Most rum drinkers really don't understand the true art and importance of wood and aging. I'll never forget my extended e-mail exchange with the Artic Dunce who expressed his view that barrels and aging were simplistic and easy, sloshing with leftover bourbon or sherry, and were really just a commodity to be purchased at the cheapest price.
Naive, and he was/is completely wrong.
As you have learned now, oak aging accounts for at least 80% of the flavors and sensations that rum drinkers value. Doing it right is expensive, sophisticated and truly a real art. I have already noted the differences in the wide variety of American and European barrels insofar as what different flavors each contributes.
But I'd like to conclude by noting the differences among individual barrels...
And to make matter even more individualistic..."Every barrel consists of dozens of individual bent planks of wood called staves. While each stave (theoretically) was harvested from the same forest, individual staves would have almost certainly originated from a tree located in a different part of the forest and from a different part of the tree. Oak trees undergo a growth spurt each Spring. Under favourable conditions, the rate of growth is hastened, leading to a coarser grained wood which is reputed to offer coarser tannins and less subtle oak characters.
So logically, each barrel is constructed from a menagerie of oaks with varying grains, each of which offers something different..."
Point: each stave dries differently, thus is differently affected by the toasting/charring to follow, thus having a different effect on the rum. I won't bore you with why, but the time and method used to dry the oak (kiln or superior air drying) also affect what each single stick of oak will ultimately contribute. And last..."After the oak is harvested it is split into staves and left to dry in stacks for a period of one to three years. The position of each stave in the stack will affect its moisture content at the point at which it is used in the construction of the barrel. The moisture content of the staves impacts on the effectiveness of the next barrel making step, toasting."
The cooper will use his or her individual judgment to deliver a level of toast specified by the distiller from untoasted, to light, medium, medium plus or heavy toast, then to charring: light, medium or alligator The amount of toast or charring, and it's depth within each stick depends on the heat of the flame, how long it was applied and the moisture content of the stave. Amazing!"Toasting is the step which probably results in the greatest variability in oak barrels. Toasting involves placing the barrel over a flaming pile of oak off-cuts. The intensity of the flame and the time which the barrel is exposed to it fundamentally affect the chemical composition of the oak and hence the flavours that it imparts to the wine. No formulas are applied here."
This final process can carmelize and release wood sugars (in modest amounts). Charring produces a level of, uh, charcoal that actually filters and smooths the rum as the barrels breath, while at the same time allowing access to inner woods and thus increases the complexity and depth of the spirit."The cooper's judgment of these factors as well as their personal opinion as to what actually constitutes the buyers' specifications can result in significant variability in the barrel delivered to the winemaker."
Note: the distillers who cheat - and there are many - try to duplicate the smoothness and complexity of true aging by simple adulterating their rums with syrupy sugar, glcerol, artificial flavors and spices and even sherry wine.
It's not all that hard to taste the difference between a phonied up rum and a truly fine, pure and unadulterated rum.
A final word about ex-bourbon and sherry barrels...
Michael Jackson is one of the very few true greats of the world of fine spirits. Ex-bourbon barrels are the most commonly used barrels used in the aging of both rum and whisky. Jackson points out that first fill with rum or whisky imparts vanilla-like flavors (wood vanilan), along with caramel-toffee, dessert apple and "a touch of tannin".
While a first fill of ex-bourbon barrels will create these flavors, the second use contributes significantly less, while the third and later use contribute very little except for smoothing.
A very few quality distillers use expensive ex-sherry barrels.
These costly barrels are much more variable, as they may have contained cheap or expensive sherry of wide variety of grapes. Most are from Palmino grapes: fino (dry, delicate), manzanilla (saltier), amontillado (dark and nutty), pao cortado (aromatic and complex) and Pedro Ximenez (raisiny, syrupy and dark). Note also that most ex-sherry barrels are made from European oak.
Like ex-bourbon barrels, the ex-sherry barrels' contribution likewise decreases after the first use. Third fill barrels are used to maintain character, and fourth fill are typically reserved for blending. Some distillers feel that the second use provides the most balanced and pleasing result.
Summary:
Each barrel is truly a living and individual entity, with no two barrels having the same effect at the same rate. The master distiller/blender is truly an artist who constantly monitors each barrel and logging the development of the rum or spirit within. Even the position of the barrel in the warehouse is important, and barrels are often rotated to obtain the most consistent and high quality result.
It is fair to say that cooperage and aging may be the primary factor in creating a truly fine rum. Certainly distillation, particularly in classic pot stills, is nearly as challenging, but account for probably only 20% of the flavorful outcome.