Hot extraction of mercury: Heating cinnabar with natron oil

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 third method, which involves heating cinnabar in a closed vessel in the presence of ‘natron oil’, as reported by Pseudo-Democritus according to later alchemical witnesses.

Greek recipes

The Philosopher Anonymous (8th-9th century CE), On Virtue and Interpretation, chap. 7

Τhis very famous philosopher (i.e., Pseudo-Democritus) said: Who does not know that the vapour of cinnabar is the mercury of which it is composed? Therefore, if anyone grinds cinnabar with oil of natron, mixes them together, puts them in the double vessels and lights a persistent fire, he will collect the entire vapour that was sitting in the bodies (of cinnabar).

Οὗτος οὖν ὁ ἀγαθώτατος φιλόσοφος· Τίς δὲ οὐκ οἶδεν ὅτι ἡ αἰθάλη τῆς κινναβάρεως ὑδράργυρός ἐστι, δι’ ἧς καὶ συντέθειται; Διὸ καὶ εἴ τις ἐλλείωσας αὐτὴν τὴν κιννάβαριν νιτρελαίῳ, ἀναφυράσας καὶ περικλείσας ἐν ἄγγεσιν διπλοῖς, ὑποκαύσας φωσὶν ἀλήκτοις, πᾶσαν αἰθάλην λήψεται ἐγκεκαθημένην [lege ἐγκαθημένην?] εἰς τά σώματα.

Byzantine anonymous recipe (the dating is uncertain)

On cinnabar. You must know that the transformation of cinnabar happens by means of the oil of natron: in this way cinnabar is melted by a light fire, as you know.

Περὶ κινναβάρεως. Δεῖ γινώσκειν ὅτι ἡ ἀνάκαμψις τῆς κινναβάρεως διὰ νιτρελαίου γίνεται, καὶ οὕτως χωνεύεται μετὰ πυρᾶς λεπτῆς, ὡς ἐπινοεῖς.

Replicating the procedure


The first-century CE alchemist Pseudo-Democritus, as reported by a reliable Byzantine author known as ‘The Philosopher Anonymous,’ mentions a procedure for the hot extraction of mercury from cinnabar. Democritus is said to have ground cinnabar together with natron oil and placed the mixture in a ‘double vessel’ in order to capture all of the vapour, that is, the mercury contained in the cinnabar.

The expression natron oil translates the Greek nitrelaion, which merges the terms nitron and oil (Gr. elaion). The exact meaning of this expression remains elusive and continues to be a subject of debate. The Greek term nitron likely refers to the mineral natron (i.e., sodium carbonate decahydrate) extracted from Wadi el-Natrun, a nearly dry lake in Egypt. However, its association with oil is uncertain. The alchemists probably referred to a mixture prepared by dispersing natron in a liquid.

In a closed vessel, the role of sodium carbonate is pivotal, as demonstrated by the following experimental evidence. We mildly heated the same amount of cinnabar both with and without sodium carbonate, without adding any liquid. In the former case, the cinnabar fully converted into mercury; in the latter, unreacted cinnabar remained.

Flasks with cinnabar and sodium carbonate and with cinnabar alone after the reaction
Flasks with cinnabar and sodium carbonate (left) and with cinnabar alone (right) after the reaction

To confirm the role played by sodium carbonate, we performed the extraction under vacuum to prevent the cinnabar from reacting with atmospheric oxygen. Under this condition, mercury was obtained, and sodium sulfate (Na2SO4) was identified as one of the products.

Extraction under vacuum with sodium carbonate
Extraction under vacuum with sodium carbonate
Mercury droplets condensed on the glassware
Mercury droplets condensed on the glassware

Once the key role of sodium carbonate (i.e., natron) in the hot extraction of mercury had been confirmed, its oily nature and specific function remained to be determined. We prepared saturated solutions of sodium carbonate in both water and vinegar, and interestingly, their viscosity resembled that of oil. It is well known that when sodium carbonate is mixed with vinegar, it reacts, forming sodium acetate and carbon dioxide bubbles. However, due to the low concentration of acetic acid in vinegar, it does not completely consume the sodium carbonate.

To conduct the extraction, we utilized sodium carbonate dissolved in either water or vinegar. During the process, the water (also present in the vinegar) evaporates, resulting in the production of sodium sulfate and mercury. In both cases, no significant improvement was observed compared to the reactions under dry conditions.

Finally, we tested a mixture of sodium carbonate and vegetable oil and discovered that even with mild heating, the oil readily ignites and its decomposition hinders the extraction of mercury. When the temperature is lowered to avoid decomposition, the extraction does not proceed.

Burnt vegetal oil whose decomposition prevents the reaction
Burnt vegetal oil whose decomposition prevents the reaction
Extraction in presence of oil at low temperature
Extraction in presence of oil at low temperature