This method of manufacture, also called "surface process" consists of placing a mixture of vinegar and wine in oak barrels, also called "vessels", to acetify.
The barrels are filled up to their widest point in order to expose the maximum surface area to the air.
On the front of the barrel there is a round opening called a "eye" which allows air to circulate and the cellar-master to check the liquid. These rooms in which rows of barrels are racked on several levels must be kept at a temperature of 25 to 30°C.
The bacteria thus take oxygen from the air and fix it on the ethanol to produce acetic acid. It takes around three weeks for this natural process to occur. This depends on the original ratio of wine to vinegar and on the temperature of the exposed surface.
A sample of vinegar is then taken for analysis to check the quality of the acetification. A part of the liquid is then extracted taking care to leave enough of a 'starter' to begin a new production. The vinegar extracted is replaced with the same quantity of wine.
The vinegar is then left to mature for at least six months in oak casks or "demi-muids" (medium-sized barrels) to develop the aroma. The vinegar is then filtered and bottled and aromatic plants may or may not be added (tarragon, mint, shallots,…).
This manufacturing method is exclusively used for wine, cider or honey vinegars, which are then given the appellation "vinaigre à l'ancienne".