Chartres

The light comes through the glass in a blue that no one has been able to replicate since the 12th-century glaziers took their recipe to the grave. Chartres in France is a cathedral city built around that colour — Chartres blue — a pigment trapped in medieval windows that turns the interior into a space that glows rather than merely shines. The effect, after 800 years, is undiminished.

Chartres Cathedral, built predominantly between 1194 and 1220 after a fire destroyed the previous structure, is considered the finest example of French Gothic architecture. Its 176 stained-glass windows, the majority original to the 13th century, cover over 2,600 square metres — the largest surviving collection of medieval stained glass in the world. The particular blue dominant in the windows, known as Chartres blue or bleu de Chartres, has resisted exact replication; the mineral composition of the cobalt-based pigment and the glass-making technique remain imperfectly understood. The cathedral's labyrinth, laid into the nave floor around 1200, measures 12.9 metres in diameter and was walked as a symbolic pilgrimage. The old town descends from the cathedral to the River Eure, where medieval wash houses and a network of footbridges create a lower town of quiet gardens and stone. Chartres en Lumières, running from April to October, projects light installations onto 24 historic buildings across the city.