What is a dunnage warehouse?

A dunnage warehouse is a low, often windowless warehouse with thick stone walls and a slate roof on wooden trusses, in which whisky casks mature lying down. The floor is of earth, gravel or cinders and there is no damp-proof course beneath it, so that the ground and groundwater largely determine how humid the air inside is.

The casks lie on heavy wooden beams, the dunnage. They are stacked in rows, called stows in Scotland, usually two or three layers high. That comes to about two metres. There are also dunnage warehouses with two or more storeys, separated by wooden mezzanine floors.

A racked warehouse stacks casks in steel racks up to eight or twelve tiers high, a palletised warehouse puts them upright on pallets in four to seven tiers. A dunnage warehouse stays at two or three tiers. The difference is therefore mainly in height and density: the higher the stack, the more casks under the same roof.

Racked warehouses came into use from the 1950s, palletised warehouses in Scotland from the mid-1980s. Together they store the greater part of the Scottish stock: about sixty percent of the casks lie in racks and almost forty percent on pallets. What matures in a dunnage warehouse is therefore a small part of the whole.

Thick walls and a low build keep the temperature in a dunnage warehouse level all year round, and the earthen floor without damp-proofing keeps the air humid. Day and night, summer and winter: the difference inside stays small.

In a racked warehouse it works differently. The casks at the top can reach around 20 degrees in summer, while it stays at 12 to 15 degrees at the bottom. The air is also drier up there. A cask that lies high therefore goes through a different maturation from a cask on the floor, within the same building.

In Scotland, roughly 1 to 2 percent of a cask's contents evaporates each year, and in the humid air of a dunnage warehouse it is mostly alcohol that is lost. That loss is called the angel's share.

Because more alcohol than water escapes from the cask, the alcohol percentage drops during maturation. In a dry environment the opposite happens: mainly water evaporates there and the percentage actually rises. Over a long maturation the volume loss adds up considerably. For a cask of twenty-five years, it amounts to roughly forty percent of the contents, and in warm countries annual evaporation can reach twelve or fifteen percent.

A dunnage warehouse gives whisky a style of its own, but the difference from a racked or palletised warehouse is in practice smaller than is often suggested. Trade journals take the view that the influence of the warehouse on the quality of the whisky remains an open question.

There are good reasons for that. Every cask is different, and the position of the cask in the warehouse counts for more than the type of building: high and dry matures differently from low and cool. Moreover, a blender combines casks from different warehouses to achieve a consistent style, so the distinction disappears in the bottle. The shop on the Brink in Bathmen stocks 3,898 whiskies, of which 2,783 are from Scotland and 983 carry an age statement of twelve years or older. The type of warehouse in which a bottle matured is rarely stated on the label.