Villa dei Misteri
At the centre of the frescoes, in "The Hall of the Mysteries" at Pompeii is the figure of Dionysus, the public cult of whom, had long been popular in southern Italy. The dramatic red of the wall panels is vermilion made from mercury sulphide (HgS), commonly named cinnabar. This is a deep red pigment used extensively to cover the background of paintings, giving a uniform red background to pictures, which, in certain circumstances, can become unstable, turning into dull grey-black shades. As early as the 1st century BC, Vitruvius, in his treatise De Architectura, mentioned the impact of light, and gave the recipe for a sort of protective varnish based on Punic wax (a waxy soap made of beeswax and soda lime):
"Those who are desirous that the vermilion should retain its colour, should, when the wall is coloured and dry, rub it with a hard brush charged with Punic wax melted and tempered with oil: then, with live coals in an iron pan, the wall should be thoroughly heated, so as to melt the wax and make it lie even, and then rubbed with a candle and clean cloth, as they do marble statues. The coat of Punic wax prevents the effect of the moon’s as well as that of the sun’s rays thereon which injure and destroy the colours in work of this nature."
Despite the Punic wax, the decorated walls of Pompeii have lost their colours in recent years. The most common explanation which has been given is that the exposure to the sun transforms cinnabar into another phase (in which the same atoms, mercury and sulphur, are arranged differently) - metacinnabar, which is black.
It is also very possible that the cause of the colour change is oxidation at the surface, beneath which the cinnabar has remained intact. Cinnabar dissociates in the presence of chlorine to produce sulphur, which then (in the form of sulphur dioxide), reacts with the calcite in the mortar to form calcium sulphate. This reaction is favoured by the presence of HgS in the pigment, explaining why areas that were not painted with cinnabar were not discoloured.
Pliny the Younger
Pliny the Younger was a witness to the eruption of Vesuvius. A detailed description of this famous event is contained in two letters of his letters, where, he appears not to notice the destruction of the city, even though his uncle perished in the immediate neighbourhood:
A black and terrible cloud, rent by snaking bursts of fire, gaped open in huge flashes of flames; it was like lightning, but far more extensive .
Soon afterwards, the cloud lowered towards the earth and covered the sea. Ashes were already falling, but not yet thickly.
When night fell, not one such as when there is no moon or the sky is cloudy, but a night like being in a closed place with the lights out.
One could hear the wailing of women, the crying of children, the shouting of men; they called each other, some their parents, others their children, still others their mates, trying to recognize each other by their voices.
It lightened a little; it seemed to us not daylight but a sign of approaching fire. But the fire stopped some distance away; darkness came on again, again ashes, thick and heavy. We got up repeatedly to shake these off; otherwise we would have been buried and crushed by the weight.
At last that fog thinned and dissipated in a kind of smoke or mist; soon there was real daylight; the sun even shone, though wanly, as when there is an eclipse.
Our still trembling eyes found everything changed, buried by a deep blanket of ashes as if it had snowed.
Fear prevailed, since the earthquake tremors went on, and many, out of their senses, were mocking their own woes and others’ by awful predictions.
But we, even though we had escaped some perils and expected others, we did not think even of going away until we should have news of my uncle.