Formulary's Printing-Out Paper (POP) Kit - also called salted paper - is a very old technique that was originally developed by William Henry Fox Talbot.
In the process, paper is first coated with a solution containing chloride ions. After drying, the paper is then sensitized with silver nitrate, which reacts with the chloride to form light-sensitive silver chloride. Upon exposure to ultraviolet light, the silver chloride is converted to silver metal, forming the image. Development is not necessary - thus the term "printing-out."
Salted paper prints have a beautiful delicacy in the lighter tones while the deeper shadows seem flat. The visual impact of the prints is controlled, in part, by the paper used for printing.