It is over 80 years since the end of Prohibition in the USA in 1933. Since that time the use of American ex-Bourbon barrels has changed the taste of Scotch whisky.

Casks have been used since time immemorial for storage and movement of goods. Coopering is an ancient skill depicted in paintings on ancient Egyptian tombs, mentioned by Greek writers, referred to in the Bible several times (1 Kings 18:33 “Fill four barrels with water.”) The Roman, Strabo, writing in AD 21 actually praises the Celts as being ‘particularly fine coopers’.

Cooper is said to derive from Cupa, the Latin for a vessel. There were three grades of coopering skill – dry, dry-tight and wet – depending on the suitability of a cask for its purpose. Wet coopers were the most skilled, being able to make casks of such precision that they could hold liquids without leaking. The whisky world alone needs around 3 million ‘wet coopered’ oak casks a year for whisky – and the UK could never satisfy that level of rapacious demand having cut down most of its oak trees by the early nineteenth century in building Royal Navy warships to fight the French.

Renowned for their short arms and deep pockets, Scottish distillers grew to rely on second-hand casks. From the seventeenth century onwards various alliances, treaties and wars, meant the fortified wines (suitable for long distance travel) of Sherry, Port and Madeira were particularly popular in Britain. Shipped from Iberia in oak casks, the wines were bottled at the port of entry (Edinburgh, Glasgow, London, Bristol, Liverpool) and the redundant vessels re-used by grateful distillers.

It didn’t take long for distillers to realize that the original cask contents – the dark, sweet, fortified wines – could beneficially mellow maturing whisky. Badly distilled whisky could be disguised, young whisky made to seem older. These prized casks were hand-coopered from European oak, Quercus robar, found across the mid latitudes of Europe. And the French used it too, for wine, but also for warships.

It took 3,400 oak trees to build one 74 gun ship-of-the- line. The strain on 17th century French forests was considerable. In 1669 Louis XIV’s finance minister, Jean-Baptiste Colbert, did what the British should have done; he expanded and reorganized France’s woodland, planting only the best and strongest oak suitable for building a navy to fight the British. This legacy was built on by Napoleon, who banned all tree felling (without authority) in 1803, and three years later decreed that only trees more than 150 years old could be felled, and replaced with a new oak. Thanks to this far-sighted forest management policy (and steel-plated warships and entente cordiale) France’s forests now produce the world’s finest oak destined for more culturally acceptable uses than warships – the planet’s greatest winemakers.

In 1920, George Saintsbury, the celebrated writer and gastronome wrote: “I have noticed, in the forty-five years since I began to study whisky, that the general style of most if not all kinds has changed… The older whiskies were darker in colour, from being kept in golden Sherry or Madeira casks, rather sweeter in taste, and rather heavier in texture; the newer are lighter in both the first and the last aspect, and much drier in taste.”

This observation heralded the changeover from European to American oak with the introduction of Bourbon barrels – with their tyloses.

The Bourbon barrel is made from Quercus alba or white oak, commonly known as American oak. It is accepted practice to use trees that are over 90 years old. The cellular structure contains bubble-like cell structures – the tyloses – that bulge into the cavities of the xylem, the tube of moisture-conducting cells, blocking water movement. These tyloses make the wood particularly watertight, even with thinner staves, and perfect for mechanized barrel-making. The dominance of hand coopered European oak casks lasted until the beginning of the twentieth century before the United States took over with machine-produced casks on a scale never before seen.

Bourbon distillers deliberately sought very dark colours, high levels of vanilla and caramel flavours that maize-distilled spirit draws out from heavily charred, newly felled, white oak barrels. After prohibition, in 1935, this long established, exclusively new oak custom was made federal law: thanks to the powerful Coopers Union the law now says that bourbon casks can be used once only. Being cheap and readily available they were eagerly snapped up by voracious Scottish distillers.

The rapid escalation of the use of Bourbon barrels coincided with the weakening in popularity of Sherry, Port and Madeira. The use of Sherry butts fell further with the outlawing of bulk shipments from Spain to the UK in 1981. Today, around 97% of all Scotch whisky is maturing in American oak.

As ex-bourbon cask prices rise owing to the increasing demands of the Scotch whisky industry, the price differential between second-hand and virgin American oak barrels has fallen. This may mean that in future distillers could be obliged to use new oak casks, increasing the potential for the ‘Bourbonisation’ of Scotch whisky. We must remain vigilant in the face of that, and also reject the alternatives of using old tired wood, or allowing oak chips, or essence, both of which are happily currently illegal.

Or could premium quality French oak be back on the menu again as it was in the nineteenth century? As far as whisky is concerned, if the nineteenth century was Europe’s era for oak, then like world history, the twentieth century has belonged to America. American oak, with its simpler, vanilla and caramel influences, has been a benign force for good.

The history and politics of Bourbon has been mutually beneficial for both Bourbon and Scotch distillers. A very positive relationship between two of the world’s greatest spirit styles.