Mixed

The casks available to early distillers for barrel-ageing was historically dependent on what they were able to acquire at a reasonable price from local merchants or ports. As a result, these casks could have been used to transport a variety of things, from spirits such as brandy, whisky, or even other rums, to sherry, wines and beer. The varying influence of each barrel on the liquid filled into it means that as with most spirits categories that can involve cask ageing, blending in rum has long been an essential part of the production process.

This is particularly true of the modern era as big brands seek to ensure consistency of the flavours, aromas and increasingly, natural colours of their proprietary releases. Today many notable distillers blend rums from a variety of casks types, such as Foursquare in Barbados who have done so to much acclaim with their Exceptional Cask Selection, and Guatemala's Ron Zacapa who use a variety of American oak and ex-sherry casks within their solera systems.

Another form of mixed maturation comes from the modern process of cask-finishing, also known as secondary maturation. This involves transferring the entire contents of a cask or casks into a new one, ordinarily of a different wood type. The idea was pioneered by Scotch whisky distillers in the 1980s, ordinarily seeing bourbon or refill cask aged whiskies re-racked into sherry wood to add additional flavour. Nowadays standard finishing casks also include wine and other spirits, while all manner of unusual experimentation has taken place in the shape of, for example, maple syrup and even ex-Tabasco hot sauce barrels. Secondary maturation founds its way to the rum world mostly via the independent bottlers of Europe. It was the key feature of the Maison Ferrand's Plantation rum line when launched in 1999 and already a core practice of those primarily engaged in whisky sales. However, the relative rum boom of the 21st century has seen an increasing number of distillers of rum from all classifications increase their engagement with the practice, as more and more consumers have come round the the idea of rum as a sipping drink rather than merely for mixing cocktails.