Amongst woodworking, the cooper’s craft stands unique. The ability to turn wood into a waterproof vessel, capable of holding water or other liquids, without the use of any adhesives, requires extreme accuracy and excellent workmanship. Yet in times past, this was done with hand tools, working mostly by eye. Today, the art of the cooper still remains, as those wood barrels are used in the making of whiskey and wine.
The most common wood for making barrels is white oak, specifically American white oak, although the French also make oak barrels, using their own oak trees. White oak contains tilos, which helps seal the grain in the wood. While white oak is the preferred wood, red oak, acacia, hickory, maple, redwood, walnut, cherry and chestnut are also used. White oak is preferred for the flavor it infuses into the liquor that is being aged in it. Bourbon whiskey, in particular, must be aged in new oak barrels to be called bourbon. In contrast, other types of whiskey can be aged in barrels that have been reused. Even then, the lifespan of a barrel is only 8 fills, after which they need to be replaced.
Although whiskey and wine are the main spirits aged in oak barrels, they aren’t the only ones. Scotch whiskey, Irish whiskey, Canadian whiskey, rum, reposado, anejo tequila, brandy and even beer can be aged in barrels, either made of white oak, or one of the other hardwoods mentioned above.
The cost of these barrels obviously adds to the cost of the spirits that are aged in them, especially when we take into account that a new barrel can cost anywhere from $900 to $2,000. Those each hold 53 to 55 gallons, so at a minimum they add $3.27 to the cost of a bottle of bourbon or $0.41 to the cost of a bottle of wine, assuming the wine is aged in a barrel; not all wine is.
Today, there are somewhere between 1,500 and 2,000 coopers in the world. There is no school in the United States which teaches this craft, so one needs to apprentice at one of the existing cooperies to learn it. Coopers are notoriously secretive about their craft, in an attempt to avoid adding to the competition. But in Europe there are still colleges teaching coopery, which requires a three year course of study, after which there are still three years of apprenticeship before one can be considered to be a journeyman cooper.
Modern coopery has changed from what it once was, in that modern machinery has been brought into the craft. Whereas a cooper once had to shape the staves totally by hand, today’s cooperies use specialized machinery to accomplish those tasks. Even so, the process of fitting and assembling the barrel is still done by hand, although final fitting of the hoops is done by a huge hydraulic press.
Making Barrel Staves
A barrel is primarily a bunch of staves, held together. The staves must be precisely formed, so as to eliminate the possibility of leaks. standard barrel size is 21” at the ends and 26” in the middle, by 36” high. To make this, 31 to 33 staves are used, not all the same width. Selecting a set of staves which will fit in the bands, giving the desired barrel diameter is part of the process.
The stave is rather complicated, geometrically speaking. It is rounded on both the outside and the inside, across the board’s width. It is also tapered towards both ends, with the center of the stave being the widest part. At the same time, the sides of the stave have to be cut at an angle, so that the staves will sit snugly against one another to seal.
It may seem only like a peculiarity of the design, but the curved profile of the barrel is critical to it sealing properly. By tapering the staves and then forcing them together and holding them together with hoops, it provides the pressure necessary to cause the staves to seal against each other. If the staves were straight, rather than tapered, it would be impossible to generate the pressure needed for the oak staves to create the necessary seal to make the barrel water-tight.
In olden times, the cooper cut, or more accurately split, their barrel staves from trunks of oak trees they felled themselves. The wood was split in a quarter-sawn pattern, which is why the staves vary in width. Rough shaping of the stave was accomplished with a special axe, known as the “cooper’s side axe”, followed by a drawknife to finalize the shape. Those aren’t what anyone would call precision tools, yet the cooper used them to produce precision work.
The oak that most cooperies use today comes from a mill and is aged for somewhere between 24 and 36 months. It is not kiln dried, but rather dried in the open air. One coopery refers to the wood as “export grade,” meaning that it is pure heartwood and has no sap or knots whatsoever. That’s important, both from the sense of flavor and that leaks can cause the barrel to leak. When selecting wood for making a barrel at home, it is essential that it be the best, clearest wood possible.
Fortunately for us today, at least some of the work for shaping the staves can be done with power tools, rather than having to do them by hand. However, some steps require rather specialized equipment to do them with power tools. For the sake of this article, I’m going to stick to tools that can be found in a woodworker’s shop.
Individual staves must cut to exactly the same length. The length is critical for making a straight barrel. If staves vary in length at all, it is likely that the barrel will try to imitate the Leaning Tower of Pizza during assembly. For our purposes, the staves should also be exactly the same width, although they weren’t in olden times, nor are they in modern cooperies. To calculate the width of the staves needed for the barrel, divide the circumference of the barrel by the number of staves. Then rip all the staves to that width.
After planning the boards to 1” thick, the inside of the staves need to be rounded convex. This is one of those steps which would be done with equipment that isn’t found in a home workshop, either a shaper or a molding cutter, so we’re limited to doing this with a radius spokeshave. The curvature doesn’t have to be exact, as nobody will see it; but the concave surface is necessary to ensure the barrel’s volume when completed. The outside of the staves is then shaped convex with a straight spokeshave. It doesn’t have to be shaped perfectly, because the finished barrel will be sanded to get the outside smooth, but the closer it is to the right curvature, the easier it will be to fit the barrel together and fit the hoops over the ends.
Tapering the staves and angling the cut is actually something that is fairly easy to do in the home workshop, as long as a router table is available. That’s going to take the place of a specialized CNC joiner. Start by making a template for the curvature of the staves, which will form the desired barrel profile. This is one of those trade secrets that coopers don’t share, but the angle of the taper is only about six degrees. In addition to the template and a straight bit with a bearing, it will be necessary to create a jig which alters the angle of the table.
So what angle is needed for the mating edges of the staves? That depends on the number of staves. If we divide 360 degrees by 33 barrel staves, we get 10.9 degrees. If we weren’t using the same width staves, the wider staves would need to be at a slightly greater angle, while the narrower ones would need to be at a lesser angle. But for simplicity sake, since we’re using all the same width staves, we can stick with one angle.
When attaching the template to the staves and routing the edges to taper them and cut the edges at an angle, take extreme care to not cut any material from the middle of the staves. That width has already been calculated and must be maintained. This is the single most critical part of the process.
Assembling the Staves into a Barrel
Once the staves have been cut, it’s time to assemble the barrel. This is still something that is done by hand in modern cooperies, using much the same tools and techniques as have always been used. Other than not having hydraulic presses to help us, we will be using the same techniques.
Coopers normally have a set of working hoops for every size barrel or cask they build. These differ from the hoops that become a permanent part of the barrel in that they are welded, rather than riveted together. End welding eliminates the step in the circle and the rivet heads that are part and parcel with a riveted hoop. The smoother inside makes it easier to assemble the barrel. Once the barrel is assembled, these hoops are removed and replaced with riveted ones.
Lay out the staves, side by side, measuring to ensure that the overall width matches the desired circumference of the middle of the barrel. At this point, commercial barrel makers use a jig to hold the end hoop and the staves, as they are setting them in place. The jig is like a round metal basket, allowing all the staves to be set into position and held there. Once in place, a strap clamp is placed around the loose end to hold it, while the barrel is removed from the jig.
It is still possible to assemble the staves, without that jig. Rather than placing the end hoop on the ground, it is held in one hand, at what will be the top end of the barrel, while the staves are put in place with the other hand. Once all the staves are put in place, a second, larger hoop is driven farther down the staves, pushing them together and shaping that end of the barrel into its final form. It is necessary to hammer the hoops down the sides of the barrel, using a weighted hammer and a hoop driver, somewhat similar to a large cold chisel that has been ground blunt, to press the edges of the staves together.
As part of this process, the tops of the staves are hammered too, causing them to sit against the floor and evening up the barrel. At the same time, it is important for the cooper to hit the inside of any boards that are inset from the hoops, rather than pushing out against them. During this process, it is not uncommon for the barrel to start out leaning before this process, but as the tops of the staves are hammered and the hoops are driven down the barrel, the barrel straightens up.
This is where the trained eye of an experienced cooper makes a difference, as they will see which staves are problems and know how to adjust for them. Ultimately, all the staves must be seated properly against each other, with the ends exactly even, for the barrel to seal.
While bringing the staves together on the first side of the barrel is fairly easy, that can’t be said about the second side. Because the staves are all tapered, the circle made by the ends of the barrel that haven’t been worked on has a much larger diameter than the middle of the barrel. In order to bend the staves together for that end of the barrel, heat is needed.
heat bending is common in some sorts of woodworking, such as making string instruments. The curved sides of a guitar body, for example, are curved with heat. Heat is also used in some bentwood operations. The big difference here is that the entire barrel needs to be heated at the same time, as all the staves need to be bent at the same time. This is accomplished by putting a gas burner inside the barrel. As best as I can tell, there is no such device available commercially and the owner of one coopery stated in a video that those used by cooperies are all custom made. Probably the closest thing that we can find on the open market would be a gas burner for cooking or smelting, perhaps the gas burners that are used for deep frying turkeys. Some coopers use water during this process, either dipping the barrel in the water or spraying water on it, while others do not.
The barrel is placed over the heater, so that the heater is inside it and left to heat until the outside temperature of the barrel is 300°F. Once that temperature is reached, a band clamp is placed around it and used to pull the staves together. This is typically a band clamp using steel cable, rather than nylon webbing, but I believe a band clamp with a nylon strap will work. The melting temperature of nylon is 515.9°F, considerably higher than the temperature the wood is warmed up to.
Once the barrel is warm enough and the strap clamp is applied to bring the ends of the staves together, the end hoop is put on that end of the barrel. With that hoop in place, the strap clamp can be removed and the other band put in place, hammering it down to compress the staves together, just as was done on the first end. The only difference is that the ends of the staves do not need to be hit to even them up, as they should already be even.
The assembled staves of the barrel are then put back over the burner to “toast” them. The easiest way of thinking of this is just like smoking a piece of toast at home. Toasting the wood actually caramelizes it, breaking down the hemicellulose in the wood to convert it into sugars which will be absorbed into the spirits being aged in the barrel. If the barrel is going to be used for whiskey, the inside of the barrel is then charred, converting the inner surface to charcoal. This layer of charcoal acts as a filter, removing impurities from the fermenting whiskey. Wine barrels are not charred, only toasted.
Once the toasting and charring is completed, the barrel must be set aside to cool. Further work at this point will provide the staves an opportunity to lose their curve. Only when cooled will it be safe to remove the hoops and continue working on the barrel.
Adding the Barrel Heads and Finishing the Barrel
Caution: It is dangerous to remove all the hoops at one time, as the barrel will fall apart. Any work done from this point on, where I mention removing hoops, only refers to removing one or at the most two hoops (out of four) at a time.
The outside of the barrel needs to be sanded smooth, so as to ensure that the hoops make good contact all the way around their inner perimeter. This is accomplished by removing individual hoops and sanding; usually with a belt sander. As one part of the barrel is finished, the hoop is reinstalled and tightened back up before another hoop is removed.
A groove, called a croze, needs to be cut into the inside of the barrel, on both sides, for the barrel head to sit into. In olden times, this was a two-step process done with special planes. The first step in the process was to flatten the inside of the ends of the barrel, thinning them out slightly. The plane used for this is called a chiv or howell. Once that is in place, a tool called a croze is used to cut the slot, which is also referred to as a croze. Today, we can do that with a router, using a straight bit with a guide on the outside of the barrel in place of the chiv and a slot cutting bit with a bearing in place of the croze. It would also help to use a roundover bit to radius the inner edge of the barrel end, making it easier to insert the barrel head.
The barrel heads are made of edge-joined boards, ¾” thick; but they are not edge glued. Just as glue must be avoided for the staves of the barrel, so that they don’t get into the contents, glue must be avoided for the heads. So the boards must either be doweled together or they must be edge routed, in a pattern that will provide a tight, interlocking joint. One coopery in the country uses what is known as a “double z groove” to join the boards together.
When cutting out the barrel head, the outside diameter must fit exactly in the croze, allowing the barrel staves to press in on the head, pushing the edge joints together. At the same time, the barrel head cannot be oversize, as it will push out against the staves, causing gaps for the contents to leak out of. The edges of the barrel head are also tapered, something else that can be done on the router table, so that they end up the exact width as the croze, ensuring a watertight fit. The inside of the barrel head should be toasted and/or charred as needed to match the rest of the barrel. This can be done over an open fire.
With the barrel heads made and toasted and/or charred, the hoops are removed from one end of the barrel and the head tapped lightly into place. The actual hoops that will be part of the barrel are then installed on that end, hammering them into place. In commercial operations, the bands are pressed into place with a huge, custom-built press.
Compared to all the precision that goes into the making of the barrel, the hoops have more leeway in their cutting and manufacture than anything else. If a hoop is 1/8” too long, it really won’t affect the function of the barrel, it will just need to be pounded a bit farther down the barrel. Nevertheless, commercial cooperies use as much precision in making their hoops as anything else, so that their barrels will have a consistent appearance. Once the hoops are in place, tacks are driven through them to help keep them from slipping.
The hoop is a band of steel strapping. The edges need to be deburred or slightly rounded to make them safe for handling. Four holes are drilled in the strap, one pair at each end. Then the strap is bent. This can either be done by hammering it over a round arbor (or round pipe held in a vice) or with a roller bender. The holes are used for riveting the ends of the hoops together, with hammered rivets (not pop rivets).
With the new hoops installed, the barrel is ready for the final step in the process. A 1-7/8” to 2” bung hole is drilled in the side. This is different from plastic or metal barrels, which typically have the bung hole in the head. The bung hole is seared with a hot iron, sealing the end grain in the wood, so that the contents of the barrel won’t soak into the end grain.
The finished barrel needs to have some water put into it to test it for integrity. If not filled immediately with spirits, some water should be kept inside, to prevent the wood from drying out. If the wood were to dry out, it would shrink, possibly resulting in a leaky barrel. However, if that were to happen, the problem could be solved by filling the barrel with water and allowing the wood to soak up the water and swell.