In the Jura, where the most famous vin jaune appellation is château-chalon, the wine must be made from the signature local white grape variety the savagnin, grown ideally on marl. The grapes are picked well ripened, often not until late October, ideally at about 13 to 13.5% potential alcohol, to allow for an increase during the ageing process, and fermented as normal. The wine is then put into old 228‐l/60-gal casks usually not quite filled so that the local benevolent film-forming yeast, called here the voile or veil, can develop on the surface. It is similar to the flor which is responsible for fino sherries but can develop at a lower alcoholic strength and a much thinner layer, coloured grey, is considered the best. The ageing ‘cellars’ (which may be above or below ground) are ventilated deliberately to allow temperature fluctuations during which the activity of the voile will change. The presence of the veil prevents severe oxidation, but the important factor in making vin jaune is that for at least five years the wine is left in cask, untouched other than to allow regular sampling to check the amount of ethanal formed (a crucial compound for the taste of vin jaune) and for a dangerous rise in volatile acids. It may not be bottled for a full six years and three months after the harvest. Inoculation or seeding of yeasts to form the voile for Savagnin and even Chardonnay wines was introduced in the 1970s, and today this is used by some producers, but is disapproved of by purists who believe that natural methods produce a better wine and are reliable if ‘cellar’ conditions are right and barrels have previously held wine aged under the voile.