Vermilion / Cinnabar

What it is

Vermilion is mercury(II) sulfide in its red, trigonal crystal form (the mineral cinnabar). It is a semiconductor whose band gap lets it absorb everything shorter than orange-red, so it reflects a dense, warm scarlet with excellent hiding power. Mercury sulfide also has a black cubic form, metacinnabar, with the same formula — which is why the question of whether darkened vermilion has simply 'turned black' has been so hard to settle.

How it was obtained

Cinnabar ore was mined in antiquity, famously at Almadén in Spain and in China, and ground as a pigment. Manufactured vermilion, made by combining mercury and sulfur into black mercury sulfide and converting it to the red form, appears in Europe from about the 8th century and was the standard red of medieval and Renaissance painting. From the late 17th century a cheaper 'wet' route was developed in which the conversion took place in solution, and it was manufactured on an industrial scale into the 19th century.

How it ages

Vermilion is fairly lightfast yet can darken badly, sometimes to purplish grey or black. National Gallery research found the change starts at the surface and needs chlorine — supplied by dirt, including salt from skin, rather than by the pigment — forming mercury chloride sulfides that light then converts to white calomel, with a dark phase below; glazes and varnish gave some protection. At Pompeii, blackened areas proved to be calomel and gypsum crusts holding dirt, while stains on buried fragments were iron-manganese deposits and not altered pigment at all.

How it is identified today

Mercury gives a strong, unmistakable XRF signal, and Raman spectroscopy identifies the red sulfide directly. Telling mined cinnabar from manufactured vermilion is harder: impurities, particle shape and mercury or sulfur isotope ratios can help. Diagnosing darkening needs cross-sections and surface-sensitive methods to find chlorine-bearing products and crusts, since, as Pompeii showed, a black surface does not necessarily mean altered pigment.

What replaced it

Cadmium red, introduced in the early 20th century, replaced vermilion as the dense, opaque warm red of the artist's palette, and today naphthol and pyrrole reds also fill that role. These do not darken the way vermilion can and carry no mercury, though cadmium carries its own restrictions. Vermilion is still sold in small quantities for traditional East Asian seal paste and lacquer and by specialist colourmen.

What the evidence says

• FactStrong evidence

  In National Gallery paintings, vermilion blackening required chlorine from surface dirt, forming mercury chloride sulfides that were further altered to calomel by light; the unaltered vermilion contained no chlorine.

  Caveat: The identity of the dark phase itself was left tentative in this study and has been debated since.

  Source: The Blackening of Vermilion: An Analytical Study of the Process in Paintings

• FactStrong evidence

  At Pompeii, blackened cinnabar exposed to the air carried calomel and gypsum crusts, while dark stains on fragments buried before the eruption were iron and manganese deposits rather than altered pigment.

  Caveat: Findings from three Pompeian houses; burial conditions elsewhere differ.

  Source: When Red Turns Black: Influence of the 79 AD Volcanic Eruption and Burial Environment on the Blackening/Darkening of Pompeian Cinnabar

• FactModerate evidence

  Vermilion exists as natural cinnabar and as a manufactured pigment made by dry and later wet processes, and was eventually replaced by cadmium red.

  Source: Vermilion — ColourLex pigment page; Pigments through the Ages — Vermilion