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Whiskey making has a well-trodden transatlantic cycle. Americans age their bourbon in charred new oak barrels to extract flavor. They then ship those barrels across the ocean, where they enjoy a second life by being used to mature Irish whiskey and Scotch.

But lately, whiskey makers are bending time-honored, barrel-aging traditions. This cask-aging experimentation is resulting in a wave of new whiskey flavors, often coming from a secondary aging process in a different barrel than what was used to originally age the spirit. These used barrels can come from rum houses in the Caribbean, Spanish sherry, Mexico’s mezcal, and even Chardonnay and Merlot wine barrels.

This process, often known as “finishing,” can help a distillery stand apart in a crowded and competitive spirits industry, lure in new consumers with new flavors, and even change some old misconceptions. “Before the [whiskey] boom began, the interest from the media in whiskey was that the Scotch were innovative, and bourbon was not,” says Chris Morris, master distiller at Woodford Reserve. “Personally, that got me ticked off.”

In the late 1990s, Woodford Reserve began to experiment with finishing in barrels from woods made in Kentucky, but not just traditional oak. The bourbon brand began to use barrels made from cherry, maple, and ash. “We went crazy and that began the real experimentation,” says Morris. A Chardonnay-finished bourbon was the first to be released in 2006.

Woodford Reserve would use fully matured double-oaked bourbon that had sat in the new oak barrels for no less than five and half years for the secondary aging. The goal of the finishing is to augment the vanilla, caramel, and fruit notes that Woodford Reserve is already known for.

At Maker’s Mark, Bill Samuels Jr. spent six months putting just nine words on a paper to outline what he wanted his new expression to be. The first word was simply “yummy.” He also wanted bigger, bolder flavors and a longer finish. The result was Maker’s Mark 46.

It took two years and 125 experiments to land on the finalized process, taking Maker’s Mark and finishing it for nine weeks in a limestone cellar in barrels that have 10 seared virgin French oak staves. This process is almost unnecessarily complex and annoying, as French oak trees don’t yield as much as American oak and take twice as long to grow.

“The accountants weren’t happy, the warehouse guys weren’t happy, the marketers weren’t happy,” recalls Jane Bowie, director of innovation at Maker’s Mark. “It is a product I love because nothing about it makes sense, except for the way it tastes.”

Cask innovation is an ongoing commitment for both brands. Woodford Reserve last year debuted a bourbon that had been aged in cognac barrels and sold in a fancy decanter at a price of $1,500 per bottle. Three years earlier, Maker’s Mark launched the Private Select program, allowing barrel customizations for up to 1,001 different combinations depending on the use of five different finishing staves.

Jefferson’s Kentucky Straight Bourbon Whiskey Very Small Batch

The most famed experimental offering from Jefferson’s Bourbon is the distillery’s ocean series, in which hundreds of barrels of bourbon are put on boats that visit five continents on an average trip. The initial experiment resulted in a thicker, darker spirit that tastes of caramel, like a dark rum with briny notes from the sea. Jefferson’s has launched 20 bourbon voyages to date.

“There are some purists who don’t like what I do,” admits Jefferson’s founder and chief strategist Trey Zoeller. “Distilling is a science. Maturation is where it gets fun.”

The Irish are emerging as risk takers. Teeling Whiskey Distillery has 70 different maturation projects in its warehouse with some releases finished in rum or brandy barrels, as well as French oak barrels that had previously been used by a Finnish rye gin company. Slane Irish Whiskey ages in three casks: American virgin and seasoned casks and Spanish sherry casks. As a result, Slane’s Triple Casked has vanilla and spice notes, as well as ripe banana, Demerara sugar, and a bit of dried fruits and baking spice.

Method and Madness’s entire range comes with a twist. Expressions are finished in oak and chestnut from Spain or France. Last year the distillery, which is owned by Pernod Ricard’s Irish Distillers, released limited-edition bottles made with finishing from wild cherrywood and acacia wood.

“When you are playing with innovative casks, you don’t want to kill the underlying whiskey flavor,” says Jack Teeling, founder of the Teeling Distillery. “You want to find something that complements it.”

Of course, there are plenty of cask-aging misfires. Secondary casks can overpower perfectly good whiskey or result in undesirable flavor combinations. Woodford Reserve had high hopes for sassafras, but aging in the wood resulted in a weirdly minty-tasting whiskey. Method and Madness once used Tokaji casks that were too dry and had a slight sulfur note. Jefferson’s put bourbon in used Tabasco barrels, but the resulting flavors scorched the mouth, while Teeling’s use of trees from the Brazilian rain forest purportedly resulted in a drink that “tasted like a piano.”

“Some woods just aren’t suitable, they can leak. Or the whiskey just doesn’t taste good,” says Brendan Buckley, a marketing director at Pernod Ricard, which owns Method and Madness.

Seeing where the winds were blowing, the Scotch Whisky Association last year relaxed rules to allow the nation’s distillers more flexibility in the barrels they could use for finishing. The move comes as sales for Scotch whisky have faced challenges of late while demand for whiskey from America and Ireland soars.

“It has been incredibly exciting. People are looking for interesting, new, and unexpected flavors,” says Zeenah Vilcassim, global brand director for marketing at Dewar’s.

Anticipating the change from the industry’s trade organization, Dewar’s two years ago began to experiment with new barrel-aging processes it couldn’t explore previously. It has already released two new expressions under the company’s cask series, using rum and mezcal casks.

Cooper & Thief Sauvignon Blanc is full of aromatic fruit flavor, with citrus and spice notes reminiscent of a fine blue agave tequila, giving way to a toasty vanilla from the barrels.

Many barrel handoffs are between sibling brands owned by the liquor industry giants. Slane gets barrels from a Kentucky cooperage owned by Jack Daniel’s maker Brown-Forman. Dewar’s uses Bacardi and Ilegal Mezcal barrels. Corona beermaker Constellation Brands ships Casa Noble tequila and High West Distillery rye whiskey barrels to wine brand Cooper & Thief. Stateside, wine sales have softened of late, so aging in whiskey barrels can give a glass of the brand’s Napa Valley Cabernet an extra hint of sweet caramel and spiciness that consumers are gravitating toward.

Cooper & Thief cellar master Chris Leamy says six months of extra aging is the limit for spirits-barrel-aged wines. He warns against overdoing it.

“You don’t want to overwhelm this wine with the spirit,” Leamy says. “The whiskey will always win if you go too hard.”