Pompeii goes from Red to Black
Last year my wife and I travelled around the world, something I have long wanted to do. I can now guarantee, from personal observation, that there really is an International Date Line - though the churlish Air New Zealand pilot refused to wake the passengers to tell us the exact moment when we crossed it. With what nostalgia I recall Neptune clambering over the ship's railing in happier days to exact a forfeit from all those who had never crossed the Equator before. As I recall, copious quantities of shaving foam were involved, and all the damsels of his court not only had hairy chests but bore a distinct resemblance to the sailors who scrubbed the deck or served our meals.
Still, I digress.
The reason for mentioning this trip is that along the way we stopped in China where, with the help of China Highlights we were able to see everything we wanted, among which was the Terracotta Army of Xian. A vast hall covers an acre or so of bare earth, slightly more than half of which has been excavated to reveal the ranks of pottery men, all dull and dusty terracotta and looking like slightly ornate versions of Bill and Ben the Flowerpot Men.
Our guide then took us down into another large building where, in conditions of near stygian darkness, a single warrior shiny with coloured glaze stands in a glass case, fixedly smiling at the teeming tourists. Above the hub-bub of Chinese our guide yelled at us the astonishing information that all the warriors had been similarly coloured and glazed, but that the colour had faded within a couple of months of being unearthed - a fact which explained why so much of the army remains unexcavated! The Chinese authorities have halted excavations until they can work out how to preserve the magnificent colours.
A similar problem confronts the Italian authorities with regard to the frescoes in Pompeii and Herculaneum. Many of them employ a dark red paint, either to highlight part of the scene or as a dark red ground against which the figures stand out. Known as "cinnabar", the red pigment is based on mercury sulphide - not something I would care to use on my walls! - which has an hexagonal form. Unfortunately when exposed to light, the cinnabar crystals change into a cubic form, metacinnabar, which is black in colour!
The result is that nearly all the beautiful frescoes are turning black where they should be red, but what is even more distressing is that all chemical or physical processes that should turn the metacinnabar back to cinnabar have failed miserably.
Recently Marine Cotte of the European Synchroton Radiation Facility at Grenoble, France, has been using the powerful x-rays produced by the machine to examine the paintings. It turns out that light has nothing to do with the degradation of the frescoes - at least, not directly - but chlorine from the plaster reacts with the cinnabar to make a photosensitive compound. As any photographer knows - back in the days when Real Photographers [TM] did their own processing - exposing unfixed film to light results in it turning dark grey and then black. The same thing has been happening to the Pompeii fresoces.
Fortunately there are chemicals that can be applied which bleach photographic silver and turn it back to is normal hue. The authorities are now experimenting to find out what happens when such chemicals are applied to cinnabar - and not just in the short term, but long term. Meanwhile they are reducing the illumination of the frescoes still further to try and slow down the blackening.
Alas, even if they find the ideal chemical, their troubles will not be over. The same x-rays also disclosed that sulphur dioxide in the polluted atmosphere of the Bay of Naples is affecting the plaster itself, turning it into calcium sulphate. The crystals of this substance are just the right size to trap particles of dust and soot from car engines and industrial processes. If they hope to keep their frescoes in pristine condition they will have to encase them in air-tight coverings to keep the dust and pollution out.
This may be possible in a museum, but many of the frescoes are still in situ and amid the chaos of crumbling brickwork and heavy-handed tourists it is not clear how this treatment can be given to room-sized frescoes.
photosensitive compound That means, something which reacts to light. The photosensitive compound in photographic film is a form of silver which, like the ancient frescoes, turns black when exposed to light. I await with interest some enterprising novelist who will imagine a piece of ancient glass that accidentally focuses light onto a newly uncovered wall, which is then fortuitously soaked with acid rain, thus preserving the moment when the dastardly Giovanni Botelli of the Camoro murders the Duchess of Amalfi who was hot on his heels over the mountains of rubbish lying in the streets of Naples. Return
© Kendall K. Down 2009
