Two days after I took it out of the “gas chamber” the swelling due to moisture had gone down enough that I could remove the lid. I was a little surprised that the interior had darkened as much as the outside, but I guess I shouldn’t have been. The lid was certainly not airtight. What did surprise me was that I had put a coat of Danish oil on the inside prior to assembly, and the ammonia had darkened the wood right through this small amount of finish. It just shows how permeable a finish like this is, and how something like ammonia or water vapor can penetrate it. I may try an experiment with a film-forming finish like polyurethane to see it the ammonia can penetrate that.
About the box taking up moisture – I tried another experiment. I placed another small box in the gas chamber, this time with some water instead of ammonia. The box did pick up some moisture, but not nearly as much as the box with ammonia. This seems to suggest that the ammonia somehow makes the box take on moisture. On the other hand, the nights have been a bit warmer. Maybe there was just less condensation this time around because of the temps.
The chemistry of wood fuming is not well understood. A nice research article is here. This work showed that of the species tested, oak, maple, spruce and larch; all darkened, but oak darkened most quickly and intensely. The research suggested that the tannin in the oak was responsible for the darkening, but different compounds in the other species were responsible for darkening them.