Search for “Sewer deodorant sealing rings”. I can’t find link in Taiwan but attached example below.
Install on every floor drain and shower drain - it blocks odors and roaches
They are usually silicon material. The weight of the water opens the bottom of the silicon. When water flow stops, the silicon springs shut to keep out odor and roaches.
Cold also be mold or black mold.
Some wood may have got wet.
Something may have been painted over or covered up.
20 USD for a bit of plastic, I’m in the wrong business.
It could also be your black powder contains aluminum powder. One is reminded not to mix potassium nitrate containing compositions with aluminum powder as the nitrate reacts with aluminum, liberating ammonia. Then the ammonia reacts with the sulfur forming hydrogen sulfide…
what is black powder…
What are you talking about here? Nobody (else) in the thread seems to be making explosives.