Split, stolen, even stashed in a salt mine, one of the world's most mythical oils, Flemish masterpiece "The Adoration of the Mystic Lamb", is undergoing its most ambitious clean-up in 600 years.
By Flemish primitive masters Hubert and Jan Van Eyck -- though Hubert remains something of a mystery -- the 24-panel work is also known as the Ghent Altarpiece or Lamb of God, and features the first known nudes in Flemish art, Adam and Eve.
Its unusually eventful past as well as questions over its genesis pose an extra challenge to the five-year restoration project taking place in full public view at the Ghent Fine Arts museum (MSK), with details also on website closertovaneyck.kikirpa.be.
"We'll never find the exact original state, it just isn't possible!" project leader Livia Depuydt-Elbaum told AFP.
"With time, colours fade, materials alter. But we can get closer than has ever been possible before."
After a year at work, state-of-the-art analysis has also shown that the wood in two panels was carved out of the same tree. Infrared reflectography has revealed an early sketch hidden under layers of paint.
"It is a much finer work than ever said before, which uses extremely complex painting techniques," said art historian Helene Dubois after poring over St John the Baptist's robe with a special 3-D microscope at the Ghent museum.
Visitors to the museum in the Belgian city can see the work in progress behind a large glass panel.
'No catastrophic gaps'
Already the iconic Van Eyck reds and greens, the optical effects and mastery of detail such as fabric patterns and jewels are emerging from beneath old dulled varnishes.
Hopes are that the 1.2-million-euro ($1.6 million) project will clear up questions about the creation of the work -- how long it took to complete, which of the brothers or their assistants painted what?
Little is known about the Van Eyck family and although Jan's works are famous, not a single painting has been attributed with certainty to Hubert. Some even doubt his existence.
"We have noticed huge differences in painting technique," said Dubois. "There are very big differences in quality not only between the panels but also between different parts of one panel."
In the first months, conservationists researched the work's chaotic history before painstakingly scraping off coats and coats of yellowing varnish and layers of over-paint at a rate of just four square centimetres a day.
"All in all there are no catastrophic gaps, no faces or key elements have been badly damaged or attacked," said Depuydt-Elbaum. The worst problem area is a large white spot in St John the Evangelist's robe, she said.
That panel was either badly restored in the past or left too close to a window in the almost 100 years the work spent in a Berlin museum, where its side panels were sawn apart to separate back and front.
From its start, the talk of Europe
Made of 12 oak panels painted on both sides, the massive altarpiece from its beginnings was the talk of Europe, attracting kings and queens to St Bavo's cathedral in Ghent -- even German artist Albrecht Durer made the trip in 1521.
According to letters etched into the frame that make up a chronogram in Roman numerals, the immense 4.4 x 3.4-metre (yard) work dates to 1432 -- although art historians squabble about whether it was really finished by then.
The red capital letters are part of a four-line verse stating that Hubert Van Eyck, "a greater man than whom cannot be found," began the work, but that it was completed by Jan, "the second greatest artist."
With the Reformation, Protestants attacked Ghent in the 16th century and the altarpiece was hauled up to safety in the St Bavo tower.
Two centuries later, panels that had been seized by the French were returned to the church by the Duke of Wellingon after his victory at Waterloo against Napoleon.
Ghent sold them not long after to an art dealer -- with the exception of the Adam and Eve panels whose nudity was a moral shocker at the time -- from where they wound up in the hands of the king of Prussia before heading home.
In 1934, two of the panels were stolen in Ghent and one -- The Just Judges -- remains misssing to this day.
Sent to the Vatican for protection during World War II, the panels went instead to France and were seized by the Nazis, who later hid them in a salt mine in Austria.
There they were saved from planned destruction by the 3rd US Army.
Art historian and restorer Dubois said she first saw the work at age 15 and was transfixed.
"It inspired me to become who I am," she said.
"After 600 years, this work has not yet delivered all its secrets."