The other day when we visited Chateau Gaudrelle, Cyril, the maître du chai (cellar master) was fumigating the empty wine barrels. Everything has been bottled now, so he needs to maintain the empty barrels so they don't grow bacteria or fungus over the months before the next harvest.
To do that, he uses sulphur dioxide, applied by burning a small pastille suspended inside the barrel. He had a sort of hook which took the pastille at one end, which he then lit from a candle and placed inside the barrel via the bung hole. The barrel must be sealed up for a few minutes while the pastille burns and spreads its fumes throughout the interior. Then the hook is removed and the remains of the pastille, now just a block of ash, are carefully knocked off on to the floor, making sure none fall into the barrel. The pastilles contain 2.5g of sulphur and must be applied roughly once a month to dry barrels. If the barrels are wet you risk making sulphurous acid, which can lead to the barrels giving a bad taste to the wine.