text: The Calotype The "calotype", or "talbotype", was a "developing out" process, Talbot's improvement of his earlier photogenic drawing process by the use of a different silver salt (silver iodide instead of silver chloride) and a developing agent (gallic acid and silver nitrate) to bring out an invisibly slight "latent" image on the exposed paper. This reduced the required exposure time in the camera to only a minute or two for subjects in bright sunlight. The translucent calotype negative made it possible to produce as many positive prints as desired by simple contact printing, whereas the daguerreotype was an opaque direct positive that could be reproduced only by being copied with a camera. On the other hand, the calotype, despite waxing of the negative to make the image clearer, still was not pin-sharp like the metallic daguerreotype, because the paper fibres blurred the printed image. The simpler salted paper process was normally used when making prints from calotype negatives.
  Talbot announced his calotype process in 1841, and in August he licensed Henry Collen, the miniature painter, as the first professional calotypist. The most celebrated practitioners of the process were Hill & Adamson. Another notable calotypist was Levett Landon Boscawen Ibbetson.
  In 1842, Talbot received the Rumford Medal of the Royal Society for his photographic discoveries.
  In 1852, Talbot discovered that gelatine treated with potassium dichromate, a sensitiser introduced by Mungo Ponton in 1839, is made less soluble by exposure to light. This later provided the basis for the important carbon printing process and related technologies. Dichromated gelatine is still used for some laser holography.
  Talbot's later photographic work was concentrated on photomechanical reproduction methods. In addition to making the mass reproduction of photographic images more practical and much less expensive, rendering a photograph into ink on paper, known to be permanent on a scale of hundreds if not thousands of years, was clearly one sure way to avoid the problems with fading that had soon become apparent in early types of silver image paper prints. Talbot created the photoglyphic (or "photoglyptic") engraving process, later perfected by others as the photogravure process.