Which nineteenth-century French photographic process produced a single unrepeatable image?

The daguerreotype - a silvered copper plate developed over mercury vapour, giving an image of extraordinary fineness that is also unique. There is no negative, so every daguerreotype is the only copy, and it must be viewed at an angle to resolve from mirror into picture.

The French government bought the process in 1839 and released it free to the world, except in England where Daguerre had taken out a patent. That decision is why photography spread as fast as it did, and why the rival British process, Talbot's calotype, lost - it was patented and licensed restrictively, despite being the better idea. Talbot's produced a paper negative from which any number of prints could be made, which is how photography has worked ever since.

The exposures were long enough to erase people. The earliest photograph of a person, a Paris boulevard scene of 1838, shows an empty street and one man having his shoes shined, because he stood still for several minutes while the traffic did not. Portrait sitters were held in neck clamps, which is a good part of why everyone in early photographs looks so grim, and the mercury vapour made it a hazardous trade.