Cognac must be aged in French oak barrels, which includes oak primarily sourced from two forests, Limousin and Troncais. As compared to French Limousin oak, Troncais oak is known to have a finer grain. Barrels can be used and reused, as long as they have never held a non-wine product, i.e., there’s no ex-bourbon barrels being used here.

Barrels are indeed reused heavily though, from 50 to 100 years. Different houses have various grading systems for their barrels, moving eau de vies from fresh barrels to older barrels in order to control the aging and impart different flavors and characteristics.

The barrels used can come in various sizes, although they’re all substantially larger than typical bourbon barrels. For instance, while Hennessy uses mostly 350L and 270L barrels, for their special anniversary release, Hennessy 250, they specifically used 250 250L barrels. Brands are free to use their own mix, with other common sizes skewing even larger, including 450L and 540L varieties.

With no age labels on Cognac, eau de vies are left in the barrel for as long as it takes to achieve a desired result. In some scenarios it could be 40 or 50 years until you get the reveal of the full potential of the eau de vie.

Whenever an eau de vie has reached its peak maturation, whether that means it’s been aging for 10, 30 or 100 years, if it’s not going directly into a blend, it’s stashed away in demijohns.

Demijohns are glass vessels, housed in wicker containers that look a bit like topped laundry baskets, which enable an eau de vie to be kept indefinitely, without further aging, oxidation or evaporation. As such, a trip to Martell’s cellars could include sampling an eau de vie distilled in 1848, barrel-aged for 65 years until 1913, and then stored for another century in a demijohn.

Other factors affecting Cognac’s aging process include whether or not the barrels were toasted, and to what degree, and whether they’ve been stored in humid or dry cellars. Dry cellars produce higher rates of liquid evaporation, providing woodier, and spicier notes, while humid cellars produce higher rates of alcohol evaporation. A tasting at Baron Otard’s Château de Cognac of eau de vies distilled at the same time but kept in cellars of different climates showcases a stark contrast.

Another factor is the stacking mechanisms used. Courvoisier, for instance, primarily stands their barrels up vertically, whereas other brands may use horizontal rickhouse style systems, or the more traditional approach of pyramid-stacking barrels.