The word vinegar, derived from the French vin aigre, means "sour wine".

For more information on making vinegar check the following links:
"Making Flavorful Wine" by Lawrence Diggs, Winter 1999 WineMaker magazine  How to make vinegar article
Information on how balsamic vinegar gets that special flavor by oxidizing the must and aging in several different types of wood:  Balsamic.com
 

Temperatures:
Temperatures between 80 and 85 degrees are ideal. Low or fluctuating temperatures slow the process. At 75 to 85 degrees F, it will take 6 to 8 weeks for conversion. At 85 to 90 degrees F, it can take 4 to 6 weeks for conversion. Temperatures over 95 degrees F slow conversion; above 140 degrees F, the bacteria die.

An acetic film called "mother" will form. This smooth, leathery, grayish film becomes quite thick and heavy. It should not be disturbed. It often becomes heavy enough to fall and is succeeded by another formation. If the mother falls, remove and discard it. An acid test will indicate when all of the alcohol is converted to vinegar. Part of the vinegar may be withdrawn and pasteurized. The remaining unpasteurized vinegar may be used as a culture to start another batch. Living bacteria are in the liquid. A piece of the mother is not necessary to start a new batch.

Specialty vinegar contains acid as high as 7%. Beer containing 5.5% alcohol will yield about 5% acid. Wine containing 11 to 12% alcohol must be diluted to 5.5 to 7% alcohol before using it to make vinegar.

Aeration is necessary for acetobacter to turn alcohol to acetic acid.  I use an aquarium air pump and 1/8" OD vinyl tubing to introduce air into the vinegar making container. I aerate the wine for the first day or so to help get it started.  I used to use an aquarium aeration stone, but I found that the acid in the vinegar dissolved the glue used in the stone.   I have used a water bed heater with a thermostat wrapped around the plastic bucket to give the acetobacter bacteria a more ideal environment.

Preserving vinegar
To preserve vinegar, add 3 campden tablets per gallon of vinegar or Heat the vinegar to 155 degrees F and hold the temperature for 30 minutes. After pasteurizing vinegar add one tablespoon 80-proof vodka to each gallon and age it.  If desired to enhance the bouquet, up to one cup oak or beech chips may also be added. Pasteurized or sulphited vinegar can no longer produce more vinegar. Pasteurizing kills vinegar bacteria and prevents the formation of "mother" which could lead to spoilage. Pasteurized vinegar keeps indefinitely when tightly capped and stored in a dark place at room temperature. Temperatures above 160 degrees F cause a loss of acidity, flavor and aroma.

Aging vinegar

Vinegar has a strong, sharp bite when first made. It becomes mellow when aged. The esters formed during aging, like those in wine, develop after a period of six months or more when stored at a cool, steady temperature (50 to 60 degrees F is ideal). This undisturbed rest also allows suspended solids to fall, making the vinegar clear and bright. Siphon the clear, aged vinegar off the deposit of solids into sanitized bottles. Introduce as little oxygen as possible. Winemaking suppliers sell attractive vinegar bottles. Use corks or plastic caps to avoid vinegar contact with metal. If corks are used, the necks of the vinegar bottles should be dipped several times into melted wax to form an air-tight seal. The quality of vinegar improves for up to two years and then gradually declines. Fermented vinegar can be sold without the special permits or licenses required for alcoholic beverages. It costs the same as a good bottle of wine.