Traditional casks are either 350 litre fûts or 540 litre tierçons and are made of oak. Old oak plays a considerable part in cognac maturation as 72 % abv spirit could leach excessive character from a new barrel. The spirit is affected in a number of ways during cask ageing ; it slowly, but positively oxidises, extracts colour and character from the oak, and evaporates slowly through the pores in the wood giving further concentration. Old, quality cognacs will obtain their colour and complexity of character from these interactions and after approximately a generation, especially with brandies that have been distilled sur lie, one detects notes of rancio charentais, a certain oily richness discernible only in great, old cognacs. Younger cognacs can boost both their colour and character by the addition of spirit caramel and boisé—a concentrated cognac solution ; the results tend to be less balanced, less complex and less integrated.
An important part of the maturation process is the amount of cognac that evaporates each year through the wood. The region’s climate and the humidity in the chais (cellar), itself dictate the differences ; a cask in a cooler, damper chais will result in a greater loss of alcohol whilst one in a warmer, drier chais would see a greater loss of volume. The evaporation concentrates the spirit by reducing it, imparting a creamy velvety, rich character. During the ageing process the spirit extracts and makes soluble the colour, lignins, vanillins, tannins and sugars from the barrel. The spirit then breaks them down whilst interacting with oxygen and gaining colour. Ultimately, the brandy mellows and its colour and character emerges.
High-strength spirit can potentially extract too much from a cask. In order to lessen the impact, the cellar master may reduce the brandy to approximately 56 % abv immediately after distillation. Alternatively, he may put the high strength alcohol into new oak for a very short period of time, as little as one to three months, and then transfer it to old oak effecting a reduction between the two and then allowing for a second, more gradual reduction, in the years running up to bottling.