Although we are fascinated by the virginal flavours of our pure cane juice as exhibited in our Pre-Cask bottlings, the vast majority of our rum is distilled for the additional flavour, complexity and amelioration derived from maturation in oak casks.

At face value, maturation is a simple business. We fill our casks with colourless pure cane spirit and at some point in the future we disgorge the cask to find rum that has taken on a darker colour and in which the flavours of cane now intertwine with flavours both from the oak itself and from any liquid previously contained in that cask.

There is, however, rather more to it than that.

We have covered our wood policy here already, including our four main cask types: First Fill American Oak, Virgin American Oak, Premium French Oak, Vins Doux Naturel (sweet fortified wines), along with more recently a consignment of Andean Oak from Columbia. But the process of maturation and the flavours it induces are more complex than simply the type of oak into which rum is being filled.

There are three profound processes occurring in a maturing rum cask that influence the spirit’s flavour – extraction, evaporation and micro-oxidation.

Extraction is the most straightforward to understand. Maturing spirit will seep into the staves of the cask and from it the alcohol will extract the oak’s flavour compounds – vanillins (the same compound as in a pod), lignins, tannins (that provide amongst other flavours, natural colour) – as well as, potentially, nuances of the casks’ previous constituents. Spirit will extract more pronounced flavours of coconut, vanilla and caramel from an American oak cask, for example, more toasted spice and unctuous mouthfeel from a French.

Of course the level of extraction possible is dependent on the freshness of the oak and strength of the spirit: the fresher the oak the more the flavour; the higher the filling strength the greater the extraction. Casks act like teabags – the first time they are used, all of their flavour is available to be extracted. The second time there is a little less and once you are using your cask a third or fourth time, any available contributory flavour has already been leached away. This is why we see fresh oak as paramount to our rum … even though it would certainly make more economic sense, like most of the industry, to just reuse the casks over and over again.

Evaporation is only possible because of the inherent porosity of oak. The staves, varying in thickness between 18 to 25mm, act as a Gore-tex-like membrane allowing the passage of air and spirit without leaking liquid itself. The oak pores let smaller alcohol and water molecules through at varying concentrations depending on the environment – more on that in a moment – in what is colloquially known as ‘the Angel’s Share’.

However the most important process of all, so far as the quality of our spirit is concerned, is that of gradual micro-oxidation over time. Microscopic volumes of air entering through the oak’s pores, as though through the cork of a bottle of fine wine, slowly burnishing the youthful spirit flavour compounds into the mellower, smoother notes that we associate with age. This is the reverse side of the angels’ share, the loss by evaporation of alcohol in to the warehouse environment, its place taken by oak-strained air.So much for the processes – but here comes the critical point: these natural developments in maturing spirit are inevitably governed by the environment in which they are taking place. By the heat and, just as crucially, the humidity of the air around the casks.

In the cool, damp air of coastal Scotland or Ireland, for instance, the various processes of maturation take place at a slower creep, with a higher concentration of alcohol than water molecules escaping through the cask’s pores causing the alcohol strength to  fall over time. In the hotter, drier bourbon warehouses of inland Kentucky the opposite occurs – more water molecules than alcohol is lost from the cask. and spirit strength rises.