The rumour started in arts circles and the press about five years ago. Was Le Jardin a Auvers a Van Gogh or a forgery by Emile Schuffenecker? The doubt meant no one bid when it came to auction in 1996.
Now the rumour has been squashed. After rigorous analysis, specialists at the new Centre of Research and Restoration of the Museums of France pronounced the picture genuine.
A Paris exhibition last year featured works by Cezanne and Van Gogh, together with copies done by their friend, Dr Gachet. High-tech examination by the centre differentiated authentic paintings from imitations.
Authentification is just one function of the centre, formed by the merger last year of the museums of France research laboratory and restoration service. Director Jean-Pierre Mohen said the merger allowed the centre to respond more efficiently to demands from the 50 or so national and 2,000 local museums. Demands might include repairs, restoration, dating objects or identifying materials.
The centre, which deals with up to 3,000 objects a year, employs about 150 curators, chemists, researchers, computer scientists and specialised photographers, plus some 200 restorers largely employed on contract and a network of workshops and restorers. It collaborates with universities, museums abroad and other laboratories. Young researchers from CNRS, the National Centre for Scientific Research, work alongside in a special unit. The laboratories occupy a former carpark under the Louvre grounds in Paris. Artworks are analysed by X-ray, scanning and infra-red, ultra-violet or raking light techniques.
Broken statues and musical instruments are among antiques awaiting attention. The wire "skeleton" of a tiny 18th-century painted glass-paste statuette is visible on X-ray. In pictures, the artists' original sketches and mistakes are revealed under paint.
The laboratory's 30m long particle accelerator reveals information about bones, geological origins, ingredients of Egyptian cosmetics or the composition of pigments, including each colour in an illuminated manuscript or piece of Chinese pottery. It proved that the eyes of Ishtar, an alabaster Parthian goddess in the Louvre, are rubies.
Sophie Lefevre of the centre said: "When big exhibitions are planned five years in advance it allows us to restore and study the works to be shown. We can make an examination which we couldn't often do otherwise."
One project between the laboratory and cosmetics company L'Oreal analysed the composition of eye make-up worn in ancient Egypt. Traces taken from 49 pots in the Louvre's collection were found to be largely lead-based. But the presence of the compounds laurionite and phosgenite, rarely found in nature, as well as the more common galena and cerussite, indicated that the Egyptians synthesised their products and had mastered such disciplines as chemistry, optics and medicine.
While some workshops will soon be installed in the Pavillon Flore at the Louvre when renovation finishes, the headquarters of the restoration side is located at another historic site - the former royal stables of Louis XIV's Chateau of Versailles.
Craftsmen and women practise such skills as cabinet-making, textile and fine-arts restoration, gilding, marquetry and metalwork, carefully reviving delicate embroidered lace, 18th-century Tibetan silk panels, brass-and-tortoiseshell-inlaid Boulle cabinets or paintings dating from the 14th century.
The work raises ethical questions such as how much they can restore a damaged piece without it losing its integrity. Museum curators are legal custodians of their works so they decide.
Paris's Tolbiac University offers a postgraduate course on conservation skills that help avoid the need for restoration. Mr Mohen says that later generations need comprehensive records about works and their restoration treatment so that they can be reversed or further damage be prevented.