Before The Phantom Menace took over at the top of the British film chart this summer, the film that everyone was queuing up to see concerned that hardy old perennial, the mummy’s curse.

The Mummy was yet another cautionary tale about what can go wrong if you root around in the sands of ancient Egypt. The fact that we know this is not merely Hollywood fantasy and that the ancient Egyptians really did wish all sorts of nastiness on latterday tomb raiders is due to the discovery, 200 years ago this August, of the Rosetta Stone. The chance discovery of this unassuming piece of rock gave us the key which unlocked the mysteries of an ancient world.

At the end of the 18th century Napoleon had embarked on a campaign to conquer Egypt. Apart from his soldiers, he took with him experts in ancient architecture, artwork and inscriptions and even set up an institute of studies in Cairo.

While searching for materials to strengthen their coastal defences, his soldiers came across a large black slab, covered with three separate scripts. The top one was hieroglyphs, the strange pictograms which cover many ancient Egyptian monuments, and the bottom one was in Greek. The middle one had Napoleon’s experts scratching their heads.

But they didn’t have long to ponder the problem. After Napoleon’s Egyptian campaign ended in defeat at the Battle of the Nile, he was obliged to relinquish all his Egyptian treasures as part of his surrender. The Rosetta Stone was smartly transferred from Egypt to a new home at the British Museum, where another set of scholars sat and puzzled over the densely packed characters.

The first major breakthrough in cracking the stone’s code was made by a physician and scientist, Thomas Young, who studied ancient languages as a hobby. He began work on the unidentified text and quickly noticed that certain signs corresponded to royal names appearing in the Greek text. He then noticed that there were also similarities between these signs and those in the hieroglyphic text.

From here, he worked out that the mystery text was demotic, a form of everyday Egyptian in which the more formal hieroglyphs were reduced to a basic shape.

The French linguist Jean-Francois Champollion is credited as the father of Egyptology. His first sight of the Rosetta Stone was a plaster cast taken from the original. When Young’s work was published on the hieroglyphic alphabet, Champollion extended his theories and corrected some mistakes. By 1822 he claimed that by using the Rosetta Stone as a cipher, he could translate ancient Egyptian handwriting. His work on the hieroglyphic alphabet is the basis of our understanding of Ancient Egyptian history and civilisation.

It was something of an anti-climax to discover that the text on the Rosetta Stone was a rather dull and priestly decree by Ptolemy V. But from this the meaning of those thousands of pictures on Egyptian monuments was revealed at last.

Somewhat controversially, the Rosetta Stone has stayed in England to this day. It is one of the British Museum’s biggest attractions and is the star of the present exhibition, “Cracking Codes: the Rosetta Stone and Decipherment”. This coincides with the newly-reopened Roxie Walker Galleries of Egyptian Funerary Archaeology, where visitors can see coffins, mummies and grave goods from the same tombs together for the first time. Here, the logic of the Egyptians’ attitude to death is made clear. They believed that once dead the soul still needed a body (hence mummification), which needed food, and if you were a member of the ruling classes you would need your servants too.

A perfect illustration of this is a fantastic gilded coffin from the tomb of the priestess Henutmehyt. She has a food hamper, a battered wooden chest with the remains of duck and goat meat for her journey. She also has a wonderful army of small figures called shabtis who would do any manual work demanded by the gods. Her internal organs have been removed and placed in canopic jars, while the rest of the body has been mummified.

Anyone who is familiar with the old funerary galleries at the Museum will be thrilled by the new transformation. There is a dazzling array of gold-leafed and bejewelled artefacts. The multicoloured coffin decorations are also astonishing: greens, golds, reds and ochres all richly decorated with symbols and devices showing the incumbent’s hoped-for resurrection.

There are statues in all shapes and sizes, from giant striding figures from the Valley of the Kings, to pocket-sized men milling grain, or working in breweries, bakeries and butchers’ shops. There are even miniature boats with crews on the rigging.

The mummies come in a variety of guises. There are some whose every limb and digit is individually bandaged, others have their arms bound to their sides and their legs wrapped together. Scanning devices even allow us to see beneath the bandages.

British Museum, Great Russell Street, London WC1 3DG. Tel: 0171 323 8511. Website: www.british-museum.ac.uk. Cracking Codes: the Rosetta Stone and Decipherment, until January 2000. Book at least two weeks in advance. School groups up to age 18 free with teachers. The museum is updating its free teaching packs on the Egyptians

THE EGYPTIAN ROAD TO LIFE AFTER DEATH

* For several hundred years ground-up mummy was sold as a drug, said to staunch blood, and Francois I of France always carried some.

* During the first 40 days of mummification the body was packed with natron, a salt crystal found along lakes situated 40 miles North West of Cairo.

* For those who couldn’t afford top-notch mummification, where organs were removed and kept in jars, the body was injected with cedar oil to make the insides liquefy and drain out.

* Around 450 yards of bandages were used per mummy. The bandaging was an important ritual which lasted about 15 days and was accompanied by prescribed magical gestures and utterances.

* Animals were very important. Some creatures were mummified themselves, including crocodiles, baboons, ibis, eels, falcons, snakes, fish and cats.

* Other embalming methods were used: Alexander the Great’s body was said to have been preserved in honey so that, rather like Lenin’s, people could see it. Workers in the second-century AD are reported to have found an old jar of honey near the pyramids and tucked in, before they realised it contained the body of a perfectly-preserved child.

* Some coffins bear portraits of the deceased’s face.