Britain’s Most Fragile Treasure sees historian Dr Janina Ramirez unlock the secrets of a centuries-old masterpiece in glass. At 78 feet in height, the Great East Window at York Minster, often referred to as England’s Sistine Chapel, is the largest medieval stained-glass window in the country, and it was the creative vision of a single craftsman, John Thornton, the foremost stained-glass artist of his time. The scale of John Thornton’s achievement is revealed as Dr Ramirez follows the work of the conservation team at York Glaziers Trust, who are restoring the Great East Window to its original glory. The programme promises a unique opportunity to examine John Thornton’s greatest work at close quarters, and to reveal exactly how medieval artists made images of such complexity using the simplest of tools. The Great East Window of York Minster is more than a work of artistic genius, it tells a story of the medieval world, preserved in the most fragile medium of all: glass.