In the Song dynasty, Ru ware was fired using firewood as fuel, making temperature control extremely difficult. Each addition of firewood caused the kiln temperature to rise sharply, while Ru ware firing required the temperature to remain stable and change gradually. There was no highly precise firing equipment at the time, so the process depended entirely on the kiln workers’ experience.

First, let us discuss Ru ware’s distinctive glaze-making techniques. The glaze recipe for Ru celadon was extremely complex, and its color was exceptionally difficult to achieve. The glaze on Ru celadon was not simply a “glass glaze”; it might more accurately be called a “lustrous glaze,” and its ingredients were not all minerals that melted at high temperatures.

Ru ware glaze incorporated “agate powder,” and most of its ingredients would not melt completely even at high temperatures. This made the glaze highly viscous and increased the difficulty of firing. All the raw materials for Ru ware came from natural ores, with no chemical additives. The glaze color depended on the reduction of trace amounts of iron in the ores. Reducing iron oxide to the right degree could produce a rich glaze color and a jade-like luster, but this process was extremely difficult to control. As a result, the glaze on Ru ware has a gemstone-like appearance, which the ancients described as “resembling jade without being jade, yet surpassing jade.”

In both ancient and modern times, many factors affect kiln firing, including control of the kiln atmosphere, temperature fluctuations, and weather changes—all of which require extensive experience on the part of kiln workers. An old saying goes, “Of ten kilns, nine fail; shaping is the work, firing is the gamble,” aptly illustrating how difficult Ru ware is to produce. Even with every effort, the yield of high-quality Ru ware was only about 30%, lower than that of other kilns. This is one important reason why Ru celadon is so expensive.