Luster is a decorative metallic film that was applied an the surface of medieval glazed pottery. It can be obtained via the low-temperature (similar to 650 degreesC), controlled reduction of copper and silver compounds. In this paper, we show that luster is a thin layered film (200-500 nm thick) that contains metallic spherical nanocrystals dispersed in a silicon-rich matrix and has a metal-free outermost glassy layer that is 10-20 nm thick. Silver nanocrystals seem to be separated from those of copper, forming aggregates 5-100 pm in diameter. This composite structure exhibits optical properties that are dependent on both the particle size and the matrix. Luster is indeed the first reproducible nanostructured thin metallic film that was made by humans.