Kallotypes

Kallotypes are a historical photographic printing process that produces a positive image on paper. The process involves coating paper with a solution of ferric ammonium citrate and silver nitrate. When exposed to ultraviolet light, the ferric ammonium citrate is reduced to ferrous ammonium citrate. This ferrous salt then reduces the silver nitrate to metallic silver, forming the image. The paper is then washed to remove any unreacted chemicals.

Kallotypes were invented by Sir John Herschel in 1839, the same year he announced the invention of the platinum-palladium print process and coined the terms "photography," "negative," and "positive." The process was relatively simple to perform and produced prints with a rich, warm tone. However, kallotypes were not as permanent as some other photographic processes of the era, and the images could fade over time if not properly fixed and protected. Despite their limitations, kallotypes were popular for amateur photography in the mid-19th century due to their ease of use and the attractive results they yielded. They are now considered a significant part of photographic history and are appreciated for their aesthetic qualities.