Stephane Petit's patients arrive at his clinic almost unrecognizable, their skin invaded by microbes, their faces blackened by pollution, their jewel-toned robes sullied by candle smoke and the debilitating breath of millions of well-intentioned admirers.
Petit is a doctor to some of the world's oldest stained-glass masterpieces: the blue-hued mosaics of the Cathedral of Our Lady of Chartres, which stands in this medieval village 50 miles southwest of Paris. His patients -- saints and butchers, priests and blacksmiths -- have survived more than seven centuries of revolution, war and atmospheric grime buildup.
"The glass is full of mini-craters," said Petit, a man of 40 built like a cornstalk, his rail-thin body topped by a mop of thick, unruly dreadlocks. "There's lichen and algae. Microbes from the stone move in and invade the place. Sometimes only a fossil of the paint remains."
With more than half an acre of stained glass set in 175 windows, Chartres Cathedral is in the midst of the most exhaustive renovation in its modern history as millennial anniversary celebrations next year draw near.
Over the past 15 years, Petit's studio has restored 16 of the cathedral's windows, including the most famous -- the Blue Virgin window depicting Mary and the child Jesus. The artisans are now working on the glass of other churches while awaiting the chance to bid for work on seven windows that have been removed from above the cathedral's main altar and are undergoing scientific analysis.
After a fire destroyed most of an earlier cathedral on the site in 1194, the town built an even grander edifice in just under 30 years. Today, preservationists say, cleaning and restoring just the stained-glass windows is likely to take longer than that.
At a cost of nearly $200,000 for each of the largest and most intricate windows, the rehabilitation is constrained as much by finances as by the 18 months of analysis and delicate craftsmanship required to clean and repair each section.
The glass masters who created the windows used the vivid rainbow hues not merely as decoration, but to provide a history of both the spiritual and earthly realms, from Creation to Last Judgment, for the largely illiterate population of the time.
Today, the windows -- viewed by 1.3 million visitors a year, an attendance second only to that at Notre Dame Cathedral in Paris -- rank among the world's best-preserved records of medieval life, according to historians.
The glass weathered the anti-clerical fervor of the French Revolution. During the two world wars, villagers and priests dismantled the windows and hid them in locations throughout France.
The faces that have stared from the windows for centuries personalize the renovation project for fundraisers, donors and preservationists.
"Each of the windows tells a story," said Servane de Layre-Matheus, 66, who heads an international fundraising campaign for their restoration. "We explain the stories. People like to hear about the life of the saint. If we only talk about money, it's no good."
The French government pays for repairs to the infrastructure of the 87 cathedrals it considers historic monuments, but those funds cover only half the cost of refurbishing windows, according to de Layre-Matheus, who quit a government job eight years ago after surviving breast cancer and dedicated her life to helping preserve the cathedral.
De Layre-Matheus's group, the Chartres, Sanctuary of the World Association, has raised more than $1.5 million from corporations and 3,500 individuals. An affiliated group in the United States is also collecting money.
"Many of the donors have been to Chartres once in their lives," said de Layre-Matheus, who first visited the cathedral as a small child with her grandmother. "Some like art, others like history, some think Chartres is the most beautiful cathedral in the world."
The sheer number and intensity of the windows give visitors the sensation of walking through a massive kaleidoscope. Dozens of the windows soar three stories high and contain as many as two dozen extraordinarily detailed scenes. Intertwined in the religious chronologies are mosaics depicting the trades of the artisans who financed the windows.
Road and tunnel builders paid for St. Julien's window. Butchers donated the Miracles of Mary window and are recognized in an elaborate scene of customers pointing to slabs of meat on a trestle table. A calf's head sits among the cuts, and a pig carcass hangs from a nail. Carpenters sponsored the Noah window, evidenced by scenes of tradesmen shaping wooden slabs with hatchets.
Work began only this year on the highest, least accessible windows, which combine elements of the Gothic and Romanesque styles.
In the restoration process, the blue glass mosaics are first removed from their metal frames one window at a time. "Like somebody who is ill, the glass is taken down very carefully and it goes to the clinic," de Layre-Matheus said.
Many of the mosaics end up in the care of Petit, who began working in his father's stained-glass shop at age 5 to earn pocket money. His team begins the restoration process by cleaning each section of glass using chemical mixtures, gels and brushes.
The artisans then dismantle the panes, extracting each tiny piece from its lead casings with pliers that would be at home on a dentist's tray. They repair cracks using syringes filled with silicone, fix bad repair jobs from previous centuries with hair-thin copper strips and reassemble the pieces like a giant, fragile jigsaw puzzle.
"It's very emotional when we rediscover the image in the glass," said Claire Babet, a painter and restorer who has worked with Petit for the past seven years at his modern studio in the farmland outside Chartres. "You see decorative elements emerge, plants or little tassels that couldn't be seen before. When we do a hand or a face, it's incredible.
"It becomes more and more beautiful," she continued, her face animated. "But the best lies at the end, when you get to put it back in the church and the light shines through the whole thing."
Special correspondent Andrea Denham contributed to this report.
This window is dedicated to a patron saint of innkeepers and wine merchants.