These glorious drawings open a window onto Piranesi’s soul. Like him, they are endlessly inventive, astonishingly original and ferocious. Flying across the page, Piranesi’s quill pen violently scratches out one fantastical design after another. There are riots of imaginary architecture of almost impossible grandeur, as in An ornate triumphal arch with a grand staircase (c 1747–50), and intense jumbles of motifs, as in The meeting of the Via Appia and the Via Ardentina (c 1750–56). Both these designs, characteristically, are filled with invented trophies and monuments and contain dramatic contrasts of light and shade. There was great method in Piranesi’s madness, and beneath every apparently hectic drawing can be detected the solid underlying structure of a guiding grid. He never put a perspectival foot wrong.

Most of his drawings in the British Museum were bought at the sale of the collection of John Gott, bishop of Truro, in 1908. The bishop’s grandfather was the great Leeds industrialist Benjamin Gott, whose sons used their money wisely, travelling to Italy and accumulating libraries and works of art. The drawings that the Gotts acquired had often served as first ideas for Piranesi’s incomparably rich etchings, An ornate triumphal arch, for example,