Vinegar
If a cork happens to pop out unnoticed and air reaches the wine for several weeks, there is a good chance that bacterial action will begin to convert the alcohol in the wine into acetic acid. Once the presence of acetic acid can be detected (a vinegar-like odor) the wine will lose its appeal as wine. A usable vinegar can be retrieved by encouraging the process to go to completion.
Vinegar produced from an undiluted wine will be overly strong, so an equal volume of water should be added. The container should be less than three-quarters full and closed with a loose cotton plug or covered with a piece of light cloth to keep out fruit flies.
If wine vinegar is your desired goal and no wine has started to sour, use a vinegar starter. A selected strain of vinegar starter can be purchased from some winemakers’ shops, or a wild starter may be used. Frequently the water in an air-bubbler will have a vinegar-like smell. This can be used to start a batch of vinegar. The wine is diluted with an equal volume of water and the container partly filled and covered as above.
A warm, but not hot, location will speed the process. In a month or two the vinegar should be ready. The clear portion of the vinegar can be poured or siphoned off for use. If another batch is wanted, more of the wine-water mixture can be added to the old culture.