The box hasn’t darkened appreciably over the past two days, so I put in fresh ammonia in a slightly bigger saucer to give it more surface area. Here’s a funny thing. It felt like the box was damp to the touch. The lid just lies in a rabbet in the sides, and the wood had swollen so much that it was jammed in tight! It really has been picking up moisture. I don’t know if this is due to the fuming reaction, or if it is just a consequence of having the box suspended over a dish of water. The ammonia supposedly reacts with tannin in the wood to produce the dark color. If anyone knows the detailed chemistry behind this, I’d like to know.