The storage in oak barrels helps the eau-de-vie mature and get richer. There is a complex reaction by which the tannic and aromatic compounds of the oak wood slowly dissolve in alcohol. The quality of the oak and of the cellar is vitally important for achieving the micro-exchange between alcohol and oxygen.
The best barrels are made from the black oaks which are grown in the local forests in Gascony. The 'merrain' which stems from the noblest part of the tree is split by hand into thin planks, the 'douelles', in order to maintain its watertight properties. Over three to five years the 'douelles' mature in the fresh air. Rain,wind and sun clean the douelles by eliminating their excess of tannic compounds. They will then be placed side by side without being stuck together and, finally, after being shaped over a wooden fire, they will be made into barrels.
The cellar also has an essential role to play in the ageing process. Its humidity level is a key factor. The more humid a cellar the sweeter the eau-de-vie and the lower the alcoholic strength since the alcohol molecules are the first to evaporate. Conversely, the drier the cellar the faster the water molecules evaporate and the stronger the eau-de-vie, thus producing a somewhat irritating taste.
When the optimal quality is reached the Armagnac is bottled. Alchemy is then over. Wood is really the only important criterion of ageing. It is best to manipulate the eau-de-vie as little as possible while it is maturing although every year it is necessary to compensate the partial evaporation of the spirit - this is called "the part of angels" - by filling the barrel with an identically aged Armagnac (you obtain a Vintage Armagnac) or with that of the following year. This process is called the 'ouillage' and permits the natural reduction of alcoholic strength.