The mirror black patina of certain early Chinese bronzes consists of crystalline cuprite of unusual mineral structure, possibly having a thin film of tenorite on its surface. Bronzes bearing this patina are usually of silver-white or nearly white metal with chemical composition and crystallographic structure closely resembling those of what was known to the Romans and is still known to us as speculum metal. Speculum metal appears to owe its rustless property to its high proportion of fine-grained duplex (alpha plus delta structure). The description of speculum metal objects as having a quicksilver or silver patina is erroneous, the color of the object being due to the silvery color of the alloy itself. Patination deposits on some early Chinese mirrors have been tested for quicksilver but no trace of this metal has been found. Some early Chinese bronzes, however, may have been tinned, a procedure which may well have produced a surface skin of composition similar to that of speculum metal.