Fox Talbot was born in England in 1800 and died in 1877. He invented the first generally available photographic process.
He was surrounded by people who could draw and paint. So, being not particularly gifted in that area, he decided to develop a process of “photogenic drawing”. His experiments began around 1834 and by 1835 he had “salted paper” prints. He didn’t announce his process until 1839 after learning that a Frenchman, Louis Daguerre, had invented the Daguerrotype.
Fox Talbot soaked ordinary paper in a solution of table salt, Sodium Chloride, and then coated it with silver nitrate. This formed silver chloride, which visibly darkened when exposed to light. Though exposures would take hours.
There had been earlier photographic processes, but the images formed would continue to darken when exposed to light. Fox Talbot managed to find a way to stabilise or “fix” the exposure.
The image formed was a “negative” meaning that the bright parts of the scene were rendered black in the image. But once fixed the image was stable enough that it could be put in contact with a fresh unexposed piece of paper, and the pair exposed to sunlight to make a “contact print”. The second image was then an accurate positive rendition of the tones in the scene.
In 1841 he improved the process by replacing the silver chloride with silver iodide. The paper was coated on one side with silver nitrate, dried, and then dipped in a potassium iodide solution. Once dried this paper was stable and insensitive to light. Before use, it had to be activated by brushing the working side with a solution of silver nitrate, acetic acid and gallic acid.
This paper was much more sensitive to light, bringing the exposure time down to minutes rather than hours. But the image formed was a latent one. It was invisible until developed using more of the gallic acid solution and gentle heat. Once developed the image was fixed using a hot solution of sodium thiosulphate, which was called “hypo”. A term still used in film development.
This was called the calotype process after the Greek words for beauty and impression.
You can appreciate that the need to apply liquids to a piece of paper and get the resulting damp sheet into a camera in the dark made the process less than convenient.
Since the calotype produced a negative image a contact print still needed to be made. As it didn’t matter how long this took the early simpler salted paper process was used to make positive prints.
Although cumbersome this calotype process was the basis for modern film photography.