Editing Calotype (section)

==The process==
Talbot made his first successful camera photographs in 1835 using paper sensitised with [[silver chloride]], which darkened in proportion to its exposure to light. This early "photogenic drawing" process was a ''printing-out'' process, i.e., the paper had to be exposed in the camera until the image was fully visible. A very long exposure—typically an hour or more—was required to produce an acceptable [[negative (photography)|negative]].

In late 1840, Talbot worked out a very different ''developing-out'' process (a concept pioneered by the [[daguerreotype]] process introduced in 1839), in which only an extremely faint or completely invisible [[latent image]] had to be produced in the camera, which could be done in a minute or two if the subject was in bright sunlight. The paper, shielded from further exposure to daylight, was then removed from the camera and the latent image was chemically [[photographic processing|developed]] into a fully visible image. This major improvement was introduced to the public as the ''calotype'' or ''talbotype'' process in 1841.<ref>[https://patents.google.com/patent/US5171A/ US Patent 5171], Henry Fox Talbot: "Improvement in Photographic Pictures" filing date Jun 26, 1847</ref>

The light-sensitive [[silver halide]] in calotype paper was [[silver iodide]], created by the reaction of [[silver nitrate]] with [[potassium iodide]]. First, "iodised paper" was made by brushing one side of a sheet of high-quality writing paper with a solution of silver nitrate, drying it, dipping it in a solution of potassium iodide, then drying it again. At this stage, the balance of the chemicals was such that the paper was practically insensitive to light and could be stored indefinitely. When wanted for use, the side initially brushed with silver nitrate was now brushed with a "gallo-nitrate of silver" solution consisting of silver nitrate, [[acetic acid]] and [[gallic acid]], then lightly blotted and exposed in the camera. Development was effected by brushing on more of the "gallo-nitrate of silver" solution while gently warming the paper. When development was complete, the calotype was rinsed, blotted, then either stabilized by washing it in a solution of [[potassium bromide]], which converted the remaining silver iodide into [[silver bromide]] in a condition such that it would only slightly discolour when exposed to light, or "[[photographic fixer|fixed]]" in a hot solution of [[sodium thiosulphate]], then known as ''hyposulphite of soda'' and commonly called "hypo", which dissolved the silver iodide and allowed it to be entirely washed out, leaving only the silver particles of the developed image and making the calotype completely insensitive to light.

The calotype process produced a [[translucent]] original negative image from which multiple [[positive-negative film|positives]] could be made by simple [[contact print]]ing. This gave it an important advantage over the daguerreotype process, which produced an opaque original positive that could be duplicated only by copying it with a camera.

Although calotype paper could be used to make positive prints from calotype negatives, Talbot's earlier silver chloride paper, commonly called ''salted paper'', was normally used for that purpose. It was simpler and less expensive, and Talbot himself considered the appearance of salted paper prints to be more attractive. The longer exposure required to make a salted print was at worst a minor inconvenience when making a contact print by sunlight. Calotype negatives were often impregnated with wax to improve their transparency and make the grain of the paper less conspicuous in the prints.

Talbot is sometimes erroneously credited with introducing the principle of latent image development. The bitumen process used in private experiments by [[Nicéphore Niépce]] during the 1820s involved the chemical development of a latent image, as did the widely used daguerreotype process introduced to the public by Niépce's partner and successor [[Louis Daguerre]] in 1839. Talbot was, however, the first to apply it to a paper-based process and to a negative-positive process, thereby pioneering the various developed-out negative-positive processes which have dominated non-electronic photography up to the present.