The first time you arrive in Cognac the impression is that the town is dirty. The pale stone buildings are covered in an uneven black layer that climbs the walls, surrounds the windows and creeps up the trees in the garden next door.
It is not dirty. It is colonised, and what colonises it lives off what escapes from the warehouses.
01 A whole town painted black
The stain is not confined to the ageing warehouses, which is where the source is. It spreads to neighbouring houses, to street walls, to garden walls, to tree bark and to roofs.
And it is not fussy about the material. It colonises stone and masonry, concrete, galvanised sheet metal, PVC plastic and plant bark. The substrate is irrelevant as long as the condition it really cares about is met.
That condition is alcohol in the air.
Over twenty million bottles a year evaporate across the district, and something lives on exactly that.
02 The angels' share: twenty million bottles
Brandy ages in oak casks, and a cask is not airtight. Vapour escapes continuously through the wood for all the years the ageing lasts. In the trade it is called the angels' share.
The figure used as an average is around two per cent of the volume a year, with higher peaks in summer depending on ambient temperature. Over some 750,000 casks in use in the delimited area, the annual result is equivalent to more than twenty million bottles.
What evaporates also depends on the humidity of the warehouse, and that detail is the cellar master's to manage. In a dry chai proportionally more water than alcohol is lost, and the strength rises; in a humid one the opposite happens, and the strength falls. Placing a cask in one place or another is a decision, not chance.
03 Who eats the vapour
The organism responsible is called Baudoinia compniacensis. It is a black fungus that shows a preference for airborne ethanol vapour: it is not its only source of calories, but it accelerates growth and stimulates spore germination.
That is why it appears where the vapour concentrates, and why the stain draws on the façades a map of how the air circulates. There are black zones and clean zones on the same wall, and the difference is not the stone: it is the draught.
Nor is it exclusive to distilleries. It has also been found in bakeries, where yeast fermentation releases ethanol, and it can appear in association with natural fermentative processes, such as seasonal fruit drop.
04 How it survives a metal roof in the sun
One of the strangest things about this fungus is where it lives. A galvanised metal roof in full sun goes from very high temperatures to night-time ones within a single day, and that cycle would destroy most surface-colonising organisms.
The explanation is that, on exposure to ethanol vapour, it produces heat-protective proteins. That is: the same substance that feeds it gives it the resistance it needs to live where it lives.
That dual function explains why in these towns the fungus reaches the roofs and why the stain does not stay at street level. And it also explains why cleaning is not enough: as long as there is vapour, it comes back.
05 A hundred and fifty years to give it a name
The scientific history is long and full of detours. The black sooty growth on buildings near the distilleries was first examined in 1872, and the genus name was formally established in 2007.
The name honours a nineteenth-century pharmacist, Antoine Baudoin Poggiale, who left the first description of a black mould beside the distilleries. The species epithet, compniacensis, is simply the latinised demonym of Cognac.
The story continued. In 2011 the genome of one of these species was completed, at 21.88 million base pairs, and in 2015 DNA sequence analysis split what was believed to be a single species into five, with different distributions by continent.
And it has reached the courts. In 2012 lawsuits were filed by residents against producers in Kentucky over the fungus growing on properties near the warehouses: the same phenomenon that here is accepted as part of the landscape is litigated elsewhere.
06 What you see and what you do not
The most immediate practical consequence is that you no longer need to ask where the warehouses are. They are visible from the street, because they are the blackest buildings, and the gradient of the stain on the surrounding façades shows the direction of the prevailing wind.
The second is that this is one of the few places where an industrial activity leaves a visible trace that is neither pollution nor ruin, but a living thing that has moved in to feed on it. The town wears what it makes.
And the third is a change of scale. When you stand in front of a black wall, what you are looking at is not a stain: it is the accumulated record of decades of casks breathing behind it.
