All black powder attracts water. Before a hunt or shoot, I empty the powder in my horn into a shallow earthenware bowl, then set it in the oven warmed to about 200F for a few hours. My stock of powder is in the airtight cans I bought it in. black powder is one of the few products that has not been noticeably improved in the last 250 or so years. It also does not ever deteriorate in storage as long as it’s kept dry. The Lewis and Clark expedition carried their powder in lead boxes which were soldered shut. They capsized one or two of their canoes in the Salmon River in 1803 or ’04 losing several rifles and some of the lead boxes containing US government issue powder. In the 1960s (IIRC) that portion of the river went dry during the fill-up of an upstream dam at which time remains of one of the rifles and several of the powder boxes were recovered. One was opened and the powder in it was found to be as good as new. What a wonderful design for a container! Over 150 years in a wild, roiling river and still good as new! – Fred The Valmet-meister
JWR Replies: Thanks for your letter. I would recommend using extreme caution when getting any source of heat anywhere near a box of blackpowder! In the modern context, for safety I’d recommend a waterproof container has some means of pressure relief.