Whisky bond

A dunnage warehouse is the traditional type of warehouse used for maturing whisky in Scotland and some are still in use today. Dunnage warehouses are often on site at the distillery, or close nearby, and generally have a small capacity.^[2] 

Dunnage warehouses are low in height with thick walls and an earth floor – the walls are traditionally stone, however some later warehouses are brick. They often have a slate tiled roof. The size of dunnage warehouses means that casks cannot be stacked more than three high and are laid on their side; this ensures good air circulation and an even maturation.^[3]

The earthen floor results in a humid atmosphere, and combined with the thick walls the humidity and temperature remain fairly consistent due to their insulating properties.

As dunnage warehouses are small and have low ceilings, machinery cannot be used to manage the casks. This increases the running costs of dunnage warehouses.

Dunnage warehouses are often the most familiar type of warehouse due to the fact that they are often located on distillery grounds and close to the stillhouse and are often contemporary to the construction of the distillery.^[4]

As the whisky market grew and demand increased, dunnage warehouses were no longer sufficient to house the volume of whisky being produced. Racked warehouses are a response to this demand and are a modern style of warehouse that are much larger in size.

Racked warehouses have thinner walls and tin roofs, and are often much larger than dunnage warehouses. This, combined with the concrete flooring, allows casks to be stacked up to 12 high on their side on racks without the weight of the casks causing a problem.

The main problem with racked warehouses comes with the temperature and humidity regulation. The concrete floors of racked warehouses reduce the humidity compared to the earthen floor of a dunnage warehouse.

The concrete floors and thin walls allow a seasonal change in temperature in a racked warehouse which results in slight variations in the volume of the whisky during the course of a year. The expansion and contraction of the whisky in the cask can speed up the maturation process due to increases in contact with the wood of the cask.^[5]  

The concrete floor of a racked warehouse also reduces the humidity which can speed up the maturation process.  This can be a problem as the whisky may not have enough time in the cask to draw all the flavours from the wood.

In low-humidity environments, the rate of evaporation is higher for water than for alcohol, resulting in a smaller angel's share, with the alcohol strength actually rising in some cases.^[6]

The variations in temperature and humidity in a racked warehouse can result in unpredictable and uneven maturation. This is because casks at the bottom of a rack will be more humid and cooler in comparison to casks at the top which will be hotter but drier.

Racked warehouses are able to utilise machinery such as forklifts to move the casks, making the management of casks much less labour intensive. This reduces the costs of the warehouse and can make the end product cheaper too.

A pallet warehouse is a similar modern construction to the racked warehouse with one considerable difference: the casks are stored upright rather than on their side.

Evaporation rates within palletised casks are often much higher than dunnage or racked warehouses.  So much so that a Dundee based company released a product called "Scotch Bonnet" to sit on the top of the casks in an effort to reduce evaporation.^[7]

Due to the structure of a pallet warehouse, being similar to that of a racked warehouse, and the fact that the casks are upright, a greater number of casks can be stored in a single warehouse, reducing costs.