Hot Extraction of mercury: Heating cinnabar with iron

For a general introduction to the three hot extraction methods for processing mercury from cinnabar, see the section on Hot Extraction of Mercury: Sources

Here, we test the second method, which involves heating cinnabar in a closed vessel in the presence of iron, as reported by Pliny the Elder (1st century BCE), Dioscorides (1st century CE) and the Leiden Papyrus (3rd-4th century CE)

Latin recipe

Pliny the Elder, Natural History, book 33, chap. 123

It (i.e., cinnabar) is put in an iron shell in flat earthenware pans, ad covered with a convex lid smeared on with clay, and then a fire is lit under the pans and kept constantly burning by means of bellows, and so the surface moisture (with the colour of silver and fluidity of water) which forms on the lid is wiped off it. This moisture is also easily divided into drops and rains down freely with slippery fluidity.

(minium) patinis fictilibus impositum ferrea concha, calice coopertum, argilla superinlita, dein sub patinis accenso follibus continuis igni atque ita calici sudore deterso, qui fit argenti colore et aquae liquore. Idem guttis dividi facilis et lubrico umore compluere.

Greek recipe

The Leiden Papyrus (P.Leid.X), fol. 16 (passage taken from Dioscorides, On Medical Substances, 5.95)

They put an iron shell containing cinnabar in an earthenware vessel and enclose it with a convex lid smeared on with clay; then they light a fire upon (the vessel) with charcoal. The vapour that settles on the lid, when wiped off, is mercury.

θέντες γὰρ ἐπὶ λοπάδος κεραμεᾶς κόγχον σιδηροῦν ἔχοντα κιννάβαρι, περικαθάπτουσιν ἄμβικα περιαλείψαντες πηλῷ, εἶθ’ ὑποκαίουσιν ἄνθραξιν· ἡ γὰρ προσίζουσα τῷ ἄμβικι αἰθάλη ἀποψηχθεῖσα ὑδράργυρος γίνεται.

Replicating the procedure


We can assume that ancient alchemists attempted to add other ingredients before roasting cinnabar in closed vessels. Iron appears to have been used, as indicated by Zosimus of Panopolis (see The Cold Extraction of Mercury with Lead, Tin & Iron). The significant role of this metal is further confirmed by Pliny the Elder, Dioscorides and the Leiden Papyrus. To replicate this procedure, we placed the cinnabar on a small iron plate within an alumina crucible, which was then covered with a lid. Upon heating, cinnabar reacts with iron—and not with oxygen—according to reaction: HgS + Fe → FeS + Hg.

Adding iron powder to cinnabar
Adding iron powder to cinnabar

The reaction swiftly reaches completeness, and the residual powder characterized by XRPD confirms the presence of iron sulfide in the form of hexagonal troilite.