Technological affinities between qinghui and carbonaceous mortars from Baocheng and Yaotai walls at Xianling Mausoleum
This study identifies a technological affinity between the dark-grey mortars from the Xianling Mausoleum's Baocheng and Yaotai walls and traditional Chinese *qinghui* materials by demonstrating that their unique lime-carbonaceous composition, characterized by poorly ordered carbonaceous matter, was intentionally incorporated for specific pointing and finishing applications distinct from the bedding mortars.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
In the world of historic preservation, the mortar holding ancient bricks together is often seen as mere glue, a silent background material that simply keeps a structure standing. Yet, to the materials scientist, this mortar is a time capsule. It records the specific choices of the builders: what they mixed, how they mixed it, and where they decided to use it. When researchers study these mixtures, they are not just looking at dirt and stone; they are reading a technical manual written in the physical fabric of a building. A key concept in understanding ancient Chinese construction is the idea of "qinghui," a traditional material that looks like a dark grey paste. Historical records describe it as a mixture of lime and carbon-based substances, used for specific jobs like finishing walls or filling gaps, distinct from the lighter mortar used to lay the bricks. The question that has long puzzled historians is whether these written records match the actual physical reality found in ancient structures. Did the builders really mix carbon into their lime, and if so, how did they do it?
At the Xianling Mausoleum in Hubei Province, a massive imperial tomb complex built during the Ming Dynasty between 1527 and 1541, a team of researchers set out to answer these questions. The mausoleum, the final resting place of the parents of the Jiajing Emperor, features thick brick walls that have survived for nearly five centuries. Upon close inspection, these walls reveal a striking visual contrast. The mortar used to bed the bricks deep inside the wall is a pale, yellowish-white color. However, the mortar used to fill the visible joints between the bricks and to finish the outer surface of the wall is a distinct dark grey, almost black. This difference suggested to the researchers that the builders were not using a single recipe for the entire wall. Instead, they likely employed different materials for different parts of the structure, saving the special dark mixture for the parts people could actually see.
To understand what made these dark mortars different, the team collected samples from three specific sections of the mausoleum: the front wall, the rear wall, and the connecting platform. They carefully separated the dark pointing and finishing mortars from the pale bedding mortars. In the laboratory, they subjected these samples to a series of rigorous tests. First, they dissolved the lime binder using acid, leaving behind only the solid particles that did not dissolve. The results were immediate and clear. The pale bedding mortars left behind a large amount of residue, mostly made of sand and stone fragments. The dark mortars, however, left behind a much smaller amount of residue, but this residue was filled with tiny black particles. When the researchers looked at these black specks under powerful microscopes and analyzed them with light-based instruments, they found they were made of poorly ordered carbon, a form of soot or charcoal that had not been fully burned or processed.
The researchers discovered that this carbon was not just sitting on the surface of the wall, perhaps from years of smoke or dirt. The carbon was locked deep inside the hardened mortar, trapped within the lime matrix. In some places, it appeared as tiny, dispersed specks; in others, it formed small, millimeter-sized clumps that looked like composite agglomerates. These clumps contained a mix of carbon, calcite, and tiny red particles of iron oxide. The presence of these clumps, which were still visible inside the hardened mortar, told a specific story about how the material was made. It suggested that the carbon was added to the lime mixture before it hardened, but the builders did not grind it down completely. The mixture was stirred, but not so thoroughly that every single speck of carbon was broken apart. This resulted in a final product that was a complex, heterogeneous blend of lime, sand, and carbon, with the carbon existing in various sizes from fine dust to small chunks.
By comparing these physical findings with historical texts, the team found a strong connection. Ancient Chinese construction manuals, such as the Song Dynasty's Yingzao Fashi, described a material called he qinghui, which was made by mixing lime with "soft coal" or, if coal was unavailable, with coarse ink. The researchers noted that the carbon found in the Xianling mortars matched the description of these historical materials. The fact that the dark mortar was used selectively for the visible joints and surfaces, while the pale mortar was used for the hidden structural layers, confirmed that this was a deliberate technological choice. The builders were not just coloring the wall; they were applying a specific, carbon-enhanced material to the parts of the building that were exposed to the elements and the human eye.
The study also revealed that this technology was not a one-off experiment but a systematic practice across the entire mausoleum. The dark carbon-bearing mortar was found in the front wall, the rear wall, and the connecting platform, showing a consistent application of this technique across different construction phases. While the exact source of the carbon—whether it was coal, charcoal, or ink—cannot be pinpointed with absolute certainty from the chemical analysis alone, the evidence strongly supports the idea that these mortars are the physical manifestation of the traditional qinghui described in historical records. The researchers concluded that the builders of the Xianling Mausoleum possessed a sophisticated understanding of material differentiation, choosing to enhance the durability and appearance of their exposed walls with a lime-carbon composite that had been passed down through generations of Chinese craftsmanship. This discovery provides a rare, tangible link between the written instructions of the past and the physical reality of the present, proving that the dark grey lines on these ancient walls are not just stains, but a deliberate and enduring technological signature.
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