Until a spark sets one off. Shards of glass everywhere.
The article I referenced in an early post about these items addresses this:
“Thanks to Eric Bye I was made aware of an article on the Internet entitled, “Can a static spark set off black powder?” (see bibliography). The treatise is replete with photographs of each of the five steps in an experiment to ignite black powder and other carbon-containing propellants, with static electricity, failed. The anonymous experimenter’s explanation of why the sparks wouldn’t set off the powder:
“The answer comes from the fact that black powder, and other carbon- containing propellants, are fair conductors of electricity. When material conducts well, it takes a lot more current to heat it up. This is why the lamp wire stays cool and the filament in your light bulb get white hot. The same current passes through both, but because the light filament has a much higher resistance to the passage of electric current, most of the heat ends up there rather than in the wire. In the experiment, the air has a very high resistance, while the powder conducts fairly well. The passage of the spark heats the air white-hot, but the powder stays cool. A very high-current spark (like lightning!) would, of course, heat everything and cause ignition, but this would take much more current than could be provided from a static-like source.”
Implying actual experience in using a glass powder horn, Scott Grandstaff’s research (see bibliography) indicates: “In the case of the small glass priming
horns the straps were very short, nestling the horn very high and close to the body – under the arm. The small horns were for the priming powder (finely ground) used in the pan of a flintlock rifle or pistol. While the pan used the finely ground priming powder, the main charge of not-so- finely-ground gunpowder was poured down the barrel.”
Conclusion
Powder horns of glass were made. Powder horns of glass were used. Powder horns of glass were used to contain black powder. And powder horns of glass were used as decorative items.
They were made well beyond the 19th century, where they were a necessary adjunct to weapons that used black powder.”