There have been a few small APUG discussions about Hippolyte Bayard's direct positive process as described in the excerpt from Wikipedi below, but none of them actually explain how it works. Why does the addition of Iodide lead to bleaching of already formed printed out silver under influence of light, so as to reveal the positive image? Is there someone who can explain the chemistry behind this? And also how he managed to get relatively good contrast images without development, while modern day direct positive papers all need development?


From wikipedia:

Hippolyte Bayard (20 January 1807 14 May 1887) was one of the earliest photographers in the history of photography, inventing his own photography process known as direct positive printing and presenting the world's first public exhibition of photographs on 24 June 1839.
The direct positive process involved exposing silver chloride paper to light, which turned the paper completely black. It was then soaked in potassium iodide before being exposed in a camera. After the exposure, it was washed in a bath of hyposulfite of soda and dried.

Marco