Prussian Blue is the name of a dark blue pigment produced by the oxidation of ferrous ferrocyanide salts. It was the first modern synthetic pigment to exist and was used on the hands and frames of Big Ben which, in the 1930’s, were painted black in an attempt to disguise the effects of air pollution in the capital. Late last year — four years into the clock’s restoration — the scaffolding that had been covering one of its four faces was removed; the paint had been stripped and the pigment beneath was once again visible.