Coopers Craft Countless Bourbon Barrels A Year By Hand. Here's How It's Done

Today’s coopers use modern tools to craft bourbon barrels, but the process hasn’t changed much through the years. Each barrel is assembled by hand over the span of about two hours, and held together using carefully applied pressure and precise geometry—all without nails, glue, or other adhesives. Here’s a look at how it all happens.

The Birth of A Bourbon Barrel

1. Gathering the Wood

American white oak is the most commonly used barrel wood, due to its superior water retention. The trees are typically felled when 80–100 years old and 70 to 100 feet tall, and an average of one to two barrels can be produced from each tree. Logs are cut into three-foot sections (roughly the height of 53-gallon barrels), then sawn into four quarters and cut into rough staves. The staves are stacked onto pallets and left outside to air dry for an average of three to six months. Some distillers request longer seasoning periods of up to 36 months.

2. Shape It Up

Coopers run the seasoned staves through machinery to trim them to a length of around 35 inches. Brown-Forman Cooperage in Louisville has a particularly advanced setup that automates the entire process. But because they’re quarter-sawn the staves don’t all have the same width, which makes assembly like solving a jigsaw puzzle. Coopers start by adding staves one by one, inserting them into a metal ring used as a temporary base, selecting ones that will fit together precisely. Once the fit is tight (it usually takes 25–32 staves), a second ring is placed around the top to hold things in place for the next step.

3. Steaming Into Shape

The staves are then sent through a steam tunnel which heats them to around 120°F so they can be bent into the barrel shape, usually using an overhead inverted cone that presses down on the staves and replaces the ring with a temporary hoop that fits more tightly. Coopers add more of these “construction rings” until the final hoops are added later. After adding those rings, coopers set the shape by heating the barrels again.

4. Toasting and Charring Time

Now the barrels are ready to be toasted and/ or charred, which will have a major impact on the flavors drawn out of the barrel. Gas ovens, wood fires, or electrical tubes produce temperatures between 250° and 550°, toasting the barrel for 9 to 40 minutes. Duration and temperature are up to the distiller, and some skip toasting entirely. Next comes charring, which is required by law. Gas-fed fires burn the inside of the barrel, creating a layer of carbon and breaking down compounds like lignin, tannins, and cellulose that contribute key flavors. The char levels are numbered 1 to 4: No. 1 (up to 15 seconds), No. 2 (20–30 seconds), No. 3 (30–35 seconds), or No. 4 (up to 60 seconds.)

5. Put a Head on It

After charring, coopers cut notches around the inside edge of the staves at both ends of the barrel. This “V” or “U” shaped cut is called the croze, and it’s where the barrel heads rest. Once each head is popped in, coopers remove the temporary construction rings, replacing them with “head hoops”—the first two of the six hoops to be applied. The heads are pressed down and the hoops are tightened, which clamps the croze down on the heads, creating a tight seal before being riveted in place.

6. Send in the Hooper

Setting the heads in the previous step required pressure, which can heat the barrels up again. To prevent the wood from becoming pliable and potentially deforming, coopers let the barrels cool off for a few minutes before the hoopers add the remaining four hoops. The innermost two hoops are called the bilge or belly hoops, while the quarter hoops sit between those and the head hoops. These four hoops are also riveted, closing the joints in the middle of the barrel. Once the hoops are in place, coopers drill the bunghole— a hole in a single stave at the center of the barrel’s body—through which liquid can be added or removed. The bunghole will be plugged with a piece of wood called a bung, which is like a stopper or cork. Now the barrel is essentially complete.

7. Pressure Testing the Finished Barrel

Next, the coopers put the barrel to a final pressure test. About a gallon or two of water is added to the barrel along with five to nine pounds per square inch of air pressure. The idea is to use that pressure to detect any latent defects in the wood—like wormholes, knots, or mineral streaks—which can lead to leaks. Smaller defects can be plugged with cedar wedges. A faulty stave may require the barrel to be disassembled and the stave replaced—a process coopers usually can accomplish quickly. The new staves must be cut exactly to replicate the ones they replace, from the length and width to the depth and precise location of the croze cuts, to ensure the fit will be perfect. Once a barrel passes the test, it’s sent off to a distillery where it’s ready to accept whiskey.