Sheng Yao

Sheng Yao (升药), literally "ascending medicine," is a traditional Chinese mineral-medicinal preparation, not a plant. It comprises crystalline mercury compounds — principally red mercuric oxide (HgO) in the form known as Hong Sheng Dan, or yellow mercuric oxide (Huang Sheng Dan) — produced by sublimation from a charge of mercury, alum, and nitre (potassium nitrate) heated in a closed vessel. The resulting needle-like red or orange-yellow crystals were historically scraped from the lid of the reaction crucible and used externally on chronic ulcers, fistulae, and suppurative skin lesions to debride necrotic tissue. Because mercuric oxides are acutely toxic and dermally absorbed, modern Chinese pharmacopoeial use is highly restricted to external application in trace amounts; the substance is omitted from many modern formularies on safety grounds.