Freshly dumped whiskey barrels are about as sanitary as they can be because the wood has been saturated with a high proof alcohol solution for an extended period. If the barrel has had an opportunity to dry out before usage, however, the barrel must be swollen and primed. REMINDER FOR FRESHLY DUMPED BARRELS: NEVER BURN SULFUR IN A BARREL CONTAINING ALCOHOL VAPOR, IT MAY EXPLODE!
As with a new barrel, a used whiskey barrel may need to be filled with water to make the wood swell and eliminate leaks. These leaks will often seal themselves in only a few hours, or may take up to a couple of days. The barrel should be continually refilled until the leaks stop, and the water should be changed daily to prevent off flavors caused by bacteria and or mold growth. If the staves feel loose, it is also helpful to push the metal hoops toward the middle of the barrel before priming. This will help ensure a tight fit.
Generally, brewers use barrels for aging two different types of beer---strong ales and sour ales. The flavor potential of a freshly dumped whisky barrel is best suited for a strong ale such as an imperial stout, or other imperial strength beer. Aging periods are generally in the range of one month to one year. Bear in mind that beer in smaller barrels are subject to faster oxidation through the wood than beer in larger barrels. After the whiskey and oak flavors have been exhausted from usage, you may consider refreshing it with oak staves added directly to the barrel, and priming the staves or barrel itself with store bought whiskey. Alternatively, a barrel that has been used to the point where the flavor contribution is very minimal, referred to as a neutral barrel, is a great candidate for sour ales. Remember that once you make a sour beer in your barrel, which requires use of lactic acid bacteria and possibly wild yeast strains, it will no longer be suitable for aging anything but sour beer. Starting out with strong ales and then moving into sours is a great way to get the most out of your used barrel.
If it will be less than two months before the barrel is used again, the barrel may be filled with a Sulfite and Citric Acid solution. Use one teaspoon of Potassium or Sodium Metabisulfite powder, along with 1/3 teaspoon of Citric Acid for every 15 gallons of barrel capacity. Add enough water to fill the barrel and bung the barrel tightly.
Occasionally smell the solution to make sure sulfur can still be detected inside the barrel, replacing the solution if necessary. Rinse with water before refilling with your beverage.
Depending on the application, there a few alternatives to water. Since the water will take some of the spirit character with it, you may want to consider using neutral spirits or “faible” if available. Faible is a weak spirit that is a byproduct of brandy distillation used to maintain the barrel condition between uses. You can simulate this with off the shelf spirits. Use of faible is especially effective if you plan on reusing the barrel or have multiple barrels as it can be used indefinitely. Be aware that alcohol will remove more flavor than the water, and the stronger it is the more it will remove. After a few uses, though, it will stabilize and actually contribute to the character.
It is always best to keep a barrel full. When this is not possible, start by removing organic matter that has penetrated into the surface of the wood. This is done with a solution of Proxycarb, a sodium percarbonate based cleaner.
Use 4 oz. (or 8 Tablespoons) of Proxycarb for every 15 gallons of barrel capacity. Dissolve in a small amount of water, and funnel the mixture into the barrel. Fill the barrel the rest of the way with water. You may leave this mixture in the barrel for as little as 20 minutes or as much as 24 hours. Drain and rinse the barrel several times with water. Re-acidify the barrel using one ounce or 2 Tablespoons of Citric Acid for every five gallons of water. Slosh this all around and drain.
If you have more than two months before the barrel is used again, drain the barrel and leave it bung side down overnight. Next burn a sulfur strip in it, hanging it down at least 6 inches below the bung on a wire. Replace the bung. Remove the sulfur strip after about 15 minutes, and bung the barrel tightly. Burning sulfur releases sulfur dioxide gas into the barrel’s interior.
Repeat every two weeks (as needed) until a flashlight reveals no shiny dampness in the bottom of the barrel. Bung up the barrel and store it in a dry place until needed, allowing enough time to soak up and acidify the barrel before the next use.