After fermentation the grapes are then aged in small oak barrels. In many DOC regions, the wines are required to age for at least 3 years though it is not uncommon for producers to age their wines for 5 to 10 years. Traditionally the barrels were made of chestnut instead of oak, which contributed high amounts of wood tannins and was very porous which promoted excessive evaporation in the barrel. Under this same traditional style of winemaking, a large ullage or air space would emerge in the barrel and oxidation took place. This gave the wine its characteristic amber but also flavours and traits that may be characterised as wine faults. Towards the end of the 20th century, more producers began switching to oak barrels while maintaining the tradition of not topping up the barrels and filling in the ullage space. This angel's share still produces some level of oxidation, though not as severe as the style was historically made. Modern winemaking technique also calls for more temperature control and keeping the wine in rooms with a consistent temperature that promotes more fresh flavours in the wine and fewer faults.