Obviously, I know the short answer to this is just go shoot it and see. But the range I use is about an hour away. A coworker gave me some old 2F Goex (metal can) his father had stored in an outside shed for several years. It seems clumpy when you shake it. May have started to go bad.
What does bad powder look like? Are there any tell tale signs I can use to determine if it’s still good? Odor, appearance, etc. for powder that is no longer reliable to shoot? Will safely igniting 10 grains in a metal cup tell me anything (to check the burn & fouling)? Or flash it in the pan, or light a thin line of it like in the movies, or any other ideas? If I eventually shoot it, what do I look for? Your thoughts.
Yes, the short answer is to just shoot it and see. Sometimes old black powder samples will show the ravages created by poor storage. Especially if the caps were not screwed tightly on the cans. Going back into the 1980s there was a short period of time when you opened a new case of GOEX you had powder all over the tops of the cans and in the bottom of the cardboard case. For some reasons somebody on their female packing line was not tightening the caps. Just putting them on loosely. But that was a short period odd thing that was soon corrected. If the caps are not on tight the cans "breathe" with changes in atmospheric pressure. Air with moisture goes in. But the powder pulls most of the moisture out of that air. Then when the atmospheric pressure drops the can releases dry air. And this cycling goes on and on with the normal cycles in air pressure with high pressure and low pressure air mass movement. This this cycling of the moisture in the powder leaves deposits of potassium nitrate on the surfaces of the grains. You will see patches of a white deposit on the grains.
Depending on the brand and period of production you may see chemical changes in the powder that cannot be detected with the eyes. When black powder picks up moisture, especially above 1% water by weight of powder, you will see chemical changes in the powder that relate to the sulfur content of the powder. Fresh black powder is alkaline due to the mineral content of the charcoal being used. If the sulfur particles start to produce traces of sulfur acid with added moisture the alkaline minerals from the charcoal quickly kill the acid. Eventually you run out of those charcoal minerals that act as an acid buffer. The powder then starts to turn acidic from the sulfur acid formed. Then the process starts to speed up. Then what you have is what is described as a self-accelerating decomposition reaction. And I should point out that smokeless powders do the same thing.
In the black powder the sulfur acid goes after the potassium nitrate and leaves the actual carbon of the charcoal alone. So the elemental sulfur level starts to drop and the powder begins to loose oxygen. Ignition becomes slower and increasingly more difficult. And with less oxygen to give to combustion the powder looses strength. Lower velocities and more fouling and the fouling becomes rather corrosive. You can sometimes detect this by hanging a swimming pool pH strip in the can for a few hours. The air in the can gets rather acidic.
Open air flashing of partially deteriorated often will not show anything. With open burning the powder in the air the powder can use oxygen from the air around it. I learned that one real quick when looking at 100 year old BP samples. Great in the open. Shot it in the gun and figured I would best add the garden hose to the post shooting bore cleaning.
