I wondered what the flash pan was about, but now understand that's it's pretty dangerous to have a pan of exploding anything right near you.
I have no idea what the real story is, but it seems hard to believe that they actually took photos at night during those days, like Doc and Marty at the clock tower in 1885. The chemicals are only sensitive to UV light, and while there may be some UV light produced by the flash pan, it wouldn't be even close to a substitute for something like the sun. Even people today who shoot wet plate indoors have UV lights, sometimes in addition to a modern flash, and they have to sit still for many seconds, to get enough light for an exposure. The lights have to be close to the subject and it makes them uncomfortable. I wonder if they just used those flash pans at the end of the day to get some extra light. Or maybe the person sat there for like 30 seconds and they flashed multiple times while they sat still with the lens cap off.