Last summer we finished off what was originally a large tobacco can full of black powder that was at least 70 years old, and probably more like a hundred or more. My dad remembered my grandfather having it in his shop when he (dad) was eight or nine years old, which was in 1940 or 1941. Dad's pretty sure that it was his grandfather's powder to begin with.
It worked every bit as well as the brand new Schuetzen powder that I bought last spring.
And, for what it's worth, it wasn't exactly stored in optimal conditions. In Gramps' shop in North Dakota, the temps ranged from 110 in the summer to 20 below in the winter. And here in Idaho, it gets about that hot, but thankfully, not nearly so cold where we live. I found the tobacco can in the late '90s, when my folks sold their house. Far be it from me to scold my dad on how to store a half pound of explosives, but he did keep it in a more climate-controlled location after that.
Well we don't rent pigs and I figure it's better to say it right out front because a man that does like to rent pigs is... he's hard to stop - Gus McCrae