So-called "true celadon", which requires a minimum furnace temperature, a preferred range of , and firing in a reducing atmosphere, originated at the beginning of the Northern Song Dynasty (960–1127), at least on one strict definition. The unique grey or green celadon glaze is a result of iron oxide's transformation from ferric to ferrous iron (Fe2O3 → FeO) during the firing process. Individual pieces in a single firing can have significantly different colours, from small variations in conditions in different parts of the kiln. Most of the time, green was the desired colour, reminding the Chinese of jade, always the most valued material in Chinese culture.