Leading Article: A row that may test the Mona Lisa's smile

When a painting reaches a certain point of fame, the work itself can scarcely hope to live up to expectations. So it is with Leonardo's Mona Lisa. This must be the most famous picture in the world. No other man- or woman-made image can have been reproduced as frequently, in just about every conceivable form and variation.

Yet when they locate it in the Louvre, most people's reaction is likely to be: 'Oh, is that really all it is?' Enjoying Leonardo's handiwork is not made easier by dense crowds of the curious, or by the bullet-proof glass that preserves the good lady from assault by deranged publicity-seekers. Cynics may be comforted by the thought that at least she, and her sister in notoriety, the 'Venus de Milo', siphon off touristic philistines who would otherwise be clogging up different parts of the museum.

In short, the Mona Lisa is famous for being famous. She has become a kind of freak show. So hearts will sink at the prospect of a battle of experts over whether or not this icon of Western art should be cleaned. With the controversy over the restoration - or vandalisation - of Michelangelo's fresco, The Last Judgement, in the Vatican's Sistine Chapel still resonating, the prospect of yet another debate is daunting.

The question of whether restoration work should be carried out on paintings 400 or 500 years old arouses intense passions. Amid the steam thus generated, it is hard for uninvolved lay persons to weigh the arguments. The core question is: are the gains from removing darkened varnish and thus revealing the tones - generally much brighter - underneath, outweighed by the damage that may be done in the process?

With each picture the technical pros and cons will inevitably be different. With Leonardo, one of the strongest arguments against cleaning is said to be the fragile, porous nature of the paint. It would be a brave conservation department that risked damaging the world's most famous image. If one of the Louvre's justifications is really that the Mona Lisa needs to be brighter in order to show up in a new, darker room, doubts would seem to be justified. Yet the case for sensitively removing her veils of darkened varnish may be powerful. Stand by for its advocates to have their lengthy say too.