Vermilion

Vermilion is the red of the ancient and medieval world wherever somebody could pay: Roman wall painting at Pompeii, Byzantine and Persian manuscripts, Chinese lacquer and the imperial seal, Indian temple work and Mughal miniatures, and the rubrics of European manuscripts. The word rubric is from ruber, red, because the headings were done in it.

Its problems are two. First, it is mercury, and mercury poisoning was a routine occupational disease of gilders, painters and miners. Second, on some paintings it blackens. A form of it converts to black metacinnabar under light, and the mechanism is still argued about, with chloride contamination the leading suspect. Several famous paintings have grey passages that were once brilliant red.

In India the same material is sindoor, worn in the parting of the hair by married Hindu women, and kumkum used in ritual. Traditional sindoor was vermilion; most commercial sindoor now is a synthetic red dye, partly because of the mercury, and the substitution is not always declared. A colour with a four-thousand-year religious role quietly became a food-dye analogue.

Manuscript rubrics, lacquer, seals, temple painting, sindoor, Pompeian walls, and every bright red in European painting until cadmium arrived in the nineteenth century.

Modern vermilion in paint is almost always cadmium red or a naphthol substitute, because of the toxicity. The pure orange-scarlet it gives is still the reference for 'a red that is completely red' (no blue in it at all) which is the hardest red to reproduce in print and the one that makes a package look expensive when you manage it.