And now, for our non-British readers -
Here in UK black powder is classed as a Class 1 Explosive, for which you must have an Explosives License to buy, transport and store. You are limited to a total of 25kg at any one time. It lasts five years, and is free. Many of us have one running alongside our Section 1 or Section 2 Firearms Certificate for ease of renewal.
It must be stored in a Home Office-approved safe container, made of wood, with flameproof contact points around the lid to prevent flames getting in [easily]. You can buy them. or make them using plans off the internet, but it must be compartmentalised so as to hold a number of containers, each one of which may not hold more than 250gr/1 lb of black powder.