Cognac is double-distilled in an iron still, otherwise known as pot stills because of its Anglo-Saxon origins. This alembic, equipped with a direct heating system as required by the AOC of Cognac, allows a non-continuous distillation of "batch".
The first toasting transforms the wine into brouillis. The second toasting, which we like to call "la bonne chauffe" transforms the brouillis into eau-de-vie.
The art of distillation is expressed through the selection of aromas at each stage. This selection is sensitive to the distiller's choices: temperature and heating time, alcoholic concentration, shape of the still...
The cutting of the heads and tails is a symbol of the precision of his work. Even today, the distiller's work is still mostly done on the nose. The heads are the first vapours to come out of distillation, the distiller discards them because they develop solvent aromas but also contain alcohol that is unfit for consumption. As for the tails, they quickly produce aromas of leather and sweat, which no one would like to find in their Cognac.
It is traditional, in Cognac as in other spirits such as whisky, to keep these heads and tails to add them to the following brouillis.
As for the toasting heart, it expresses all the aromas and flavours retained by the distiller. On leaving the still, we seek to sublimate the floral and fruity notes that come from the grapes. To balance this eau-de-vie, which is not yet allowed to be called Cognac, to make it more supple on the palate and to give it its renowned "rancio", then comes the ageing stage.
During Prohibition in the United States, smugglers often made their own distillations. Unfortunately for their clients, they were not distillers and sometimes kept too many heads in their eaux-de-vie. This resulted in Brandy that was not only of poor quality, but sometimes unfit for consumption because of the presence of methanol.
In France, one would think that the same mechanism led to the banning of absinthe. Under the pressure of growing consumption, some batches would have been produced without all the distiller's requirements.