Re-roofing on a larger scale: Apethorpe Palace in Northamptonshire, a royal residence of James I, before being reroofed in Collyweston slate, the work being completed in 2012. Traditionally the mined Collyweston stone was left outside for three winters until the frost opened fissile layers that could be clived into slates. Faced with shortages of Collyweston slate, in the late 1990s English Heritage (now Historic England) funded a project to develop an artificial system to reproduce the necessary freeze-thaw cycle. (Photo: Chris Wood)
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