The Red Layer
The distinctive shape of a cask is surely a classic design. Its enduring advantage, as relevant today as it was 2,000 years ago, as its large capacity, superior strength and high maneuverability.
A whisky cask weighs in at 200-250 kgs, up to a fifth of a ton when full. Yet, with surprising dexterity, the cask can be rolled from A to B, pivoted on its central axis, and precisely stowed away.
Empty, a cask can be ‘bounced’ upright from the horizontal, its extraordinary strength allows for stacking several high.This strength and dexterity is achieved by the famous belly shape of the barrel. To bend inch-thick oak to that required curve, without splitting, requires heat. In the initial cask assembly stage the staves are tightly arranged at one end, open rosette at the other. It is then toasted upright over a brazier.
Now malleable, the staves can be forced to bend without splitting by a pneumatically-pulled cable round the girth as hoops are hammered downwards (also by pneumatic pressure) before being repeated from the opposite end. The hoops maintain the taper and keep the cask together.
The toasting has a dual effect. Primarily, it enables the wood to be bent into shape. Secondarily, the toasting – depending on length and intensity of burn – will activate the oak.
A cross-section of Bourbon barrel stave after toasting shows three distinct layers. On the inside, facing the spirit, is a layer of (1) charcoal up to a couple of millimetres thick; (2) a darker, reddish band known as the red layer; (3) and the outer, paler, natural wood.
The two inner bands, the charcoal and red layer, come from the toasting process. The red layer, as far as the heat penetrated, is the ‘active’ layer where the wood’s structure is broken open allowing wood tannins to be readily extracted during maturation. Without this burning the effect would be markedly reduced.
The charcoal layer, depending on the intensity of the toasting phase, may be a mere singeing for French oak or it may be carbonised for American. The barrels’ destined usage will determine the level of charring. A heavy char filters out harsher components of bourbon distillation while French casks, generally destined for wine, are only very lightly toasted by comparison.
The cooperage at Radoux, France