Il cane di Ercole scopre la porpora di Tiro

By 1636 Rubens was in his late 50s, honoured across Europe and increasingly slowed by the gout that would kill him four years later. From Antwerp he was running one last enormous job, more than 60 mythological scenes for the Torre de la Parada, a hunting lodge Philip the Fourth was decorating outside Madrid. He couldn't paint them all himself, so he worked like this, in small fast oil sketches on panel that assistants would later enlarge to full size. This one tells an old origin story for the dye called Tyrian purple. Hercules walks a beach with his dog, the dog bites into a sea snail, and its mouth comes away stained a deep reddish violet, the colour that would one day mark out emperors. It is brushed loosely and quickly, a set of instructions as much as a finished picture.