Unlike wine, the maturation of scotch does not happen if it is placed in bottles. Rather, oak casks are used for the process of maturing. But not all casks will do. The quality of the cask often determines the quality of the scotch. Experts inform that along with the making process and the quality of the barley, the quality and nature of the cask account for as much as 95% of the ultimate quality of a malt whiskey.
So, the quality of the wood that goes into the making of the cask is immensely important for the production of whiskey. Casks need to be made of oak which is both flexible and solid enough to be constructed as a cask. The oak also serves to add a distinct flavor to the whiskey. Only two types of oak are used for making casks (barrels): European oak and White American oak.
Why aging bourbon in sherry casks is not an option
Also important is that only second-hand casks are used for the preparation of scotch. This is just the opposite to the preparation of bourbon since bourbon can only be prepared in a new cask and according to American law, the cask needs to be discarded afterwards. So for example, aging bourbon in sherry casks is never a viable option. Not only that. In fact, if any distiller try aging bourbon in sherry casks, he would be held liable to the law, at least if he is living in America. And bourbon is primarily an American product. The original bourbon only comes from Kentucky. However, bourbon producers in other countries may consider aging bourbon in sherry casks.
Things are different in case of scotch, though. Scotch can only be aged in second-hand oak casks since the newness of the wood can potentially affect the subtleties that the connoisseurs relish and value in a fine scotch.
The second-hand casks that the scotch producers use generally come from the sherry or bourbon producers. This is because sherry, just like bourbon, is generally aged in new casks.
The scotch makers often buy up the oak cask from bourbon and sherry producers at a convenient price. These used casks also add a typical vanilla flavor to the whiskey.
In earlier times, sherry which is a Spanish wine was shipped to Britain from the country of Spain in oak casks. The empty casks could not be returned to Spain since that would have been too expensive and the casks therefore used to lie about in disuse. That is when the Scots came upon the idea of recycling them and use them for the maturation of their own indigenous scotch. Aging bourbon in sherry casks may not be viable, but aging scotch in sherry casks very much was.
Sherry casks however cost more than bourbon oak casks. The casks distill the flavor from the former contents. But they also give the maturing scotch a deep amber color and a heavier body. The same, however, does not happen in case of aging bourbon in sherry casks.
Unlike as in the case of aging bourbon in sherry casks, sherry and bourbon casks may also be used for as much as four to five times for maturation of scotch whisky. It is natural that the later fillings will have less transferred quality of wood to them. This is the reason why one bottle would differ from the next bottle depending on the sherry or bourbon cask the content of the bottle was in.