Barrel Production

This article takes a close look at the barrel production of American Standard Barrels (ASB) for the production of Bourbon and Tennessee Whiskey. The end product will be an airtight 52 gallon (200 liter) charred barrel made of 31 to 33 staves with two lids (ends) made of American white oak (Quercus alba) held together by six steel hoops and twelve rivets.

The pictures have been taken at the Brown-Forman cooperage in Louisville Kentucky. A plant dedicated to produce barrels since 1945. Most of the barrels are produced for the Jack Daniel's distillery.

1. Raw Material

The wood for the barrels is American white oak from the middle to eastern United States and parts of East Canada. The raw material comes straight from sawmills around the country. A fresh cut tree has a moisture content of about 50%. This is far too much for the production of barrels. The lumber (boards) are dried to a residual moisture content of 12% in several steps. Some of the boards are matured for several month to change the flavour of the wood. In the picture below you see the colour difference between fresh and seasoned (air dried) wood. The seasoned wood contains far less tannins. These are responsible for the bitterness. Bourbon is very sweet and not bitter at all, so the staves have to rest for at least six month in the open air.

Stacks of American white oak
Stacks of American white oak

The next requirement is the quarter cut. This cut displayed in the picture below ensures that the orientation of the growth rings is in the right direction and the barrel has enough stability and leakproofness. This cut also divides the boards into wide and narrow pieces.

The Quarter Cut of American white oak
The Quarter Cut of american white oak

The next cut of the boards is into short and long pieces. The short pieces will become ends and the long will become staves from which the curvature of the cask is made.

2. Head Production

First of all the surfaces of the wood are planed. This brings back the lighter colour of the wood and also the barrel becomes more resistant against outside influences. Afterwards a machine drills holes into the sides of the planks and inserts wooden dowel pins on one side. Now you are left with wide and narrow pieces of wood that can be fitted together.

Pins in oak boards for the ends
Pins in oak boards for the ends

In the following step these boards are dowelled to a square shape of flat oak wood. This square is cut into a perfect circle with a rounded edge. This is the raw end of the barrel.

The raw ends of oak barrels
The raw ends of oak barrels

The heads then go into the oven and are charred under open flames for about 90 seconds. You will learn more about the charring of a barrel later in this article. The final step for the lids is the dipping of the edges into paraffin wax. The wax ensures the airtightness between the lid and the hull of the barrel. Finally the lids are stacked and transported to the next department.

3. The Staves

The long staves are also planed for colour and resistance against outside forces. Now they are milled into a very complex shape. The cross-section of the final stave is a trapezoid, because the barrel inside circumference is smaller than the outside. The top and bottom of the stave is narrower than the middle, because the barrel is wider in the middle than on the ends. The expert calls it a convex curve.

Milling of boards into barrel staves
Milling of boards into barrel staves

4. The Barrel Raising

This is the heartpiece of the barrel production. Every barrel raiser assembles 31 to 33 wide and narrow staves into a temporary steel ring that holds the staves into place. He or she has to make sure that the wide and narrow staves are distributed evenly around the circle otherwise the forces that hold the barrel together will also be uneven. In that case the areas with less pressure are likely to leak.

Raising of oak barrels
Raising of oak barrels

5. The Steaming

If you would just bend the barrel into shape, the staves would break and the barrel would be lost. So the raised barrels are put upside down with the wide end to the bottom. Then hot steam is blown trough the staves to make them flexible.

Steaming of raw barrels
Steaming of raw barrels

A worker can then put on the second temporary steel ring. This is the first time the barrel has its final shape. Another two thick steel rings are added and now it is heading for the heat treatment.

Bending of the staves into barrel shape
Bending of the staves into barrel shape

6. The Toasting

In front of the ring fixation picture you see the inside of a raw barrel. It is as bright as the outside of the barrel is. The barrels enter a toasting area where the inside of the barrel is heated to a specific temperature. The exact temperature, time and process is a secret as it is one of the most important factors in making a good Bourbon or Tennessee whiskey.

The wood has a sugar content to supply the different parts of the tree with nutrients. The best way to experience this is at a maple syrup harvest. The oak tree has a less sugar content, but also contains a substance called tyloses. This ensures the waterproofness of the barrel. The oak tree is the perfect combination of tyloses and sugar. By heating the barrel from the inside the sugar is liquefied and expands quite a lot. This forces the sugar to come to the surface and turns it into caramel also. These tasty caramel compounds are easily dissolved by the raw whiskey (alcohol) and therefore added to the flavour profile of the whiskey. The caramel is also the reason why the matured whiskey has its brownish colour. This is called additive maturation, because you add flavour to the whiskey. 

This toasting process is not required to make a Bourbon or Tennessee whiskey. Although most of the Bourbon are known for their sweet and caramel flavour profile and so most of them do toast the barrels. In the next picture you can see that the inside of the barrel is now a dark brown. They have been toasted.

Toasting the oak barrels
Toasting the oak barrels

7. Charring of the Barrels

During the charring of the barrels they are being burned from the inside with an open flame. This is done far hotter and shorter than the toasting. And while the toasting goes deep into the wood the charring only burns the very top of the wooden surface.

Charring of oak barrels
Charring of oak barrels

This charcoal layer acts as a filter. The charcoal reacts with the sharp substances of the white dog (raw whiskey). The sharp and unwanted flavours are taken out of the whiskey. The expert calls this subtractive maturation, because you take away unwanted flavours. The Bourbon and Tennessee whiskey laws require a charred barrel for maturation. But however there are different kind of chars. The intensity of the char is counted from one to four, with four being the highest. The number four char is often referred to as an alligator char, because the surface of the burned wood breaks and it looks like the rough skin of an alligator. The char is the reason for the smoothness of the Bourbon and Tennessee whiskey. Although the Tennessee whiskey still has a trick up its sleeve.

8. The Hoops

The hoops are formed in the iron department. They are cut from steel coils into three different lengths. These form the three different kind of hoops that a barrel has. First is the head hoop on the ends of the barrel. The quarter hoops are the next ones to the inside and then there are the two bilge hoops. They are the ones furthest to the inside.

Name Inch Centimeter
Bilge Hoop 79 200
Quater Hoop 75 190
Head Hoop 71 180

All hoops are riveted together. During the riveting process the cooperage presses their signature into the rivets. So everytime you see a barrel with BB on their rivets, you know it comes from Brown-Forman Barrel. The barrels with MO on their rivets come from a different company in Missouri.

The rivets on the hoops of the barrels
The rivets on the hoops of the barrels

9. The Hooper

Before you can continue with the production you have to cool the barrel down. During the cooling the barrel shrinks. If you would put on the hoops before you cool it, the barrel would shrink and the hoops would fall of, resulting in the collapse of the barrel. After the cooling the trenches for the lids are milled along the circumference and the prepared lids are joined with the rest of the barrel. Finally the temporary hoops are replaced by the real head hoops followed by the other hoops. They are pressed down by a machine called the hooper.

The hooper
The hooper

10. Bunghole and Check

The bunghole is the entrance and the exit for the whiskey. It is placed on the side of the barrel right in the middle between the heads. The bunghole has to be placed directly into the middle of a wide stave. This way you minimize the risk of breakage of the stave at the hole and leakage at the line between two staves near the hole. At the Brown-Forman company a laser is used to mark the point where the bunghole will be drilled.

Drilling of bungholes
Drilling of bungholes

Right after the drilling a gallon (3.7 liter) of water is filled into the barrel. It is then rotated so that the water touches all staves and the barrel becomes really wet in the inside. Now air pressure is added and you can see if there are any leakages. If there is a leak you will see bubbles of water forming at the leak. It is important to keep the barrel wet. If the barrel dries out the wood shrinks and the stability and airtightness of the barrel is in danger. This is why most of the distilleries keep a bit of water or whiskey in their empty barrels. Finally a temporary plastic bung is added and the barrel is off to the distilleries to mature whiskey.

11. Cooper Station

If a barrel does not pass the airtightness test it goes of the the cooper station. Here the most experienced workers of the plant repair the broken barrels. Heads are being replaced as a whole or the trenches are cut a bit deeper. If a stave is cracked they replace the individual stave. Small leaks are repaired and the cask is returned to the original testing station.

Repairing of a barrel
Repairing of a barrel