathering and primitive farming coexisted (2.9 - 2.0 cal ka BP), and an intensive farming age (last 2.0 cal ka BP) (Ju et al., 2024; Yu and Cui, 2021). The intensive farming age began when farmers and agricultural techniques from the Central Plains reached the Pearl River Basin through the military campaigns and political control of the Han dynasty (202 BCE - 220 CE) (Gong, 2019).
The Huashan rock art was made in the equivalent period from the Spring and Autumn Period to the Han dynasty (770 BCE - 220 CE) (Guangxi Zhuang Autonomous Region Institute of Ethnic Studies, 1987), which belongs to the transitional period of the Pearl River Basin. The ethnic groups inhabiting the Zuojiang River basin during this period were referred to by the Han Chinese as the Luoyue (骆越), as recorded in history books. Thus, the Luoyue people are believed to be the creators of the Huashan rock art (Wang et al., 1988). The Luoyue community practiced matrilineality, as recorded in the Book of the ‘Later Han · Biography of Ren Yan’(后汉书·任延传) (Fan, 446). The original Chinese text is as follows: ‘又骆越之民无嫁娶礼法,各因淫好,无適对匹,不识父子之性,夫妇之道’ It means that the Luoyue community lacked the concepts of marriage institutions and patrilineal kinship, and the Luoyue people did not have marital relationships or fixed sexual partners; their sexual relationships were based solely on personal preference. This is a typical matriarchal society model (Bachofen, 1861; Morgan, 1877; Engels, 1881). Actually, most indigenous peoples in China's southwest and Lingnan regions remained in matriarchal societies until the Han dynasty, according to the ‘Book of the Later Han: Biographies of the Southern Barbarians and Southwestern Savages’ (后汉书•南蛮西南夷列传) (Fan, 446). Ethnographic research has revealed that even in modern times, ethnic minorities in south-western China still adhere to many aspects of matrilineal systems (Zhang, 2002; Guan and Liao, 2006; Du, 1980).
Therefore, the creators of the Huashan rock art were indigenous peoples who inhabited the Zuojiang region between the 5th century BC and the 2nd century AD. They may have subsisted through fishing, hunting, gathering, and primitive farming, and lived in a matrilineal society.