The Romans roofed their Welsh houses with slate and Edward I used it extensively in his Iron Ring of castles around Snowdonia, but it wasn’t until the Industrial Revolution that the demand for Welsh roofing slates rocketed. For the 1862 London Exhibition, one skilled craftsman produced a sheet of slate 10ft long, 1ft wide and a sixteenth of an inch thick – so thin it could be flexed – firmly establishing Welsh slate as the finest in the world. By 1898, Snowdonia’s quarries were producing half a million tons of dressed slate a year. At Penrhyn and Dinorwig quarries, mountains were hacked away. Workers often slept through the week in damp dormitories on the mountain, and tuberculosis was common, exacerbated by the slate dust. At Blaenau Ffestiniog, the seams required mining underground, with miners having to buy their own candles, the only light they had. In spite of this, thousands left their hillside smallholdings for the burgeoning quarry towns. Few workers were allowed to join Undeb Chwarelwyr Gogledd Cymru (the North Wales Quarrymen’s Union), and in 1900 the workers in Lord Penrhyn’s quarry at Bethesda went on strike. They stayed out for three years, but failed to win any concessions. Those who got their jobs back were forced to work for even less money as a recession took hold, and although the two World Wars heralded mini-booms as bombed houses were replaced, the industry never recovered its nineteenth-century prosperity, and most quarries and mines closed in the 1950s. What little slate is produced today mostly goes to make floor tiles, road aggregate, and an astonishing array of kitsch ashtrays and coasters etched with mountainscapes.