Losing around two per cent of a cask’s contents through evaporation can seem a depressing annual statistic for distillers (and even more so for accountants). But this type of regret is a pointless emotion, as evaporation is an essential enabler of the aging process.Often summarised as ‘concentrating’ the spirit, evaporation of water and alcohol is also about developing complexity. But it’s also about losing certain undesirable elements such as sulphur compounds (although they are mainly absorbed by the layer of char in the cask).The process of evaporation effectively begins as soon as the cask is filled, although the initial rate may be insignificant. Moreover, the evaporation rate is variable and hardly monitored on an annual basis. Checking casks focuses on the quality of the spirit as it develops, rather than the quantity of evaporation losses (although older casks need careful monitoring to ensure they don’t go below the minimum bottling strength of 40% ABV).Exactly how varying evaporation rates affect the way malts mature is still not fully understood, though quality is not considered to be commensurate with higher rates. Acask experiencing two per cent evaporation per annum for example, won’t automatically be superior to a cask showing lower losses.Various factors influence the evaporation rate, including the type of aging warehouse and a cask’s position within it, the ambient temperature and humidity levels, as well as the filling strength.Although 63.5% ABV is the standard filling strength this isn’t a universal figure, and during the 1960s-80s filling at higher strengths was hardly unusual. The filling strength reflects certain practicalities, such as the amount of casks required for aging, but it also influences the rate and the manner in which whisky matures.Higher filling strengths result in alcohol evaporating at a slightly faster rate than at a lower strength. The opposite applies to water, which evaporates at a relatively smaller rate at a higher strength, and a greater rate at a lower strength. Meanwhile, the overall evaporation rate slows down during aging as the alcoholic strength declines.A higher strength can also result in a relatively higher level of alcohol-soluble flavour compounds being derived from the oak (promoting vanilla flavours for example). Meanwhile, a lower alcoholic strength, and relatively higher proportion of water in the spirit, may see a higher level of water-soluble extracts such as tannins being extracted.The rate of evaporation is also influenced by the environment in the aging warehouse, or more specifically by the differences between the interior of the cask and the exterior atmosphere. This is because the concentration of vapours within the cask is continually trying to establish an equilibrium with the exterior environment.Correspondingly, the rate at which water evaporates is also influenced by humidity levels in the aging warehouse. If the atmosphere outside the cask is very humid, this mitigates the level of water that evaporates into the surrounding atmosphere, as it’s already ‘saturated.’ (That’s why a warmer, drier environment promotes a greater proportion of water loss, while cooler, more humid conditions see more alcohol than water evaporating).Exactly how humidity levels affect maturation is still not fully understood, though some distillers believe higher humidity promotes a slower, and more preferable, rate of maturation.Dunnage (also known as ‘traditional’) warehouses tend to have higher, and more stable humidity levels than racked warehouses. However, some racked warehouses restrict the amount of concrete to pathways between racks, retaining earthern sections under the racks to promote humidity.While dunnage warehouses also have more stable temperatures than racked warehouses, this needs to be qualified. Barrels at the bottom of a racked warehouse, for example, experience similar conditions to a dunnage.However, ascending the racks, which may go 12 barrels high, shows a graduation of microclimates, with the greatest temperature differentials and evaporation losses at the top. This is because reactions within the cask, such as evaporation, accelerate as the temperature rises.“Racked warehouses generally have concrete floors and we tend to find that casks keep strength very well, so they’re probably loosing more water in evaporation than alcohol. In a dunnage warehouse strength goes down that bit quicker, though we consider that dunnage gives us a more level maturation profile, and so greater consistency,” says Gordon & MacPhail’s Ewen Mackintosh.But even though a dunnage warehouse offers the most stable temperature and humidity levels, there are still micro-climates within an individual warehouse.“Even in one warehouse there can be a special corner where evaporation rates are different, it could be a bit damper for example,” says Ian McWilliam of Glenfarclas, where casks are aged exclusively in dunnage warehouses.“Air vents in the warehouse can also create a movement of air that could affect some casks. Similarly, casks on either side of the passageway may experience more air movement than those further back, which could influence the evaporation rate. But how much of the differences between casks can be attributed to evaporation, and how much is down to the nature of the cask, is difficult to say,” adds Ian McWilliam.In addition to the type of warehouse, another aspect to consider is the location of the warehouse.“We’ve compared casks filled at the same strength into American oak casks and aged in dunnage warehouses, and we see more evaporation losses on the Islands than in the Highlands and Speyside. What this can be attributed to is uncertain, possibly due to more constant breeze on the Islands than on the mainland,” says Euan Shand of Duncan Taylor.Evaporation also plays another vital role during aging by developing the headspace within a cask, which in turn influences the exit of vapours.When the temperature rises the spirit expands within the cask, while gases (air being composed of various gases) in the headspace expand at a greater rate than the spirit. As the headspace can’t expand into the liquid, it ‘exhales’ through the cask’s pores, joints and bung.Correspondingly, when the temperature falls the spirit contracts, creating a larger area for the headspace to occupy within the cask. Meanwhile, the gases in the headspace contract at an even greater rate than the spirit, which results in fresh air being drawn into the headspace from outside the cask.In this way air that has become saturated with vapours from the spirit (which includes sulphur compounds, alcohol and water) is exhaled, and fresh air is inhaled.Meanwhile, the process of evaporation is a continual factor, as even without the cask ‘breathing’ vapours exit the cask through the pores and joints.Whether evaporation rates are also influenced by the type of oak is another consideration. As European oak is more porous than American oak, some distillers believe the evaporation rate is correspondingly higher. However, this is mitigated by another theory, that the joints and bung are a more important ‘escape route’ than the pores, with the croze (ie. where the ends of the cask sit into the staves) providing an additional way out for vapours.Depending on the length of aging, the total amount of evaporation can be monumental. A cask laid down at The Balvenie on January 26, 1952, and emptied on September 6, 2002, lost 77 per cent of its original volume through evaporation. This equated to a loss of 173 bottles, leaving a total of 83 bottles which were released as The Balvenie Cask 191, aged 50 years.But then for one type of loss, there’s another type of gain, as it’s a magnificent dram. Delicate, and elegant yet rich, it opens with a creamy, dark chocolate note, a hint of spicy oak in the form of cloves and ginger, before dried fruits with apricots and raisins emerge, followed by more creamy chocolate. The finish builds slowly and majestically (after all those years why should it hurry), with a combination of subtle spice, nuttiness and dark chocolate. Definitely worth waiting for, particularly as it goes on. And on. Sensational.