Armagnac gains much of its character and flavour during aging. Traditionally, it is matured in 400 litre black-oak casks made from locally grown Monlezum wood. However, due to the scarcity of this wood, oak from Limousin and Tronçais forests is now more commonly used.
An Armagnac's quality is largely dependent on the period of time it spends in maturing in wood. Its age is calculated from the end of August in the year of its vintage. It is deemed a year old on 1st September in the following year.
A good maître de chai (cellar master) will start aging newly distilled distillate in new oak casks, usually for the first six months with first-fill casks and up to two years for 'new' second fill casks. The spirit will then be pumped to progressively older casks to prevent over extraction of tannins from the oak.
In Armagnac cellars it tends to be the spirit which is moved, pumped from cask to cask, while the casks themselves tend to stay in place. Maturing Armagnac greatly benefits from aeration and this is attained during the pumping process from one cask to another. The moisture level and temperature of the cellar is also key. Traditionally, thick stone walls and bare earth floors ensure year-round cool temperatures and high moisture levels. Modern cellars use air-conditioning and sprayers which constantly generate a fine mist.
As the spirit matures it turns from being a clear spirit to a mahogany amber colour as it absorbs vanillins, tannins and other flavoursome substances from the oak. The aging spirit will also lose 0.5-3% alc./vol. each year through evaporation, known as the angel's share. Monitoring oak extraction and the effects of aging on the spirit is the work of the maître de chai and after some fifty years it is usual to transfer the Armagnac to large glass jars called dame jeannes to ensure the spirit does not over-extract oak flavours and lose any more of its alcoholic strength.