Found this over at http://winebarrels.com/carenew.html
I'm pretty sure I have read similar things on homebrew forums in the past.
STORING AN EMPTY BARREL
The very best way to maintain a wine barrel is to ensure it is never empty of wine! However this is not always possible so something must be done to keep the barrel sweet.
An untreated empty barrel will quickly begin to produce moulds and bacteria. Replacing the air in the barrel with an atmosphere high in SO2 will effectively eliminate this problem.
- Make sure the barrel is tight.
- Rinse the barrel well and drain, bung down, for 24 hours.
- Use one of the following methods to ensure bacteria free storage.
Method 1 - Sulphur Sticks
These should be burned in a device that will collect any drips of molten sulphur. Place the stick in the burner which is suspended from the bung. Light the stick, place it in the barrel and allow it to burn completely. Re-sulphur every 6-8 weeks.
Quantity: 225L Barrel approx. 1/4 - 1/3 stick
100L Barrel approx. 1/4 stick
< 100 L Barrel approx. 1/8 stick
Method 2 - Sulphite Solution
- Make up a 2% KMS solution and place a small amount (about 10% of barrel capacity) in the barrel.
- Replace the solution every 6-8 weeks.
- 2% Solution -- 100 gr per 4L water or
16 tsp. per 1 Imp. Gallon
- 100 ppm Solution -- 6gr per 23 L water
or 1 tsp. per 5 Imp. Gallons
Standard Cleaning Solution
- 2/3 - 3/4 cup per full 225L barrel
- 1/3 cup - 1/2 cup per full 100L
- 1/4 cup per full 46L or 50L barrel
- 1/8 cup per full 23 or 25 L barrel
- 3 cups per full 225L barrel
- 1 1/2 cups per full 100L barrel
- 2/3 cup per full 46 or 50 L barrel
- 1/3 cup per full 23 or 25 L barrel