Copper lustre is a metallic glaze obtained in a third, low-temperature firing. The result is not a flat gold: it shifts with the angle of the light, so the wall seems to catch fire as you walk past. Applied over Moorish geometry it gives the most sumptuous finish in the catalogue, which is why it is kept for the main rooms.
See also: Moorish copper mosaics, Moorish colour tiles and Sevillian copper tiles.