The Louvre on Friday unveiled a newly-restored Leonardo Da Vinci masterpiece, the 'Saint Anne', hoping to lay to rest an art world row that saw the Paris museum accused of endangering the precious oil.

'The Virgin and Child with Saint Anne', which the Renaissance master left unfinished when he died in 1519, goes on display on Thursday as the star of a major exhibit exploring the work's genesis, and its place in art history.

Da Vinci began painting the 'Saint Anne', which depicts the baby Jesus grappling with a lamb beside his mother and grandmother, in 1503. When he died, the work was acquired by France's King Francis I, the artist's last patron.

"The story of the 'Saint Anne' is the story of the last 20 years of Da Vinci's life," said Vincent Delieuvin, curator of the exhibit dubbed 'Saint Anne', Leonardo Da Vinci's last masterpiece.

The show brings together 130 drawings, preparatory studies by the master, earlier versions of the work by his workshop, writings referring to the 'Saint Anne' and works influenced by it -- including by Raphael or Michelangelo.

Delieuvin explained at a press preview he built the exhibition like a "police investigation, with all the clues to understanding the painting."

Sketches by Da Vinci, full size cartoons -- pinpricked drawings used to mark out the form of the composition -- and 'Saint Anne' paintings carried out by his atelier, show how he how moved through three versions of the religious scene.

Each time he made minor adjustments, until the final composition in which the Virgin Mary seems to pull the baby Jesus away from the lamb -- symbol of the sacrifice he is to make -- as the wise Saint Anne urges her to let him go.

The show is the culmination of a high-sensitive 18-month restoration project.

After six months in a laboratory, the 'Saint Anne' spent a year in the workshops of the French Museums' Centre for Research and Restoration, the C2RMF near the Louvre, in the hands of the museum's chosen restorer Cinzia Pasquali.

Pasquali stressed the restoration was not for aesthetic reasons.

"This was about caring for a sick patient," she said. "From a conservation point of view we had to intervene, primarily to address a cracking of the varnish that could leave the paint exposed to damage."

That said, the 'Saint Anne' has been tranformed by the work.

Ageing varnish had left it disfigured by stains, now all but gone, with the overall effect one of lifting a yellow-brown veil, to reveal the soft blue of the Virgin's dress -- and a wealth of detail like a rocky pool of water bathing the subjects' feet.

"The build-up of varnish had as if flattened out the painting," said Pierre Curie, head of paintings at the C2RMF. "Restoring it has given it volume again, like a sculpture set in a landscape."

From details like the lamb's tail or the draping on the subjects' dresses, "we can see plainly that the work is unfinished," Pasquali said, "something we knew from other sources, but now we can actually see it."

"Some believe Mary's face is so diaphanous it may be unfinished too -- but I think he wanted it that way, to suggest her purity."

Pasquali's restoration for the Louvre hit a rock last year over concerns that a solvent used to thin the varnish could remove actual paint, with two experts resigning in protest from the committee overseeing the work.

"I don't really understand the reasons for the row," Pasquali said. "We stayed a long way from the pictorial layer -- you can't damage something you are not touching."

One of the dissenting experts, Segolene Bergeon Langle, interviewed in the Louvre's in-house magazine, said she had since been reassured on some aspects of the restoration.

But she remained unhappy about others, such as the decision to remove a white patch on the body of the baby Jesus, which she believes may have been added by Da Vinci's hand.

More generally, critics' fears centred on the faces of Saint Anne and Mary and on the 'sfumato', layers of clear paint used by Da Vinci to create smoky, soft contours.

To defuse tensions, the Louvre chose to leave an extra thickness of varnish on the subjects' fragile faces. It also ruled out removing some elements, like a group of tree trunks, even though they are thought to have been added in the 19th century.

"The Louvre has a very moderate cleaning policy," said Curie. "When we return to the painting, which we inevitably will do in 30, or 50 years, stopping at this level will give us a safety margin."