A daguerreotype’s evocative tones are created by nanoparticles that scatter mixtures of red and blue light.

Invented in the 19th century, daguerreotypes were one of the earliest types of photograph. They were produced using copper plates coated with a silvery mixture. During the taking of an image, light exposure caused the coating to form silver clusters. The plate was then treated with mercury vapour, producing a diverse array of nanometre-scale silver-mercury crystals. Each crystal scattered a colour that was composed of a combination of red and blue tones; together, the scattered light made up the image.

Alejandro Manjavacas at the University of New Mexico in Albuquerque and his colleagues analysed daguerreotypes with modelling and microscopic imaging. The team found that the oblong shapes of the photos’ silver–mercury nanoparticles tend to scatter blue light along their vertical axes and red light outward. A photo’s nanoparticles scatter a combination of these hues to create the image’s characteristic tones. The nanoparticles’ properties also explain why daguerreotypes take on a reddish tint when viewed from an angle.

The findings could advance printing technologies that form colour images from metallic nanoparticles, the authors write.