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Daguerreotypes - Discovery

Louis Jacques Mande Daguerre (1789-1853), collaborated with Joseph Nicephore Niépce from 1829 in attempts to make a workable photographic process. Niépce had earlier produced a rudimentary photographic process, heliography, using a light coloured bitumen on a metal plate in 1822 (the first camera-made image probably dates from 1826.) They decided to concentrate on methods based on the known sensitivity to light of silver salts, and in particular silver iodide. Niépce made little progress and probably abandoned these experiments a couple of years before his death in 1833, but Daguerre continued his work despite two problems - the exposures needed were extremely lengthy and the images rapidly disappeared as the plate darkened all over when viewed in light.
A fortunate accident supposedly occurred, when a briefly exposed plate was taken from the camera and placed in a drawer for later use was found to have produced a visible image. Daguerre apparently experimented with various chemicals present in the drawer, eliminating them, before finding that the development was caused by mercury from a broken thermometer.
Daguerre also found that the images could be 'fixed' or made more or less permanent, by bathing the exposed plate with its image in a strong solution of sodium chloride (salt) until the yellow colour of the silver iodide disappeared as it dissolved in the salt.
Daguerre interested the French government in the process, and presented it to the French nation in exchange for a pension. He also took out a patent in England, so he could exploit his invention commercially there, as well as working with others to market 'Daguerre' cameras and other equipment in France and across the world. His manual of the process was published very shortly after his first public demonstration.
There were soon improved versions of his process, including the use of bromine (J F Goddard, 1840) and chlorine (A Claudet, 1841) as well as iodine to produce the light sensitive silver salts on the plate surface (silver bromide and silver iodide respectively.) These made the plate more sensitive to light, and together with faster lenses made it possible to reduce exposures from the 5-30 minutes used by Daguerre down to 5-30 seconds.
Sir John Herschel, the British scientist best known as an astronomer, heard the news of Daguerre's process (and had also seen some of William Henry Fox Talbot's early 'photogenic drawings') 3 weeks after it was announced. He immediately set to work - without knowing the details of either process - and in a week had managed to produce his own photographic images, in a way similar to Talbot. Herschel made one great improvement, that was taken up immediately by Daguerre when he learnt of it. Both Talbot and Daguerre had used a strong common salt (sodium chloride) solution, to 'fix' their images, but it was not very effective. Herschel had 20 years earlier discovered that 'hyposulphite of soda' (now called sodium thiosulphate) was effective in dissolving silver chloride and bromide, and used that to fix his images. Daguerre was quick to adopt this new method for his process, and Talbot slowly followed. Generations of photographers have used 'hypo' since that time.
Gold toning, introduced by Hippolyte Fizeau in 1840, soon became normal practice, as it intensified the images and gave them a greater brilliancy. It also created more permanent images that tarnished less rapidly.
Also in 1840, Alexandre Edmund Becquerel found that exposed daguerreotype plates could be developed without the use of mercury, simply by exposure to visible light. To prevent fogging of the plates it was necessary to completely exclude the wavelengths in the ultraviolet (and possibly blue) regions of the spectrum. This is most easily done by using a deep red filter for the development exposure. Although this method seems to have been put to little practical use at the time, with our increased recognition of the health hazards of mercury vapour it is now used by most contemporary practioners of the process.
The English patent taken out by Daguerre meant that the use of the process was restricted to a few licensed portrait studios in England, but elsewhere, particularly in France and America, the use of the process developed rapidly. It was particularly suited to studio portraiture, although the long exposures - usually from 30 seconds to several minutes - made sitting quite an ordeal. However, processing times using mercury development were relatively short, and because the light sensitive material was just an incredibly thin layer on a metal surface, only brief washing etc was needed. By the time the sitters had recovered from the ordeal - perhaps with a cup of tea or something stronger - they could be presented with the case containing their likeness.
The detail in the images was limited largely by subject movement and lens performance. For portraiture, wide apertures were a priority, and some distortion and lack of resolution towards the edges of the image was acceptable. For landscape and architecture, slower 'rectilinear' lenses giving low distortion and high resolution across the plate was needed.
Many daguerreotypes were hand-coloured, a practice probably introduced by Johann Baptist Isenring in Switzerland.
The daguerreotype was the main process in use in commercial photography from 1839 to the early 1850s, and literally millions of daguerreotype portraits were made. The collodion (wet plate) process generally replaced it very rapidly, as it allowed multiple copies of images to be made. There were also no patents on this process, allowing its rapid adoption.
The American daguerreotypists were noted for the quality of their work and daguerreotypes continued to be important until around 1860 there, the rapid nature of the process without the need for printing being important. However, similarly rapid direct positive processes based on wet collodion, such as the tintype, soon replaced them for cheaper work because the iron plates used for this were considerably cheaper than silver-coated copper. 
This section is courtesy of Peter Marshall
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