Barrels are reactive containers that require some form of maintanence and care both prior to use and once emptied of wine. The wood expands and shrinks, develops deposits, and can even become host to unwanted microbial growth. Empty barrels must be hydrated to expand the wood and seal any small gaps that could lead to leaking before being filled. Barrels must be cleaned both prior to filling and after being emptied of wine. Empty barrels also need some regime of microbial prevention. Typically small discs of sulfur dioxide are burned in the barrel every couple months, or the barrel is filled with acidic sulfur dioxide water. Barrels suspected of microbial contamination should be discarded as there is a large potential for the contamination to spread.