Until 1991, the whereabouts of Raphael's original painting from the 1500s was an intriguing mystery for art historians and only copies were known.

However, during a visit to Alnwick Castle in Northumberland, Dr Nicholas Penny spotted an intriguing painting which demanded closer examination. With infrared reflectograms (right), the underdrawing was revealed beneath the paint surface, proving authenticity.

Subtle differences between this underdrawing and the finished painting, particularly in the costume and background landscape, showed the artist had changed his mind as he worked. No copyist wishing to pass a painting off as the original would have departed from his model in such a way. Chemical analysis confirmed that the pigments used were typical of Raphael's distinctive palette, including some that ceased to be used after the 16th century.

Close Examination: Fakes, Mistakes and Discoveries opens on June 30 and runs until September 12.