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Bending over a mediaeval parchment, the curator carefully applies her "butterfly stitch" of minute strips of tissue to a rip in the age-browned paper.

The adhesive dabbed on the tissue is made from the bladder of a sturgeon, one of the centuries-old techniques used by the alchemists of the British Library to restore its ancient books. The library welcomes about 4,000 visitors a day to its reading rooms, and many of its 13 million books require remedial treatment because of wear and tear, age and vandalism.

For the first time, the library's 50 conservators have been brought together under one roof and its conservation centre is now open to the public once a week.

In a large air-conditioned room, where uninterrupted daylight falls on every bench, conservation methods as old as mediaeval binding and as modern as laser technology are used to protect a collection built up over 250 years.

Of overriding importance is maintaining, and sometimes restoring, the integrity of the books, photographs, stamps and sound recordings and preserving their aesthetic value, said Vicki Humphrey, the head of conservation at the library.

"We are custodians, keeping the collections in perpetuity for the British nation and the world," she said. "We have to decide what to leave and what to remove."

One copy of every publication produced in Britain must be deposited at the library, which moved to a new central London home next to St. Pancras train station in 1997. The main reading room of the British Library, which used to be located in the British Museum, was used over the years by Karl Marx, Gandhi and Lenin.

The scale of its possessions is breath-taking – there are 625 kilometres of shelves and they grow 12 kilometres longer each year. Obviously the conservation centre cannot hope to satisfy every demand, so it uses a points system to decide the most worthy cases.

Collection areas in the library nominate items that are then picked according to how rare and heavily used they are, whether they can be copied digitally and whether there is more than one copy.

"The British Library has to care for everything in its collection. We don't allocate value," Ms. Humphrey said.

Many ancient books and manuscripts have been repaired or rebound over the centuries. The library's experts try to return them to their original condition, particularly manuscripts from Asia which were often rebound or turned into books in the West.

On one bench, a sales catalogue from 1847 is being cleaned and having a new spine attached, a restoration that will take up to 100 hours to complete.

On another, a manuscript is being resewn after its tattered leather spine was taken off and cleaned.

The curator uses a technique many centuries old, based on a wooden press, to resew the manuscript with linen thread and return it to the binding it had in the 12th century.

But spines are also rebuilt using aerocotton, a strong material developed in the aircraft industry.

"The techniques change, but the esthetics remain the same," Ms. Humphrey said.

One senior curator has spent more than three years on a single task, restoring the Diamond Sutra, the oldest printed work in the world to bear a date – from the ninth century – that was found in a Chinese cave in 1900.

"It's a highly complex job and he has to move away from it sometimes to clear his head," Ms. Humphrey said.

Restoration can involve just washing pages to clean them, to regilding the spines of ancient books with gold leaf, using the sweat of the restorer as adhesive.

The gold leaf restorers have a separate room and are the stars of the tour. Moisture makes the leaf crinkle beyond use, so the restorers move the wafer-thin metal sheet by waving a knife above it and catching the floating leaf on the blade.

The curators then heat one of the 3,000 finishing tools at their disposal to sizzling point and press the leaf into a book's leather spine or cover.

While ancient books and manuscripts lure most visitors to the conservation centre, the British Library is increasingly having to turn its attention to how it will be able to preserve modern materials for the future.

Colour photographs are "a nightmare," curators say, because they lose their tones, and the non-lick adhesive on the back of modern stamps also poses problems.

"The older pages are stable and strong, the real challenge comes from materials since the Industrial Revolution," Ms. Humphrey said. "Papers have become more unstable – mass production has its costs.

"Take a scratch-and-sniff book – how do you conserve that? I have no idea."