As mentioned last month, bourbon barrels can be charred to different degrees. Not only does that fire influence provide a charcoal layer which acts as a natural filter, smoothing out the harsh and undesirable qualities in the whisky, but it also crystallizes the sugars in the wood, enhancing the already naturally sweet character of American oak (caramel, vanilla, coconut, etc.). 

As bourbon barrels are legally only allowed to be used once, they’re readily available, less expensive than an ex-sherry cask, and now make up the majority of casks being used to age Scotch whisky. Some Scotch companies even own oak forests in America and rent the barrels to bourbon makers before bringing them over to Scotland.

Sherry is aged in a solera system, meaning that they use a series of casks where the sherry inside each vessel is different ages. Sherry to be sold is taken from the oldest casks, which are then topped up with the next oldest sherry, and so on. The barrels are never fully emptied as it’s a continual process of homogenizing the wine. Because of this, casks from an actual solera system would never be taken out of rotation and sold to the whisky industry. Also, consider that in creating sherry, these large wooden vessels are old wood (sometimes over a century old) and do not impart any wood-y characteristics to the sherry.

Nowadays, a whisky company needs a special relationship with a sherry producer to create bespoke casks for them. These casks will be lightly toasted (compared to charred) and seasoned with young sherry for several months to a few years, before being utilized for whisky maturation - which does want the actual wood influence along with the sherry notes. That seasoning sherry will be turned into vinegar, as it’s not of a quality for drinking. While sherry casks used to dominate the Scotch whisky aging field, they now make up only about 10%.

There are increasing experiments with other types of casks, such as Port, Rum, high end red wines, and even Champagne, although these are often more for finishing a whisky rather than as a long-term aging vessel. But each one will, of course, add their own particular influence to the whisky.

Evaporation and Maturity 

Wood is semi-porous, and the weather plays an important part in the aging of spirit. Wood expands and contracts in the heat and cold, and during this, the spirit goes into and out of the wood, which is where it gains its colour and character. It’s also during these temperature fluctuations that spirit aromas are “exhaled” from the cask and the surrounding environment influences (such as heather, ocean brine, forest, etc.) are “inhaled” and proceed to influence the spirit within. In warmer climates where the temperature fluctuates more intensely, the whisky will age faster than whisky matured in a cooler climate.