Crafts of the Cooperage #2 - The Bending Operation

Bending is the keystone of crafting an oak cask, and it takes place just after the raising (“mise en rose”). During this stage of production, the cask takes on its definitive shape thanks to mechanical force combined with the joint action of heat and water. The hoops fitted during bending hold the whole structure together, thus ensuring the strength and watertightness of the shell.

 

There are several bending techniques used in our cooperages: traditional bending over a wood fire, immersion bending*, and steam bending**. Whatever the method, bending requires that the oak be both heated and moistened in order to take its final form. Heat softens the oak, making it easier to bend, while moistening the staves, both inside and out, helps the heat penetrate the oak fibers. Gradually, mechanical force brings the ends of the staves closer together until the cask takes on its characteristic elongated shape, with the bulge—known as the “bouge”—at the widest part of the cask.

 

TRADITIONAL WOOD-FIRE BENDING Like the cask over the fire Heat is provided by a brazier (or heater) placed inside the raised staves, fueled with oak offcuts. Moisture is maintained by spraying the inside and outside of the cask with a water gun. Beyond time management, it is the cooper’s eye and touch that set the rhythm and determine how long the bending must last. On average, this step takes around twenty minutes, during which the cooper monitors the temperature by hand and ensures the staves remain moistened.

 

Tightening with the capstan This is the cooper’s other essential tool, used to marry the staves together. Softened by fire and water, the staves are bent at the base with the help of a capstan. The cooper encircles the base of the cask with a steel cable attached to the capstan, then works the lever to gradually tighten the staves and give the cask its final shape. Steel molding hoops consolidate the structure before moving on to the next stage: firing, also known as aromatic heating, toasting, or bousinage.

 

*IMMERSION BENDING This pre-heating technique for bending is perhaps the least commonly used in the cooperage world. At our Brive cooperage, Saury developed this unique process of pre-heating casks by immersion. The raised staves are plunged into a bath of hot water. The water penetrates the wood, eliminating the hardest tannins. The process softens the oak in preparation for bending.

 

**STEAM BENDING After raising, the casks are placed in a steam tunnel. This method is used exclusively by Tonnellerie Berthomieu / Ermitage. Steam penetrates deep into the oak and dilates its pores. The aromatic heating that follows thus penetrates very effectively. Passing through steam also helps remove the last of the harshest tannins (a 2014 study by the Exact Laboratory demonstrated a 94% reduction in the quantity of ellagitannins).

 

After the bending process, the cask moves to a second heating station for the key stage of bousinage, during which the aromas of the oak are revealed.