"Imagine you have 4,000 puzzles of 1,000 pieces, you mix them all, you burn 90%, and then you try to reassemble them with the ones you have left." Santiago Alcolea, a Catalan art historian, describes with this image the work of medieval art historians. Knowing the author, the place of origin, or reconstructing an altarpiece from the surviving compartments can be a race full of obstacles.