First off, one should never burn a sulfur wick in a wet barrel; only do this (which releases the antimicrobial SO2 gas, which we want) when your barrel is well dried-out. This is because the sulfur gas will combine with the water in the wet barrel and form sulfurous acid, which can lead to a bad taste in the wine. That being said, in any container, the only thing I can think of that would extinguish a burning sulfur stick would be the following: lack of oxygen in the atmosphere in which it’s burning, or a wet or defective sulfur stick. If I’m understanding your letter correctly, you’re telling me that when you lowered a burning sulfur stick into the headspace of your barrel, the stick went out. If it was due to a lack of oxygen, that’s actually a good thing and not a sign of an infected barrel. It means you’re doing something right with your barrel-storing method; having an oxygen-free environment will help prevent bacteria and yeast from gaining a foothold in your stored cooperage, hopefully enabling years of healthy winemaking to come. However, I’d be cautious if I were you because I’m not 100% sure that your stick went out due to lack of oxygen; it could be that it’s a defective stick or that it’s wet. To see if you really have a lack of oxygen in your barrels, you can try an old Burgundian cellar trick and light a long match and lower it into your barrel or vessel. If the flame goes out, you’ve got an oxygen deficit. Otherwise, your sulfur stick may be too damp to burn or may be defective in some other way.
Storing empty barrels is one of the trickiest things we winemakers do. You can keep on doing what you’re doing with “wet storage.” Many of us have good success with storing our emptied barrels with a solution of acidified sulfur dioxide inside them. I recommend an aqueous solution of 1 g/L of citric acid and 2 g/L of potassium metabisulfite. You can also clean your barrels, drain them, air dry them, turn them bung-up and then gas them with sulfur wicks or sulfur dioxide gas.