The definition of spirits was vague in the Middle Ages. People drank distilled whisky directly or with spices and herbs. Whisky was thus not much different from vodka and gin. There are two sayings on the origin of using oak casks for maturation:

1. In early times, Spanish sherry was commonly sold to Scotland in barrels. Empty barrels were left and sold to distilleries for containing whisky. Few months later, distilleries found the colour and flavour of whisky had both changed. To produce whisky with richer flavour, people started to use sherry casks to mature whisky.

2. In the 18th century, the UK government levied unreasonable taxes on alcohol, which resulted in the surge of illegal distilleries. To escape from heavy taxes, people used different barrels to store and deliver whisky and discovered the change in whisky flavour.

At the beginning people relied on sherry casks. When it came to a shortage of Spanish casks, American bourbon casks were used as a substitute.

Maturing in a cask has three major effects on whisky: removing unwanted flavours, adding more flavours and providing a stable environment for oxidation. Casks are named after what kinds of alcohol they contained before. Barrels contained bourbon whisky are called Bourbon Casks, while Sherry Casks are the barrels contained sherry wine. A whole new cask is called Virgin Oak.

Casks can be divided into First Fill and Refill. The former are casks which have been used to age other wines and is now being used to age whisky for the first time. The latter casks have been used to mature whisky and refilled a second time. There’s no absolute standard of good or bad between the two kinds of casks. In the eyes of blenders, they are just two kinds of containers to produce whiskies of different flavours. It only matters how to precisely create the best taste by blending these flavours.

Among the categorisation of barrels, Hogshead is also confusing. Sometimes it refers to the barrels of 225 to 250 litres. It might also refer to the restructured barrels that have been dismantled to remove unwanted staves and add new staves.

As a matter of fact, the role of First Fill and Refill casks in maturation is different. First Fill casks can give lots of special flavours to whisky, but if the maturation is too long, the strong flavour will conceal the house character of distilleries. Also, the whisky might be too woody to drink. While Refill casks can maintain a stable flavour while maturing old whisky. It takes a higher cost to use First Fill casks for maturation and thus they are rarer and more expensive but that doesn’t mean they must taste good. You can keep trying and comparing to explore your favourite flavour.

Many people will use the whisky colour to judge its age. In fact, whisky matured in bourbon for over a decade will still be pale golden; while whisky matured in sherry for only a year is already dark amber. Colour can be a reference indicating whisky flavour, but not a proof of its age.

The bourbon casks used by Scotch Whisky industry mainly come from bourbon distilleries in Kentucky and Tennessee, USA. Whisky maturing in bourbon casks will have flavours of cream, vanilla, coconut, caramel, honey and various spices. Compared with sherry casks, bourbon can generally present the characters of every distillery. Its maturation is also more stable and easier to control. Plus the quantity and supply are more stable, bourbon becomes the most common kind of matured whisky.

There are many kinds of sherry, mostly referring to Oloroso casks. Whisky matured in Oloroso will have sweetmeat, dried fruits, nuts, chocolate, cocoa and coffee flavours. Owing to its darker colour and richer flavour, it is good for food pairing and thus highly welcomed by Asian customers. In short, producing sherry is mainly about flor controlling, oxidation and timing of adding spirits:

Fino: In the process of biological ageing, flor floats and covers the top of sherry, shielding it from excessive oxidation and producing sherries of lighter colour. Whisky matured in Fino sherry casks is slightly acidic; it also has yeast aroma and a fresh taste. However, its colour is rather pale which makes it unfavourable by most distilleries.

Manzanilla: A type of Fino. Whisky matured Manzanilla sherry casks will have richer floral and fruity flavours. Especially good for Speyside whisky to maximise its characters.

Oloroso: The most popular sherry casks. Highest ABV on average. Taste sweeter. With dried fruits, cocoa, coffee and dark berries flavours.

Amontillado: A surprise when the old distilleries failed to control alcoholic strength and develop enough flor for producing Fino sherry. The sherry produced accordingly has an ideal sweetness with notes of tropical fruits, berries and nuts.

Pedro Ximénez: The sweetest sherry. Distillers have to pay close attention; the whole barrel of whisky would be ruined if it’s slightly over-matured.

The Singleton of Glen Ord 12 Year Old Single Malt Scotch Whisky Sherry Cask Edition initiates the unique triple cask maturation process. Creating an unprecedented roundness and mellowness, it also creates a new standard for sherry flavour.

All in all, regular sherry casks are always in short supply. Most whisky distilleries are customising their own sherry casks.

In recent years the whisky industry has been evolving, including using cognac, rum or port casks to mature whisky. Nevertheless, they are all trial products or limited release, unable to replace bourbon and sherry whisky.

You can try adjusting the maturation and blending process, most importantly, the Finishing, or Wood Finish. It refers to a whisky, after maturing in A cask, again matured in B cask. The whisky produced can have characters from two kinds of casks. There are also examples of maturation in three casks.

The Singleton Special Release 2018 14 Year Old Cask Strength Whisky is a ‘blend of five barrels with triple maturation’, creating a rich and layered texture.

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