The artist might have just knocked off for a tea break. The colours in these 2000-year-old Egyptian paint pots, with their careless dribbles of paint, are as fresh and bright as the day they were mixed. Excavated in 1888 from Hawara in Lower Egypt, the pots are now in the British Museum. They all look alike, but one-the blue one-contains a puzzle that took a century to solve. The vibrant 鈥淓gyptian blue鈥�, a paint splashed around the ancient world for millennia, was the world鈥檚 first synthetic pigment. Artists first began to use it about 4000 years ago, but some time around the 9th century AD, the secret of how to make it was lost. When a pot of blue paint was found in the ruins of Pompeii in the early 19th century, some of Europe鈥檚 most eminent chemists set to work to recreate the beautiful blue that no one had ever been able to match.
ANCIENT Egyptian artists liked the colour blue. They were partial to bright reds and yellows too, but blue was divine: it had links with the gods. Gods were not like mortal beings. Their flesh was gold, their bones were silver and their hair was blue. But while it was easy to prepare red and yellow pigments from common minerals-iron oxides and hydrated iron oxides-blue was trickier. Blue minerals are rare. Lapis lazuli, a semi-precious stone found in Afghanistan, was scarce and costly, and couldn鈥檛 be turned into a paint. Yet, for thousands of years, the Egyptian artist鈥檚 palette included a blue as stunning as this rare azure stone.
Egyptian blue appears in wall paintings, on coffins and sarcophagi, and on the masks of mummies. Inside the pyramid of Unas at Saqqara, hieroglyphs stand out a vivid blue against a bright white wall. Sometimes, the powdered pigment was mixed with a binder and shaped into figurines, beads and sacred scarabs, which were then fired to harden them.
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The technology travelled to Knossos and Mycenae in Bronze Age Greece, and later spread to all corners of the Roman Empire. Egyptian blue was so widely used that it must have been made in large quantities. By the 9th century, though, production had stopped and the recipe had vanished.
The Egyptians left no written clues at all. Theophrastus, the Greek philosopher of the 4th century BC, described the pigment as a manufactured stuff from Egypt, where it had been invented by a king. Three centuries later, the Roman architect Vitruvius made a note of how it was made but failed to mention a vital ingredient.
Then, many centuries later, archaeologists digging in the ruins of Pompeii uncovered the remains of a paint workshop, buried during the eruption of Vesuvius in 79 AD. 鈥淚n an excavation made at Pompeii, in May 1814, at which I was present, a small pot containing a pale blue colour was dug up,鈥� wrote Humphry Davy, the great English chemist, who just happened to be on the spot.
Davy, like many other chemists at that time, was intrigued by this manufactured material. He wanted to find out what it was, not just to solve an ancient puzzle, but because anyone who could recreate Egyptian blue would have a valuable product to market.
Armed with samples from the paintshop and a few flakes of blue peeled from Roman frescoes, Davy set about analysing the material. Davy worked out that what went into the pigment was mainly sand, lime and copper, but it was another 75 years before a French geologist, Ferdinand Fouqu茅, identified the blue crystals that gave the material its colour. It took several decades more to unravel the details of how to make it.
Since the discovery at Pompeii, archaeologists have unearthed enough samples of Egyptian blue to track down the fine details of the manufacturing process. Large lumps of raw material turn up in the rubbish of ancient 鈥渇actories鈥�, and Roman artists were often buried with unused samples of their paints. The pigment was probably made in a sort of ancient glassworks, one of a range of products made from silica. Quartz sand was mixed with lime and some form of copper and heated to between 850 and 1000 掳C. There was some alkali in the mix, too. The amount of alkali was important: too little and not much happened, too much and the whole mixture turned into a useless lump of glass.
The right stuff
If the mix and the temperature were right, what came out of the kiln was a chunk of hard blue stuff, a mixture of unaltered quartz, some pieces of greenish-blue glass, formed when some of the sand fused with copper, and the crucial ingredient-the blue crystals of calcium copper silicate identified by Fouqu茅. These crystals are a synthetic version of a natural mineral discovered in 1938 called cuprorivaite. The natural mineral is so rare it could never have been the source of the pigment used so prolifically by ancient artists.
Those who knew the secret of making Egyptian blue had their own recipes and slightly different techniques for making it. The sand was probably just desert sand and the calcium probably came from crushed limestone or as an impurity in the sand. The essential alkali might have been added as natron-the soda used to preserve mummies-or as ash from desert plants, or again it could have been an impurity in the sand. The source of copper varied. Vitruvius describes fine metal filings going into the mix. Recent analyses picked out trace metals identical to those found in alloys made at the time, implying that the craftsmen were recycling bits of old bronze or copper alloy.
The precise colour of the pigment depended on a workshop鈥檚 particular methods-the proportions of the different ingredients, the temperature they were fired at, and how long it was all left in the oven. The colour depended in part on the size of the blue crystals that had formed during the firing process. Large crystals gave a deep blue, smaller crystals gave paler hues. Similar differences could be achieved by grinding the pigment into finer powder. Whatever the shade of blue, like most silicates it was extremely stable, surviving unchanged for millennia even when exposed to Egypt鈥檚 heat and sun.
There probably isn鈥檛 much left to discover about the technology for making Egyptian blue. But there are still some unsolved mysteries. Why, for instance, did the secret vanish so completely? Did it disappear when the last of a family of paint-makers died? And who really invented the pigment? The chances are that it wasn鈥檛 an Egyptian, still less an Egyptian king. Archaeologists suspect it came from Syria or Mesopotamia, but with far fewer ancient materials surviving in the wetter climate there, the blue is likely to remain 鈥淓gyptian blue鈥�.