IODINE

The element was discovered by the French chemist Bernard Courtois in 1811, and was named two years later by Joseph Louis Gay-Lussac, after the Greek Ιώδης "violet-coloured".

Here is what the Photo-Lab Index from Morgan & Morgan has to say about it:

PLIp62

It has been involved in photographic processes since the very beginning, starting with Nicephore Niepce's Heliotypes, where its fumes were used to darken the heliotype's silver substrate for better contrast in the 1830's. His business partner, Louis Jacques Mande Dagurre, later fumed a silver surface to provide a light sensitive coating of silver iodide for his Daguerreotype process..

A century and a half later, the Multiplex Company the bleach that they used for their holographic processing scheme was simply some crystals of iodine dissolved in methanol to use as a rehalogenating bleach.

Here are the effects of iodine fumes on the metal lid of a glass jar that contaned the crystals. Don't let this happen to your lungs!

Evil stuff!