When the x-rays came back from the lab, conservator Caroline Glover couldn't believe her eyes. The pastoral scene of nymphs and satyrs by 17th-century French artist Nicolas Poussin concealed not one but two earlier works; the overlapping lines of a bridge and a group of angels clearly visible on the black and white transparency.
Caroline had noticed the faint outlines of several figures beneath the painting's surface. Her suspicions were confirmed when she removed a tiny fragment of the paint for examination under the microscope.
"Before you start work on any paint you have to start doing quite thorough research and analysis," Caroline says. "Every picture is different and you have to do extensive tests to make sure that solvents are safe for that picture. The Victorians experimented more with their paint methods which makes cleaning them more difficult than with an old master where you know it's going to be oil paint."
There was a story behind the Poussin as well. After years of doubt (Anthony Blunt, the Queen's art historian, Russian spy, and an authority on the artist, refused to believe it was the work of Poussin) the painting's veracity is now established. It is one of his earliest works - which explains the hidden images.
"As a young man," Caroline explains, "Poussin was very poor so he probably revised the canvas until he reached an image that was sellable."
He wouldn't have that problem today - his works fetch millions at auction.
The high-ceilinged studios of the Conservation Centre in Liverpool are a hive of quiet activity. Forty conservators work in this converted railway warehouse, delicately repairing the damage caused by pollution, neglect and time. It's a kind of intensive care unit for Merseyside's art treasures. Everything from Egyptian mummies and Japanese armour, to local valuables like Beatles gold discs and the statue of Eros - a copy of the archer in Piccadilly Circus that was rescued from Sefton Park and now perches above the cafe - come here for treatment.
It's also European Museum of the Year, an accolade awarded for its user-friendly ethos - visitors can see conservators at work by live video links or on studio tours - as well as its pioneering approach to the art of conservation. But science, always the foundation for a conservator's work, has an increasingly important role to play.
Conservators depend on an x-ray machine and microscope for their preliminary detective work as much as they do on chemicals and cotton wool in their cleaning. In the glass and ceramics studio, conservator Lynne Kelly relies on all kinds of science to make the job easier. She knows that the pottery warrior on horseback that her assistant Gaynor is patiently repairing (its legs were cut off when it was smuggled out of China) is more than 1,000 years old because thermoluminescent dating has told her that it was fired sometime between the 7th and 9th century, during the Tang dynasty.
Fluctuations in temperature and humidity can wreak havoc on antique Venetian glass which can become distorted if it absorbs too much moisture. "The only way of controlling that is to put it in a specific, controlled environment, " she says, pointing out two large, glass cabinets at one side of the room that serve the purpose.
"Materials that have been used in past repairs can give off vapours when they break down. You have to be aware of that." The scientific training that she received on her conservation course is as important as her art school training, she says.
Her job requires a patient and methodical approach, a steady hand and an appreciation of form and colour, but her every action depends on scientific knowledge of the materials she is working with.
"You can't do the job without it. You will find painting conservators who have degrees in chemistry and there are conservation scientists who dedicate their lives to seeing how things react to treatment."
But nowhere is the influence of science on conservation more apparent than in a room at the top of this old warehouse, where an anonymous-looking box on wheels is revolutionising the restoration of sculptures.
Traditional methods of cleaning using chemicals or high pressure washing have left damaging residues or scarred the surfaces of works. But this box contains a portable laser which, when directed through the pen-like attachment on the end of an articulated arm, can remove the grime of centuries from marble without harming the stone beneath - the holy grail of sculpture restoration.
Dr Stephen Fowles, one of the centre's laser technology experts, explains: "The great virtue of marble is that it is easy to carve and once you have carved it, it hardens. But it is very prone to attack from pollutants and gases in the atmosphere which change the surface into calcium sulphates. A black crust of pollution builds up.
"The laser makes use of a simple physical property - that is, the difference in absorptivity between the pollution and the marble underneath.It vaporises the dirt but the process of cleaning stops when you get to the stone. It's a powerful and flexible tool for conservators."
The statue of Eros in the centre's cafe was cleaned using the technique, as was the 12-tonne Spirit of Liverpool which dominates the foyer. Another application of the laser enabled conservators to measure and make a replica of the Spirit which now sits above the city's Walker art gallery in place of the original.
The technique, which has made the centre a world leader in laser conservation, is now being applied to parchment, vellum, ivory, bone, plaster, aluminium and bronze, and a heavy duty laser is being developed for exterior use on buildings.
"The laser is wonderful," says Dr Fowles. "It enables you to pick out the grains of dirt from between the grains of marble. The conservator's skill is still paramount, but this is very much an environmentally friendly tool for the future."