The first classification of Sherry Wines takes place before the must is racked of lees. The first cold days of Winter have helped to decant all of the solid particles to the bottom of the vats and the wine has now become clean and transparent, with an alcohol content of between 11 to 12% vol. On the wines' surface a type of cream or veil of yeasts has formed. This veil is called "Flor" (flower).
Wine tasters now sample all of the individual butts and take the first of many important decisions which will determine the type of ageing which that year's different wines will undergo. Some of the palest, lightest wines that are clean to the nose will be set aside for ageing "bajo flor", under a veil of "flor", in order to produce FINO and MANZANILLA. Other wines, also clean on the nose and palate, but with more body will be classified as OLOROSOS.

The wines classified as Finos or Manzanillas will be fortified with wine-distillate to bring their alcohol content up to 15% vol. while those destined to become Olorosos will be fortified to 17% or slightly higher, according to each houses' individual style. These different levels of fortification and alcohol content will determine the type of development undergone by the wines in the next phase of their ageing.

The wines, with their new alcohol content, are transferred to typical oak barrels called butts, or "botas" for the first stage of their ageing known as "sobretablas". Different to other wine Regions were the wines age in hermetically sealed vessels in order to prevent their oxidation, the ageing process for Sherry and Manzanilla takes place in 600-lt. American oak butts which are filled to 5/6 of their capacity, leaving a volume of air inside the butt "two fists high".

In the case of Finos and Manzanillas - at 15º - the veil of "flor" remains constant and covers all of the wine's free surface inside the butt. The veil, formed by live yeasts belonging to the genus Saccharomyces, isolates the wine from the air and prevents its oxidation. Moreover, the yeasts are interacting constantly with the wine, consuming its alcohol and other nutrients and endowing it with its characteristic aromas and flavours. This type of ageing process is known as "crianza biológica" or biological ageing.
In the case of Olorosos, fortified to over 17% vol., this higher strength impedes the development of the "flor" and it disappears. The wine is therefore in permanent and direct contact with the oxygen in the air. This ageing is known as oxidative or physical-chemical ageing.

Sometimes, in the case of Fino or Manzanilla wines which are aged biologically, the veil of "flor" might reduce or even disappear. This might be due to natural causes or according to a decision taken by the head cellarman who fortifies them above the yeasts' maximum limit. As the protection of the "flor" disappears, these wines, in direct contact with the air, begin a new stage of oxidative ageing. This gives rise to the class of Sherry Wines known as AMONTILLADO, a jewel among the world's wines.