Raw Materials and Body-Glaze Technology in Qingbai Porcelain Production at the Fanchang Kiln

For centuries, the story of Chinese porcelain has been told as a tale of two distinct worlds: the north and the south. In the north, potters mastered the art of making hard, white vessels by mixing two specific ingredients: a soft, clay-like material rich in aluminum and a hard, stone-like material. This combination, known to scholars as a "binary formula," allowed them to fire their wares at incredibly high temperatures without them warping or melting. In the south, however, the tradition was different. Potters relied on a single type of rock, a weathered stone that could be ground down and fired directly. The question that has long puzzled historians and scientists is when and where the southern potters first learned to mix these two ingredients together. Did they adopt the northern technique early on, or did they stick to their single-rock method for much longer? The answer matters because it changes our understanding of how technology spread across ancient China and how potters learned to control the very chemistry of their clay.

At the Fanchang kiln in Anhui province, a major production site active during the Five Dynasties and Northern Song periods, researchers recently uncovered a cache of pottery fragments and raw materials that finally allow us to test this theory. The site is famous for producing qingbai, a type of porcelain known for its delicate, bluish-white glaze. For decades, some experts believed that the potters at Fanchang had already discovered the "binary formula," mixing in extra aluminum-rich clay to strengthen their bodies. This belief was based on the observation that the finished pots contained higher levels of aluminum than the local rocks seemed to have. However, a new study published by researchers from Peking University and Nanjing Normal University challenges this long-held view. By examining the actual sludge left in the potters' processing tanks, alongside the finished shards, the team has reconstructed the entire production chain to see exactly what the potters were doing with their hands.

The researchers did not just look at the final products; they went to the source. They collected sediment samples from the bottom of the very tanks where the potters had washed and settled their raw materials centuries ago. These tanks represent the intermediate stage of production, the moment when the raw rock had been processed but not yet fired. By comparing the chemical makeup of this tank sediment with the chemical makeup of the finished porcelain bodies, the team could see if the potters had added a separate ingredient like kaolin clay. If they had, the finished pots would show a distinct jump in aluminum content that the raw materials simply did not possess. Instead, the data told a different story. The chemical composition of the finished bodies followed a smooth, continuous line with the raw materials from the tanks. There was no sudden jump or break in the pattern that would suggest a deliberate addition of a second ingredient. The aluminum levels in the pots were high, yes, but they were high because the raw stone itself was naturally rich in aluminum, not because someone had mixed in extra clay.

To understand why the aluminum levels were so high without any added clay, the team looked at the minerals inside the rocks. They found that the raw stone was mostly quartz and a mica-like mineral called muscovite, with only tiny, scattered amounts of kaolinite, the clay in question. If the potters had been systematically adding kaolin, the mineral mix would have been consistent and rich in that specific clay. Instead, the kaolinite was unevenly distributed, appearing in some samples and missing in others, which suggests it was a natural part of the rock's weathering process rather than a deliberate ingredient. Furthermore, the microscopic structure of the fired pots showed large quartz grains with melted edges, a sign that the rock had melted easily and naturally. If the potters had added extra clay, the mixture would have resisted melting more, resulting in a different internal structure. The evidence points to a single-material system where the potters were simply selecting the best, most weathered parts of the local stone.

The study also sheds light on how these potters achieved their famous colors. The qingbai wares from Fanchang appear in two main shades: a cool, bluish white and a warmer, yellowish white. The researchers found that this difference was not due to different recipes, but rather to the firing conditions and tiny variations in the raw materials. The pots that turned blue were fired in an atmosphere where oxygen was limited, which changed the iron in the glaze to a bluish hue. The yellowish ones were fired with more oxygen, keeping the iron yellow. Additionally, the presence of titanium, a metal that is usually invisible to the naked eye, played a crucial role. Even small changes in the amount of titanium in the glaze could shift the color from a pale blue to a grassy green or a yellowish tone. The potters were working with a high-fired lime glaze that included plant ash, a traditional southern technique, and they were mastering the subtle control of temperature and atmosphere to create these beautiful variations.

Perhaps the most significant finding of the study concerns the timeline of the kiln's operation. The researchers noticed that as time went on, the aluminum content in the porcelain actually decreased. If the potters had been learning to add more clay over time, the aluminum levels should have gone up. Instead, the opposite happened. This suggests that in the early days, the potters were using the most intensely weathered stone available near the surface, which happened to be very rich in aluminum. As they continued to mine the area, they exhausted this top layer and began digging deeper, where the stone was less weathered and contained less aluminum. This natural shift in their raw material supply explains the chemical changes in the pottery without needing to assume a change in their technology. The potters were not inventing a new formula; they were adapting to the changing geology of their own quarry.

The study concludes that the potters at Fanchang did not use the "binary formula" of mixing clay and stone. Instead, they perfected a single-material technology, relying on the natural properties of their local porcelain stone and their skill in processing it. This finding clarifies a major debate in the history of Chinese ceramics, showing that the southern tradition of using a single rock was robust and sophisticated in its own right. While the potters at Fanchang were certainly influenced by the northern white porcelain traditions, particularly in the shapes of their vessels and their firing techniques, they did not adopt the northern method of mixing ingredients. They developed a unique technological system that allowed them to produce some of the finest early porcelain in the world, proving that innovation does not always require mixing two things together; sometimes, it is about understanding one thing perfectly.