Dry maintenance of barrels.
After use, the barrels should be washed very carefully while hot, then with cold water. The barrel can be considered clean only then, when the water, which runs off after rinsing, it is tasteless and odorless. If good wine was stored in the barrel, further handling is not difficult and it does not require special care. It just needs to be preserved. According to the traditional method, pieces of sulfur are burned inside the barrel (to buy at the drugstore). Gas is formed during combustion, Sulphur dioxide (SO2), which prevents the growth of bacteria on wooden walls. This method is called "dry" maintenance.. They used it before with the top 2000 years the Greeks and Romans and has remained unchanged to this day.
The sulfur's effectiveness decreases more or less 4-6 weeks, then the barrels have to be sulfurized again. The sulfur is set on fire with a match, It is suspended in a barrel and allowed to burn out completely, then the barrel is tightly blocked with a plug.
I know winemakers, who want to make their wine as naturally as possible and give up not only dry wine, but also from the "wet" maintenance described later. Their method is more expensive: namely, every four weeks they fill the barrels with fresh water, and "used" they pour out, having previously checked its taste and smell, to meet, whether the barrel is healthy, or not. Only when the result is negative, they use sulfur as their last resort.
Sulfur oxide inside the barrel and on its inner walls decomposes within a few weeks, it is neutralized and has no effect on the new must, which the barrel is filled with, as long as it was properly soaked before autumn. If the water is clear and has no taste or smell, this can be said, that the sulfuric acid and its salts were washed out.