Black Angels
| Mosul | 36 21 12.91N 43 07 24.32E | The coordinates are of a Christian church complex on the bank of the Tigris and almost opposite a large mosque. Christians and Muslims coexisted happily for centuries until radical Islam was invented. |
Mosul will be familiar to readers of these pages as the modern city in northern Iraq, on the opposite bank of the Tigris river to Nebi Yunis, the ruins of the ancient Assyrian capital of Nineveh. Nineveh was excavated by Sir Austen Henry Layard, who discovered gigantic carved stone bulls and beautiful decorative reliefs amid the ashes of the burned out palaces.
Now it appears that the inhabitants of Mosul carried on the artistic traditions of the ancient Assyrians long after that empire had vanished and been forgotten. Robin Clark and Peter Gibbs, of the department of chemistry at University College, London, have been studying a Christian manuscript dating from AD 1220 and produced in Mosul. One of the most valuable surviving Byzantine productions as well as one of the weightiest tomes — the volume weighs about 22 lbs — the book contains a series of readings based upon the Gospels. What makes it particularly valuable are the illuminations that decorate nearly every page and illustrate the events of the Gospel story.
Curiously enough, however, all the figures are depicted with coal black faces. Now a negroid archangel might pass without comment in something produced in Abyssinia, but is entirely unexpected in a picture painted by Byzantine artists in northern Iraq. This feature was even more distressing for the curators of the British Museum, of whose collection the manuscript forms a part, in that records from last century, when the book was acquired, make it plain that the human and divine figures all had more usual pale coloured features when the folio first arrived in Britain. They therefore commissioned Clark and Gibbs to investigate the phenomenon and, if possible, put it right.
Needless to say, Clark and Gibbs were under no circumstances to damage the manuscript: holes, no matter how tiny, were not to be punched in it; scrapings of paint, no matter how minute, were not to be taken. Fortunately the two experts had a trick up their sleeve: Raman microscopy involves shining a laser down a microscope, which is able to focus the tiny pin-prick of light onto single grains of the paint. You then examine the spectrum of the light that is scattered off the grain of paint and, by a technique which is well known to astronomers, identify the different elements precisely.
This yielded some interesting results. The yellow pigment in the paintings was a form of arsenic sulphide known as orpiment (chemical formula As2S3). This was normal and expected. However there were other areas of bright golden yellow in the pictures and these turned out to be a different form of arsenic sulphide with the chemical formula As4S4 which, unless the chemists of Mosul were several degrees more clever than we believe, was formed by the sun shining on realgar, a naturally occuring orange-red rock. The hot sunshine produces a light dusting of yellow powder known, not unnaturally, as pararealgar. The task of collecting this powder must have been tedious in the extreme and gives some idea of the loving labour that went into illuminating books like this.
Meanwhile the mystery of the black-faced archangels remained. Once more Clark and Gibbs shone their laser down their microscope and this time discovered that the white paint used by the illuminators was based on white lead. This was used neat for clouds and the background to some text, it was mixed with red vermilion to produce the pink cheeks of the cherubim. The colours so produced survived for hundreds of years and then some explorer brought the book back to Britain to be lovingly stored in the British Library, where experts eagerly consulted it by the light of that wonderful modern invention, the gas lamp.
Unfortunately the gas was coal gas, rich in hydrogen sulphide, and this insidiously attacked the white lead — lead carbonate — turning it into lead sulphide, which is pitch black. Mystery solved, the next question was, could the damage be reversed? Nothing more simple, says Clark. All you have to do is brush the lead sulphide with hydrogen peroxide and it will automatically turn the black to white and restore the pictures to their pristine beauty. Unfortunately, as anyone with a mother addicted to using the stuff on childhood cuts and grazes will know, hydrogen peroxide "bites" and Clark and Gibbs are still trying to determine what the likely effect of the stuff will be on the rest of the book. After all, it is no good restoring the pink cheeks to the cherubim if you then turn their haloes green or, indeed, eat holes in the page of the book.
