The Vinland Map is a document that appears to be a 15th-century mappa mundi and which shows a landmass in the Atlantic Ocean, directly south-west of Greenland, labelled Vinlanda Insula ("Isle of Vinland").  It first came to light in 1957 and was acquired by Yale University in 1964.  The map's authenticity would have established the awareness of a part of the American continent by European cartographers, before the voyages of Christopher Columbus.  McCrone, already reputed for his expertise in authenticating ancient documents and works of art, was asked by Yale to analyze the map in 1972.  In 1974, he published evidence that the ink of the map contained synthetic anatase (a form of titanium dioxide), a substance not used as a pigment until the 1920s.  According to McCrone, the anatase is present in the yellow ink that the forger used to simulate the natural discoloration that appears over long periods of time around lines drawn on parchment in medieval iron gall ink.
McCrones work on the Vinland Map led to a protracted controversy, with other researchers continuing to argue for the documents authenticity and discounting the presence of titanium as insignificant.  In 2021, Raymond Clemens, the curator of early books and manuscripts at Yale's Beinecke Rare Book & Manuscript Library where the map is housed, declared that it had been conclusively shown to be a fake.  That judgment was largely based on the presence of synthetic anatase in the ink, as first identified by McCrone.