Moon and silver: the lunar metal in the alchemical tradition

In the language of the alchemists the word «Luna» meant not only the light of the night but also a metal: silver. The correspondence between the seven planets visible to the naked eye and the seven metals known in antiquity is one of the most enduring ideas of the Hermetic tradition. The Sun was gold, Mercury quicksilver, Venus copper, Mars iron, Jupiter tin, Saturn lead and the Moon, because of her white and shining colour, silver.

Origins of a correspondence

The link between heavenly bodies and metals is already attested in late antiquity. In the second century the philosopher Celsus, quoted and refuted by Origen in Against Celsus, described a ladder of seven gates associated with the mysteries of Mithras, each made of a different metal and dedicated to a planet; silver corresponded to the Moon and gold to the Sun. The scheme was not yet entirely stable, since the attribution of some metals varied from one source to another.

With the Greco-Egyptian alchemists, including Zosimos of Panopolis who lived around 300, and then with the great age of Arabic alchemy, the correspondences settled into a canonical form. In medieval Latin manuscripts the metals were indicated by the symbols of the planets: a circle for gold and a crescent for silver. These signs long survived in pharmacy and chemistry, until the reform of chemical nomenclature at the end of the eighteenth century.

Luna cornea and the infernal stone

The use of the word Luna for silver left precise traces in chemical vocabulary. Silver chloride, which when melted forms a semi-transparent mass resembling horn, was called luna cornea, «horn silver». Silver nitrate, used for centuries as a caustic, was known as the infernal stone or lunar caustic. Many silver preparations in old pharmacopoeias bore names derived from the Moon, witnesses of an age when chemistry and astrology shared the same vocabulary.

For the alchemists silver was an almost perfect metal, inferior only to gold. According to the sulphur-mercury theory, set out in the Arabic texts attributed to Jabir ibn Hayyan, all metals arose from the union of these two principles in different proportions, and silver represented a stage close to completion. Transmuting base metals into silver was therefore a lesser but real goal, alongside the more ambitious quest for gold.

Whiteness and purification

In the symbolism of the alchemical work, the Moon is linked to the stage of albedo, the whitening, which follows nigredo, the black stage of dissolution. Lunar whiteness signified the purification of matter, the passage from darkness to reflected light, awaiting the full light represented by the Sun and by rubedo, the red stage. In many texts the «white stone», meant for silver, was distinguished from the «red stone», meant for gold, in a hierarchy that mirrors that of the two luminaries.

This symbolic reading flourished in the Renaissance and the seventeenth century, when alchemy took on spiritual and philosophical meanings. The Moon became the image of a soul made clear, able to receive a higher light as the satellite receives that of the Sun. In the circle of Paracelsus, in the sixteenth century, the correspondences between stars, metals and bodily organs also became the basis of a new chemical medicine, in which the brain was associated with the Moon.

The Moon of the alchemists is thus at once a heavenly body, a metal and a symbol. Her silver shone in laboratories as in illuminated manuscripts, and the lunar name survived in pharmacies long after alchemy had ceased to be practised. It is one of the clearest examples of how the sky lent words and images to the emerging science of matter.