The Essen Domschatz is not, strictly speaking, a cathedral treasury - it is an abbey treasury that became a cathedral treasury when the diocese was created in 1957. From the ninth century until 1803, Essen was run by an unusual community of secular canonesses: aristocratic women who lived under a written rule but did not take the vows of nuns, who could leave to marry, and whose abbess functioned for centuries as both spiritual head and territorial princess of a small imperial state. These women - Mathilde, Theophanu, Sophia, Ida - commissioned almost everything in the chamber. The four golden crosses, the gospels, the seven-armed candelabrum that still stands in the minster nave: they were ordered by women who could afford the very best Ottonian goldsmiths and who used the resulting objects in their own liturgy. The Liber Ordinarius, a service book that has miraculously survived alongside the gold itself, records exactly which piece came out for which feast day.

The single most important object is a small wooden figure, only 74 centimeters tall, carved from poplar and sheathed in beaten gold. The Golden Madonna of Essen - the Goldene Madonna - was made around the year 980, almost certainly during the abbacy of Mathilde, and it is the oldest fully sculptural figure of Mary anywhere in the Western world. Mary sits on a throne with the Christ child upright on her knee, an apple in his hand, her eyes set with enamel and her face perfectly composed. The figure is meant to be looked at from every side, which makes it different from the relief Madonnas that preceded it and the painted icons of the Eastern tradition. It was carried in processions for a thousand years. It was hidden underground twice. It was restored in 2004, and you can stand in front of it today in a chapel of the minster a few meters from where it was first installed.

Gold this old does not stay still in difficult times. During the Ruhr Uprising of 1920, when communist militias and Reichswehr troops were exchanging fire across the surrounding industrial cities, the entire treasury was smuggled out to Hildesheim and kept there until 1925. After the Second World War the recovered objects spent another four years on the move - first the State Museum at Marburg, then a depot at Schloss Dyck near Rheydt, then a tour of Brussels and Amsterdam in 1949 - before they finally came home to Essen. In 1953 they appeared in an exhibition at the Villa Hügel, the former Krupp family seat. In 1957 the newly created Diocese of Essen took formal ownership. In 1958, at the wish of the first bishop Franz Hengsbach, the treasury chamber opened to the public free of charge for the first time in its history.

Between September 2008 and May 2009 the treasury closed for a structural expansion. While the new chamber was being built, the Domschatz mounted what may be the most photogenic exhibition in its history: it loaned the entire collection to the freshly opened Ruhr Museum, housed in the converted coal washery of the Zollverein colliery. The exhibition was called Gold vor Schwarz - Gold Against Black - and that is exactly what you saw. Ottonian crosses and the Golden Madonna stood inside the soot-stained shell of an industrial monument, the medieval and the industrial Ruhr held in a single frame. When the renewed treasury reopened in May 2009 it was over seventy percent larger than before and built to modern conservation standards, ready for whatever century arrives next.

Among the manuscripts you can find the Great Carolingian Gospels - known to scholars as Essen Cathedral Treasury Hs. 1 - a book that matters both for the beauty of its illumination and for the early Germanic language preserved in its margins. The Liber Ordinarius and the Necrology of Essen sit alongside it, the latter listing the names of every canoness whose anniversary the community was obliged to keep. Sixteen Burgundian fibula brooches from the fourteenth century glow in their own case - the largest surviving collection in the world. There are mitres and croziers from the modern bishops of Essen on loan from the Diocesan Museum. The whole chamber is run not as a museum in the usual sense but as a working liturgical store: most of these things are still, in principle, in use. They are simply, when not in use, available to be seen.