Scientists found these old photographs contain metallic nanoparticles

And no two daguerreotypes are exactly the same."Daguerreotypes, unlike other types of photograph, rely on light scattering by metallic nanoparticles to create an image that projects off a reflective silver substrate," the authors wrote. Those nanoparticles change how the electrons in the cup vibrate in response to light so that the color one sees shifts with the observer's vantage point."The Romans knew that if they added salt when they melted the silicon (sand) to make the glass, those salts contain gold atoms," said Manjavacas. That's how any stained glass is made." It's similar to how the precise lattice-like structure of photonic crystals produces iridescent colors in nature, like the wings of butterflies, or opals.Like the Lycurgus Cup, the unusual color properties of 19th-century daguerreotypes are the result of the scattering properties of nanoparticles, according to Manjavacas. "In our case, the scattering of the particles has a strong dependence on the substrate," said Manjavacas.The team also discovered that if they coated the surface of the daguerreotype with a layer of gold, the colors shifted more to the red side of the spectrum. That's in keeping with a 19th-century trick called "gilding," in which the photographer would apply a solution of gold salt to the image to achieve warmer tones and sharper contrast.Plasmonic nanoparticles (gold, silver, and platinum, most notably) are "tunable," because one can alter their size and shape to tweak the optical properties of the metallic material.