Rusty, Suppurated, and Discharged like Sēpía Ink: Scientific Knowledge, Animal Lore, and Colour Classification in Plutarch’s De Sera Num. 26, 565b–d

Abstract The theological and eschatological dialogue De sera numinis vindicta (“On the Slowness of the Divinity to Punish”) by Plutarch of Chaeronea (first–second century CE ) contains precise references to scientific and technical notions of his time, primarily within analogies and symbolic images. Assuming that understanding these symbolic elements requires an examination of the terminology and concepts that inspired them across scientific and broader cultural contexts, this article analyses one passage to gain insights into early imperial zoology, ink usage, colour classification, and metallurgical terminology. After introducing selected medical, zoological, and metallurgical references within the dialogue, the article focuses on a symbolic sentence describing “ill-will” and “envy” as a “rusty” (or “poisonous” or “violet-like,” ἰώδης ) and “suppurated” (or “hidden” or “treacherous,” ὕπουλος ) fluid emitted by envious souls, compared to the manner in which “cuttlefish” ( σηπίαι) emit their ink. While clarifying the meaning and chromatic implications of this image, the article explores zoological knowledge of cuttlefish, ancient sources on cephalopod ink usage, and documented associations of these inks with the colours and corrosions of bronze and iron. Personal observations of the natural colours of three cephalopod inks are also presented alongside those of corroded copper and iron powders.