Del Vaga’s work was considered the perfect teaching tool for Renaissance painting, and was the first purchased addition to Samuel Courtauld’s collection. When you look closely, it has everything: there’s a graphic outline in pen and ink, and the thinness of the paint means there are visible brushstrokes on the head of the figure and pricking marks. There is a clear build-up of glaze as well as touches of highlight. In terms of explaining technique it has it all. To the left of the main figure you can just make out an arrangement of small figures — probably a rendition of the Massacre of the Innocents before del Vaga decided to add St Joseph instead. It’s fascinating to see the artist working things out directly on the canvas.