Cognac must be distilled between the range of 67–72 degrees and many producers will try and get it to the higher end of the range to maximise on the purity of the distillate. Whatever the strength of the vapours, the temperature is quite likely to be very hot and the vapour will rise quickly from the boiler to the chapiteau, the onion shaped pot on top of the boiler. The smaller the chapiteau, the more efficiently the wine vapours are carried on up to the col de cygnet (swans neck). The shorter the col de cygnet, the less spirit will be rectified before it goes round the top bend - at which point it will start to condense as it passes back through the super heater or chauffe-vin, this time to cool the spirit rather than to heat the wine. The final stage of bringing the vapour back to a liquid is through a large condenser with huge coils in a water filled tank , cooling it to a level where it can be tasted by the distiller to judge the quality of the distillation and where the strength can be measured.
The final eau de vie or water of life, a water clear liquid is, once the burnt heads are removed, diverted into a new oak barrel.
Typically barrels are made from oak from the Limoursin Forests, some 70 km to the east of the town of Cognac. This has not always been the case though. During the 17th and early 18th centuries other woods were used, since travel was more difficult and transporting heavy timber arduous. There is some evidence to suggest that elm or possibly even ash may have been used from the local forest which would account partly for the dry hazel nut taste of very early cognacs. These timbers would never have been highly successful, as the grain in elm is neither straight or easy to bend and oak would even in early days have been preferred.
It is the Forests of Limoursin, to the east of Angoulême, that was the predominant reason for the success of Cognac. The trees grow large and thick and the wood is relatively porous allowing both physical and chemical processes to occur in the brandies. Oak from the forest of Tronçais near Burgundy is also used, but the grain is much tighter and the maturing process is considerably increased. However, it suits some houses who seek to develop their cognacs for shorter periods, since the wood contains more lignin and less tannins than Limoursin.
Physically, oak is hard, supple and watertight, making it ideal for storage and transportation, but it is also dense, thus allowing only slow evaporation. Wood for the barrels has to be carefully chosen, ensuring that it is free from knots and imperfections. The wood comes from old trees, greater than 50 years old and the staves are split rather than cut to ensure the grain is straight. The staves are air dried outside for about six years before being taken and shaped into curved staves and formed into barrels. Sun and rain remove many of the soluble elements in the wood, including the bitter tannins which would introduce bitterness into the brandy.
Chemically, oak is two thirds neutral and crucially contains little or none of the of the resinous substances found in other woods that can pollute the spirit with undesirable tastes. Like cognac, the actual making of the barrels is a great skill passed down from generations and one that deserves recognition. It can never be mechanised and visiting a cooperage is a memorable experience. Each stave is shaped carefully and then placed together using a metal ring to hold the staves in place. After a complete circle of staves is formed, further rings are added. A groove is then formed at the end of the staves to hold the end plate, held in with pegs. No glue or nails are ever used in the barrels construction.
New oak from either the forests of Limoursin or Tronçais contains many harmful chemicals which need to be removed before the brandies are allowed to be placed inside them. The completed barrel cannot be used until it has been toasted to remove some of the harmful wood tannins which can introduce bitterness to the brandy. There is however a small percentage of useful tannins left in the wood and a larger level of Lignins which, when combined with the hemicellulose which makes up most of the rest, gradually dissolves into the maturing spirit, imparting the agreeable sweetness found in older brandies.
The Tannins impart colour to the maturing spirit, but too much exposure can create a bitter harshness to the brandy. Young brandies are usually only kept in new barrels for perhaps between 6 – 12 months before being transferred to older and mainly neutral barrels, thus creating a more readily drinkable brandy at a younger age. The process of ageing is slow. Naturally the more the brandy comes into contact with the barrel the more effectively it matures, so smaller barriques will tend to provide a better cognac over time than the larger 350 litre barrels. Different variations in the wood, toasting and size of the barrel will change the style of the cognac during its long ageing.
But there is one more ingredient that will provide a major variation in the flavour of the cognac - water, or more significantly the dampness of the cellar in which the cognacs are stored. Many old cellars have earth floors and those closer to the Charente will have a greater degree of dampness than others in the Bois crus. The water molecule is larger than that of spirit, so will block the quick release of the spirits through the wood, allowing more time for the tannins and lignins to dissolve in the spirit, which in the case of brandy aged for 40 or more years will develop the much sought after “rancio”.