In the middle of the 20th century, whisky makers in Scotland found themselves in predicament. The barrels they’d depended on for decades for aging their whisky had more or less ceased to exist.

These treasured casks hailed from Spain’s Sherry region, shuttled north to quench the Brits’ notorious thirst for the wine. Once emptied of their contents, used barrels were often bought up by Scotch distillers for aging.

Sherry casks, distillers soon discovered, performed a quiet magic on aging whisky. It removed the burrs and rounded the sharper edges. It added depth of flavor. Mostly, this was due to sherry being absorbed deep into the wood during the six to nine months it typically spent in transit; the higher-proof whisky coaxed it out.

“Oloroso-matured whisky is much darker and much richer—caramel, toffee, raisins, sultanas,” says David Turner, distillery manager at Bowmore on Islay in Scotland. “That’s the kind of flavor you get off Spanish oloroso casks.”

The sherry trade worked well for the whisky industry. Whisky distillers got cheap casks; whisky lovers got great whisky. Then the 1950s happened.

By then, much in world had changed, both in commerce and consumer taste. Liquor producers no longer shipped in casks. Pallets and steel containers allowed wine and spirits producers to ship everything in bottles, ready for the shelf. Where bulk shipping persisted, containers of high-density polyethylene were on the horizon. Then Spain passed regulations requiring all sherry to be bottled on its shores.

Scotch distillers, who had grown dependent on sherry casks for their flavor profile, travelled to Spain to hunt up discarded casks. But even that proved untenable, as makers of sherry were loathe to give up their aging casks. Unlike shipping casks, aging casks were prized, each with its own biome following years of local yeast and microbes occupying the wood’s pores. A good sherry cask was like a well-seasoned cast iron frying pan—no one would give one up long as it were usable.

“Sherry casks virtually last forever,” says César Saldaña, director general of the Jerez Consejo Regulador, sherry’s governing trade council. “The older, the better.”

Compounding the problem for Scotch makers, sherry consumption plummeted in the mid-20th century. Regular table wines could be shipped more efficiently and under better conditions, which meant that fortified wines no longer held the edge for export. And tastes changed—the dense flavor of sherry fell out of favor as the age of vodka and light beer dawned. Sherry became the drink of musty dowagers and fusty garden clubs.

The Scotch industry was forced to revamp its barrel supply chain, and found a willing new partner in Kentucky. By law, bourbon must be aged in new charred oak barrels, and thus bourbon makers regularly sell off used barrels. Scotland lined lined up contracts to maintain a supply, and altered the flavor profile of Scotch as a result.

Still, distillers weren’t willing to give up sherry casks entirely. So they started off down several paths toward maintaining supply—some legitimate, and some not so much.

Among the not so much: blasting new casks with pressurized steam laced with ammonium hydroxide to leech out some of the tannins that would overpower the Scotch if not culled out. The cask would then be briefly seasoned with sherry.

Another cask conditioning shortcut involved something called “paxarette” (sometimes spelled “pajarete”). Originally a sweet sherry, this was boiled down to create a rich and flavorful concentrate to give depth to certain sherries. About a half-liter would be put into a cask, which was then subjected to about ten minutes of high pressure to force the additive into the oak’s pores. The residue was then flushed and the cask filled with Scotch. (The process was eventually banned by the Scotch Whisky Association, which deemed paxarette an additive.)

The more sustainable solution: Scotch makers got into the sherry cask business.

Consider The Macallan. The distillery has about 250,000 barrels aging at any given moment at its Speyside compound, mostly ex-sherry casks with some ex-bourbon. Their portfolio also includes a well-regarded line of Scotch aged exclusively in sherry casks, with expressions ranging from 12 to 40 years old.

To sustain this production, The Macallan currently imports an estimated 25,000 sherry casks per year from Spain. About 80 percent of those casks are used only once, and then passed along to sister distilleries owned by parent company Edrington, which includes Highland Park, The Famous Grouse and Cutty Sark, along with Brugal rum.

How does it source all those sherry casks? Since supply and demand for sherry casks has essentially flipped on its head over the past two centuries, The Macallan takes ownership of the casks from the outset. They purchase staves—some of American oak, some of Spanish oak—and contract to have them air-dried for two years. They’re then used to construct sherry casks, which are then essentially lent to Spanish sherry-producing bodegas for conditioning.

“They’re making the sherry and, in essence, we’re renting it—or borrowing it—from them to season our casks,” says Charlie Whitfield, manager of brand education for The Macallan. The brand specifies relatively dry oloroso sherry for their conditioning, and also decrees the optimal time—about two years on average before the casks are emptied and shipped to Scotland. “The bodegas love us, because they’re making money from us—as well as from the end result,” Whitfield says.

The aged sherry emptied from the casks is typically used to season the next batch of new whisky casks, although sometimes it’s distilled into brandy or used to make high-end sherry vinegar. Given the complexity of the process, a used sherry cask today costs about ten times as much as a used bourbon barrel, according to Whitfield. (Prices of used barrels have varied of late, but generally used bourbon barrels ordered in quantity can be had for under $100.)

Gordon Motion, master blender for Highland Park, says that the rising popularity of sherry is what has led his company into the business. “Because the supply of sherry could be a potential issue, we are sourcing our own sherry as well,” he says. “We now have control of the wine, the wood and seasoning time, so we have pretty much complete control from start to finish.” At Highland Park, about 43 to 45 percent of the first-fill barrels are sherry casks (butts or hogsheads), and virtually every bottle sold by Highland Park contains sherry-aged whisky. A lighter, sweeter flavor is what consumers expect when they buy a bottle of Highland Park.

That’s in contrast to Scotches noted for being heavier and peatier. “At Laphroaig, we are using bourbon casks predominantly because they highlight the smoke,” says Simon Brooking, Laphroaig Master Ambassador. “We love the smoke. Sherry casks generally tamp down the smoke.”

The sherry cask sourcing process is similar for Bowmore on Islay, says distillery manager David Turner, where about 20 to 25 percent of their aging stock is in sherry casks. Like The Macallan, they chiefly use casks conditioned with oloroso sherry but have experimented with fino and Pedro Ximénez, with solid results. (Commercially, this has included the Bowmore PX Cask Feis Ile.)

For two centuries, sherry has been Scotch’s silent partner, but the terms have certainly changed. While market might adapt to the flavor profile of a Scotch that’s been aged solely in bourbon barrels, few distillers seem to want to risk all to find out. “We like the traditional style of aging whisky in ex-sherry wine casks,” says The Macallan’s Whitfield. Which is good news for those who not only like the hint of sherry in their Scotch, but appreciate the incomparable taste of history.

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