'The drawing was a page from Leonardo da Vinci’s Codex Atlanticus, and I had been invited to witness the painstaking process of its conservation. One morning last winter, I descended to the conservators’ laboratory, which occupies a room just outside the steel-and-glass doorway to the Ambrosiana’s gleaming vault. At the bottom of the stairs, I was stopped by an attendant, who took a coffee cup from my hands and placed it out of harm’s way.'