Introduction
With the advent of disposable film cameras and cheap digital equipment, photography has become a popular hobby which is accessible to virtually anyone. It is hard to imagine a world without photographs - a world in which all images had to be hand-drawn or painted and where the printing press was the only means of reproducing them. And yet it is only around 200 years since the first photograph was made.
This article presents a brief history of photography, and information on some of the photographic processes that have been used over the last two centuries.
Early Experiments
In the 18th century it had been observed that certain preparations of silver compounds would darken on exposure to sunlight, and in 1800 Thomas Wedgwood produced 'sun pictures' by placing opaque objects on a piece of leather soaked in silver nitrate solution and exposing it to sunlight. These shadow pictures were possibly the first photographs, but they were unstable and would darken rapidly when exposed to anything brighter than candlelight.
The Search for Stability
Prior to this period painters had used the 'camera obscura' (literally 'darkened room') to project pictures of the outside world onto a surface from which the scene could be traced, so in 1816 the French inventor Nicephore Niepce placed a sheet of sensitized paper, coated with silver nitrate, at the image plane of a camera obscura and obtained the first true photographs of a scene - the view from his window. These photographs were also unstable, but after lengthy research he found a technique in 1828 whereby stable images could be produced on a silver plate.
Niepce's method involved dissolving Judea bitumen in lavender oil and spreading a very thin layer onto a polished silver plate, where it would dry to a reddish color. On exposure to daylight (for a period of a few days!) the bitumen would undergo a change and become insoluble, so on washing the exposed plate with more lavender oil a negative image could be seen - where the plate had been exposed to light, the surface would remain coated with bitumen, while the unexposed areas would show the underlying silver plate. To convert this to a positive image, Niepce exposed the plate to iodine vapour which reacted with the silver to produce silver iodide, which darkened on further exposure to light so producing stable positive images.
The Need for Speed
This remained the state of the art for a few years, but in 1834 Henry Fox Talbot made negative images on sheets of paper coated with silver chloride, and fixed with solution of 'hypo' to prevent them darkening after exposure. He converted these to positive images by contact printing them onto a second sheet of sensitized paper. The 'calotype' process, patented in 1841, used this technique to produce the final positive images from negatives created by an improved process, quite similar to modern photography, in which the paper was primed with silver iodide and sensitized with a solution of silver gallo-nitrate. This paper required exposure times between ten seconds and ten minutes, and produced a deep brown or black negative image.
Commercial Success - the Daguerrotype
Meanwhile Niepce had joined forces with another Frenchman, Louis Daguerre, to investigate new photographic techniques. After Niepce's death in 1833 Daguerre continued to work with Niepce's son Isidore, and in 1837 Daguerre discovered a method of producing fixed images on silver plates with exposure times of a few minutes. In 1839 the process was presented to the Sciences Academy and rapidly became a huge commercial success, since it was now practical to produce portrait photographs. The images were developed using mercury vapour, which posed a considerable health hazard to Daguerrotype photographers.
Cost Reduction - the Collodion Process
In 1851, a London sculptor Frederick Scott Archer improved the resolution of photographs by using a solution of iodized collodion (nitrated cotton dissolved in a mixture of ether and alcohol) spread onto glass and then dipped into a solution of silver nitrate, in order to produce silver iodide in the plate. This process was much cheaper than any other contemporary photographic method, and after many legal battles with Talbot, Archer published the details of his technique in 1854 as the 'Collodion Process'. He chose not to patent the method, which rapidly took over from the Calotype and Daguerrotype in popularity.