To obtain the title of Scotch whisky, by law, the new spirit must be matured in oak casks, with a capacity no greater than 700 litres, in a warehouse in Scotland, for a minimum of three years.
The new spirit, at approximately 68%abv is reduced to 63.5%abv with spring water, in the filling store. The new spirit absorbs flavour compounds from the oak casks more effectively at 63.5%abv, than at other strengths.
Glenfarclas is matured in 100% spanish sherry casks, which have matured Oloroso or Fino sherry in Seville, Spain. With a capacity of 500 litres sherry butts are widely used at Glenfarclas, along with 250 litre sherry hogsheads.
Charring the casks releases carbon, which helps mellow the whisky, and hydrolysable tannins promote oxidative reactions such as ester formation from acids. Lignin flavour compounds such as vanillins, produce vanilla aromas. Simple sugars are responsible for a slight sweetness, whilst malty flavours come from the release of furfural, 2-methyl furfural and maltol, due to pentosan degradation.
In the traditional style 'dunnage' warehouses, dating from the 1880's, and consisting of thick, stone walls, with earth floors, the casks lie sleeping for a minimum of 8 years. During this time the casks are not disturbed, and the spirit evaporates at 2% volume, which also reduces the alcohol content by an average 0.4%abv per year. This is known as the 'Angel's Share'.
In a 'dunnage' warehouse, the casks are only stacked three high. The temperature is stable, does not fluctuate by more than 6C. There are 30 traditional style warehouses at Glenfarclas. The casks can be filled several times, and if required, will be re-coopered between refills.