Woodcut illustration of the 'graet and small cauldron and furnace' from Xingming guizhi (Pointers on Spiritual Nature and Bodily Life) by Yi Zhenren, a Daoist text on internal alchemy published in 1615 (3rd year of the Wanli reign period of Ming dynasty).
The illustration depicts a man seated cross-legged on a stool. Eyes closed and in meditative posture, he is practising internal alchemy. The 'cauldron' and 'furnace' of the title are not to be understood literally; in the figurative language of internal alchemy, these terms apply to the inner workings of the practitioner's body. The 'cauldron' (ding) refers to the head and the 'furnace' (lu) to the abdomen. The 'great cauldron' refers to the creation of elixir by refining jing (essence, semen), Qi and spirit. The 'small cauldron' is a place between the locations of Original Qi (yuan qi) and Vapour (yinyun).