This part of the Daguerreotype equipment is very important (but then again, what part isn't?). This is where the plate is sensitized, or becomes like film - sensitive to light. The final surface is an amalgum of silver and mercury. The first fuming is over Iodine Crystals. The plate is fumed until you have a rose color (checked under reflected white light and fumed again for a few seconds to mitigate that exposure) and then it's off to the Bromine for fuming. No Bromine fume, no "speed" or ISO.
These are very special fuming boxes. They are designed with NO metal in them and to seal extrememly well when fuming and in storage.