The earliest practical photo process by Louis Daguerre, from 1839. To make a daguerreotype a silvered copper plate is polished to mirror finish, sensitized with iodine fumes, then exposed in a camera and developed with heated mercury. Daguerreotype is perhaps the most challenging of photographic processes, and most toxic too. Fortunately there is an alternative to mercury development: with the Becquerel process only red light is used for developing. Most of my plates are Becquerel-developed.