The incident I remember reading about years ago, with black powder deer rifles/muzzle loaders, was with people wearing those nylon snowmobile suits in the winter months. Dry air and the nylon contributed to the static and I have no idea what voltage was required to ignite black powder.
A place I used to work considered as low as 6,000 volts of static electricity being enough to light off they solvents they worked with. When the dispensed solvent from a pipe/valve to a bucket they had the bucket filling stations outside in a gravel area and a grounding cable/clip was supposed to be used to connect the bucket sitting on the gravel/ground to ground the bucket to the beams the piping was bolted up to.
Static is some weird stuff. That place had been in operation since the 1930's using solvents and they didn't have a deep understanding of why static worked the way it did on their products. They knew what to do to keep it from causing a fire but still had fires at about the rate of once per year while I was there. We brought in tech/engineer reps from more than company that made static electricity measuring/suppression systems and I never talked to one who could answer my questions about why we had some of the issues we had.
Their equipment worked as long as we kept it in good operating order, but they couldn't explain what caused static to form where/how it did in our product. Spent some frustrating days in meetings/presentations with customers, vendors, engineers and scientists. Came back to a previous conclusion regarding electricity I'd come to many years before - why does electricity do the things it does? Because it can.
