During the second world war, the painting was transported 250 miles from central London to north Wales to be hidden in a slate mine, tucked away from the perils of Nazi invasion and Luftwaffe bombings. Eighty years on, there has been a sort of homecoming for Canaletto’s masterpiece, The Stonemason’s Yard, as it returns to form the centrepiece of a show opening this week at the National Library of Wales in Aberystwyth. The painting, an early work of the 18th-century Venetian artist, was among the treasures moved from the National Gallery in London to the cavernous Manod slate mine, near...