Hsiao Chu-Chieh checks a tenmoku (tianmu) ware in New Taipei City, southeast China's Taiwan, Feb. 20, 2017. Tenmoku, known for its variability, became widely popular during the Song Dynasty. During its heating and cooling, several factors influence the formations of iron crystals within the glaze. Hsiao began his tenmoku study in 1985. It took him at least 15 days to make a finished product of colorful glaze tenmoku. (Xinhua/Ou Dongqu)