Cut into a Gansu cliff from 366 CE onward, the Mogao caves hold some 45,000 m² of Buddhist wall painting laid down by every dynasty that held the Hexi Corridor. The postcards sell Silk Road orange. The walls do not have it: the lead minium oxidised into brown-black lead dioxide, and the faces of Cave 275 now read as dark earth.

What did survive is copper — malachite and atacamite still carrying drapery, canopies and the ribbons of the 飞天, with azurite blue in the sky bands above. This system is built from that surviving half: dark plaster ground, mineral green, and colour that sits inside the surface rather than on top of it.