Kiln change (Chinese: 窑变, yao bian) is a term that refers to the natural changes in color or texture of the glaze or body of ceramics during firing due to fluctuations in temperature, atmosphere, glaze composition and other factors. Kiln change is a unique phenomenon in the history of Chinese ceramics, which reflects the Taoist philosophy of adapting to nature, not opposing it, and achieving unexpected beauty.

The history of Chinese kiln change ceramics began in the Song dynasty (960-1279). This was a golden age of Chinese ceramics, when many famous kilns emerged in different regions, such as Ru Yao (Chinese: 汝窑), Guan Yao (Chinese: 官窑), Jun Yao (Chinese: 钧窑), Ge Yao (Chinese: 哥窑), Ding Yao (Chinese: 定窑), Yaozhou Yao (Chinese: 耀州窑), Longquan Yao (Chinese: 龙泉窑) and so on. These kilns were characterized by their pure and elegant glaze colors, fine and strong porcelain body, simple and graceful shapes, high level of craftsmanship. Among these kilns, the most vividly expressed the art of kiln change was Jun Yao.

Jun Yao was located in Yuzhou city of Henan province and was one of the imperial kilns of the Northern Song period. Jun Yao was famous for its thick layer of glaze and various colors of glaze. Jun Yao used copper oxide as the main coloring agent and was fired in a reducing atmosphere at high temperature. As a result, the glaze acquired different shades of blue, purple, red and other colors, sometimes rare colors such as sky blue, moon white, pink and so on appeared. Jun Yao also had a special effect, which was the appearance of spots or stripes of different sizes and shapes on the surface of the glaze, which were called "iron rust spots" or "roe stripes". These spots or stripes were formed because the glaze contained a small amount of iron, which was precipitated at high temperature. These spots or stripes added extra charm and variability to Jun Yao.

The process of making Jun Yao was very complex and delicate, requiring multiple kneading of clay, forming, processing, applying glaze, firing and so on. The most important stages were applying glaze and firing. Jun Yao glaze consisted of many components, including copper oxide, iron oxide and other metal oxides. When applying glaze, it was necessary to pay attention to the thickness and uniformity of the glaze layer, usually applied three to five layers. When firing, it was necessary to control the temperature and atmosphere, usually fired at high temperature above 1200°C in a reducing atmosphere. In this process, due to subtle changes in temperature, air flow, fire intensity and other factors, oxidative or reductive reactions of metal oxides occurred in the glaze, resulting in different colors and patterns of glaze. These changes were difficult to control and predict, completely dependent on the experience and luck of the master. Therefore, the art of kiln change Jun Yao had a very high randomness and uniqueness, each Jun Yao was unique.

The art of kiln change Jun Yao not only reflected the Taoist philosophy of adapting to nature, but also corresponded to the aesthetic taste of literati in the Song period. The literati of Song valued nature, pursued mood, emphasized individuality, did not stick to formalities. They liked the pure and elegant colors of Jun Yao glaze,