Advent Calendar: Day 23

By Circe -

Old photographs are literally precious. Nobody thinks about that, but it's true. Until the advent of modern color processes, which use dye emulsions, all photographs were taken on paper or film coated with a light-sensitive solution of silver halide. Black and white negatives are coated with the same precious metal we make into cheap rings and expensive forks.

An even older technique, the daguerreotype, is even richer. Daguerrotypes are taken directly onto a copper plate which has been surfaced with silver and polished to a mirror finish. The latent image is brought out with iodine, built up with mercury vapors (yikes!) and fixed with a solution of gold chloride. 

The result is quite striking, and unlike any photo you're used to, although it does bear some resemblance to some kinds of holographic stickers. The image emerges not from development of a pigment, but as degrees of reflection among clouded areas of the mirror. Areas of the plate that have been exposed to light will catch the mercury vapor, and develop a misty, non-reflective finish, where unexposed areas can be rinsed clear. To view a daguerreotype, position the plate so that the surface reflects a dark backdrop. The reflective areas of the image will clearly reflect the dark backdrop, thus appearing darker, and the hazy areas will scatter light, appearing lighter. Things reflected in a mirror will retain their apparent distance from the viewer, so the lighter parts of the image appear to float in front of the shadows.

The down side is that daguerreotypes are remarkably fragile. The image is an incredibly thin layer of deposited mercury, which can be destroyed by rough handling, and air and moisture can tarnish the silver surface of the plate. 

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