If we comment on the cleaning and restoration of paintings, it is important to remember that the conservator of today is on the end of a very long chain of events. When a cleaned painting is put back on the walls of a museum, or returned to its owner from the restoration studio, our rush to judgement should be tempered by the fact that this work may have been through the cleaning process a dozen times - and that its present condition may have little to do with the last hand that touched it.

If we comment on the cleaning and restoration of paintings, it is important to remember that the conservator of today is on the end of a very long chain of events. When a cleaned painting is put back on the walls of a museum, or returned to its owner from the restoration studio, our rush to judgement should be tempered by the fact that this work may have been through the cleaning process a dozen times - and that its present condition may have little to do with the last hand that touched it.

Ever since paintings have been made, they have been cleaned and repaired. Archives and other documentary sources record numerous examples of paintings washed and restored within little more than a century of being finished - when the paint would still have been relatively soft and vulnerable. The first restoration of Van Eyck's Ghent Altarpiece was carried out by Jan van Scorel in 1550, just 110 years after it was painted, and that of Rogier van der Weyden's Descent from the Cross, after only 130 years, was done by Philip II's court painter, Navarrete the Dumb.

Nor were these restorations immune from criticism. When Signorelli's Circumcision was damaged by damp, the child was repainted by the painter, Sodoma. Giorgio Vasari, the chronicler of Italian Renaissance art, tells us Signorelli's painting had been of great beauty, but the child was now "much less beautiful than before."

Until the 17th century, restorations were carried out by practising painters in their spare time. Rubens and Velazquez are probably the two most celebrated painters to have been involved in restoring paintings, stepping in when disasters struck the pictures in their care.

It was all pretty haphazard. Then, in the 18th century, something significant emerged - the specialist commercial picture restorer. Paintings were suddenly in greater peril than ever.