[greglepore]

They're like the polymer beads that get mixed with potting soil, they hold water and gently release it.
Cigar nerds have moved on to something similar-a thing called a Boveda pack (brand name). Its a polymer behind a membrane that is permeable and both absorbs and releases humidity. They come in specific humidity ratings, use 60% for cigars (and your stash of weed if that's your thing)