As you walk into Gallery 625 of the European Paintings department in the Metropolitan Museum of Art, you confront a rather puzzling work. Madonna and Child, by Italian painter Duccio di Buoninsegna, is much smaller than its neighbours, barely the size of a standard US letter. While its peers float in ethereal realms of shining gold, Duccio’s gilding lacks lustre, the tracery of cracks on its surface evincing the painting’s extreme age. Even the colors are not as bright. Mary’s dress, which dominates much of the scene, appears to be painted in somber azurite, rather than the brilliant ultramarine that…