How does Huangdi Jiuding Shendan Classic record the formation of mercury oxide?

Shen Dan Classic Collection of Huangdi Jiuding, a book written by the Tang people, records: "Mercury, digestion and so on. Separated and crushed, put into bronze vessels and cooked day and night. Come out and add water to charcoal, so it is purple and red. Honey pills are like beans, and their strength is like cinnabar for a hundred days. " According to modern chemical knowledge, mercury oxide can be generated by steaming mercury and pebbles in equal parts.