Ageing whisky is a complex process that, to be honest, few people fully understand. Distillers use a variety of techniques to try and create a new make that is unique to their distillery but Scotch isn't Scotch until it's spent three years in a cask and It's in the cask that the magic really happens.

Once laid down to age, the interaction between the spirit and the wood is a key part of the process with extraction of lignin, vanillin, lactones and tannins from the wood into the spirit but also adsorption of various compounds from the spirit into the wood. This can provide a cleansing effect to the malt, which is typically accentuated with a heavier char, but these elements can then alter the wood too and feedback different flavours into the spirit. As you can see, the interaction between the wood and the spirit becomes complex quickly, but it’s also only part of the full equation.

As the wood and spirit mingle happily together, there is evaporation of water and alcohol from the cask, reducing the spirit in strength and volume, whilst esterification is doing its thing too, creating more complex molecules from the soup of compounds (organic acids and alcohol) that were already present. The effect of evaporation is affected greatly by the external temperatures and humidity levels. The higher the humidity the more alcohol is lost preferentially to water, whilst in a dry climate, relatively more water will be lost. The overall external temperature will basically speed up the whole process and this is why we see aged malts from the likes of Amrut in India costing so much. Of course, producers can play with these factors too, increasing airflow in the warehouse and placing casks in warmer spots. There are now distillers experimenting with temperature control and even those who are playing with the idea of rapid aged whisky.

So there’s plenty of “unseen” chemistry going on that to a certain extent is in the lap of the gods and is hard to control. But there’s plenty about the oak that the distiller can control, namely the type of oak, the previous contents, the level of char and the number of previous uses of fills that the cask has experienced. All these affect the final malt to a huge degree. European oak adds more tannins and xmas cake flavours than its American counterpart - white oak - which adds greater vanilla and coconut flavours. Seasoning a cask will reduce its tannins and charring and toasting all help degrade lignin to release components such as vanillin. A full char will also remove some of the unwanted congeners found in spirit - cleaning the spirit and helping to smooth out the edges.

So ageing for longer potentially adds more flavour and smooths out a whisky. But Scotch just isn’t that simple, especially when you consider all the factors at play mentioned above. First fill casks can add flavour quickly and perhaps keep adding for longer as there's simply more that hasn’t been extracted. On the flip side, people talk about the subtleties that arise when ageing in second fill casks for longer. It’s probably fair to say that ageing adds more complexity but as cask extraction continues there’s always the chance that tannins levels, for example, become too extreme, or even that more water is lost over alcohol… then we’re into the realms of subjective preference. As ever, Scotch is such a tricky thing to pick, but then that’s what makes it so magical.

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