Oak barrels!

"The use of oak plays a significant role in winemaking and can have a profound effect on the resulting wine, affecting the color, flavor, tannin profile and texture of the wine. Oak can come into contact with wine in the form of a barrel during the fermentation or aging period. It can be introduced to the wine in the form of free-floating oak chips or as wood staves (or sticks) added to wine in a fermentation vessel like stainless steel. The use of oak barrels can impart other qualities to wine through the processes of evaporation and low level exposure to oxygen. 

Effects on wine

The porous nature of an oak barrel allows some levels of evaporation and oxygenation to occur in wine but typically not at levels that would cause oxidation or spoilage of the wine. In a year, the typical 59-gallon (225-liter) barrel can lose anywhere from 5½ to 6½ gallons (20.8 to 24.6 liters) of wine through the course of evaporation. This evaporation (of mostly alcohol and water) allows the wine to concentrate its flavor and aroma compounds. Small amounts of oxygen are allowed to pass through the barrel and act as a softening agent upon the tannins of the wine.  The chemical properties of oak itself can have a profound effect on the wine. Phenols within the wood interact with the wine to produce vanilla type flavors and can give the impression of tea notes or sweetness. The degree of "toast" on the barrel can also impart different properties affecting the tannin levels of the wine as well as the aggressive wood flavors. The hydrolyzable tannins present in wood, known as ellagitannins, are derived from lignin structures in the wood. They help protect the wine from oxidation and reduction." Source: Wikipedia

Barrels Curing

First rinse the inside of the barrel by filling and emptying 2 or 3 times to remove any wood debris. Prior to use, your barrel must be filled with water or submerged for approximately 3 to 5 days. This will allow the oak to expand and retain liquid. (The barrel may leak until cured) After this initial curing period, place the barrel on the holder in a dry area. Liquid will continue to slowly drain through the wood for the following 3 to 7 days. Keep the barrel filled during this time. When the barrel stops draining, empty the contents and rinse again. You are now ready to fill with your spirits.

Filling with Spirits

When filling your barrel with spirits fill the barrel completely removing all air. Place the cork in tightly. Air inside the barrel can spoil the aging process.

Reconditioning Your Barrel

In preperation for reusing your barrel after the initial use, the barrel may need to be reconditioned. This should always be done when aging wine. Fill your barrel with a cleaning solution such as Soda ash or Barolkleen (454g of Barolkleen per 5 gallons of water) and let it soak in the barrel for 2 to 3 days. This solution will remove excessive tannins.

Storage

Barrels should be stored in a cool place and away from sunlight. This will protect both the exterior of the barrel and the aging spirits inside. In time liquid will be absorbed by the barrel or evaporate, generally about 5% over 2 years, this is called the "Angels' Share". Dry climates or heated homes tend to cause more evaporation.

Maintaining your Barrel

Over time the exterior of your barrel will show wear. Lightly sand your barrel with a fine sandpaper. Wipe off any debris with a clean cloth, and apply a coat of water based varnish on the outside of the barrel. Barrels left without liquid will dry quickly and no longer hold liquid without leaking. If this occurs, begin the curing / reconditioning process again.