The Becquerel process daguerreotype lets you remove two of the more toxic chemicals from the process, bromine and mercury. However, you still need vapors of iodine (do NOT breathe this stuff). Becquerel plates are not as light sensitive as mercury developed plates because the bromine is an accelerant. Exposure time for a Becquerel plate is going to be multiple minutes. They also lack the contrast and sharpness of mercury developed plates. Becquerel plates are developed by re-exposure of the plate to sunlight, through a rubylith filter. The development can be done by inspection, but it will require say an hour or more in sunlight, and can take overnight using a uv-rich artifical light source.