PARIS.- The sale at auction of old masterpieces is extremely rare, with most such works already held by museums or public collections.

Of 311 pieces ever to have surpassed the $30 million (26.6 million euro) mark at auction, only 13 were by Old Masters, the leading painters in Europe before 1800, according to an AFP database.

These include paintings by Leonardo da Vinci, Raphael, Rembrandt and Vermeer.

A painting believed by some experts to be Caravaggio's "Judith Beheading Holofernes" is being presented in Paris on Friday ahead of its auction in the southern city of Toulouse.

It was discovered in a Toulouse attic in 2014 and is could join this elite club.

In comparison, 142 of those that fetched more than $30 million are by Impressionist or modern artists and 132 are contemporary.

They include 32 paintings by Pablo Picasso, 27 by Andy Warhol, 23 by Francis Bacon and 21 by Mark Rothko.

Already in museums
Old masterpieces are rare at auction today because many entered museums or public collections in the 18th, 19th and early 20th centuries, French art market expert Patrick Michel told AFP.

Bought by museums or acquired through donations, such pieces generally do not return to the market.

Another factor is that many Old Masters, such as Leonardo and Vermeer, did not produce a body of work as large as that of some contemporary artists.

An example is American painter Jean-Michel Basquiat, who died aged 27 in 1988 but has had hundreds of works sold at auction for more than a million dollars in recent years.

Doubts
Authenticating the artist behind older works sometimes poses problems, Michel said.

"Many old paintings are not attributed with certainty to one single painter, which often gives rise to disputes between experts," he said.

This is the case with "Judith and Holofernes".

Some believe it is a long-lost masterpiece by the Renaissance painter (1571-1610) but others think it may be a fake, perhaps a copy by one of his Franco-Flemish disciples.

Some art experts have also expressed doubt about the authenticity of a painting attributed to Leonardo, "Salvator Mundi", which sold for a world record $450 million at auction in November 2017.

Michel said the market for old art was "filled with works of average or poor quality for which there are no longer any buyers".

By contrast, the "very rare" masterpieces that come along attract enormous interest.

"As soon as a painting that seems important arrives on the market, all the major institutions of the world -- museums in the United States, Britain and now the Emirates -- sharpen their knives, financially speaking," he said.

© Agence France-Presse