The History and Science of the Barrel - Lesson 3: Physical Properties of Oak

Taking A Close Look At Oak

Regardless of the variety, oak wood is formed as cells in a layer called “cambium” are regenerated. The cells produced can be:

• Vessels designed to conduct sap (Xylem)

• Fibers to help support the weight of the tree or

• Storage receptacles (for water, waste, etc.)

Each year’s growing season (from Spring through Fall) generates a cambium layer comprised of an arrangement of all three cell types listed above. Together, these annual growth patterns form an annual “growth ring,” which is often identifiable by the naked eye.

Grain Coarseness

In the U.S.A., the number of annual growth rings per inch determine the coarseness (and quality) of the wood:

• Coarse oak wood contains less than 8 rings per linear inch

• Medium oak wood contains between 9 and 14 rings per linear inch

• Extra-fine oak wood contains more than 14 rings per linear inch

American vs. French Oak: A Matter of Tylose

Barrels made from French oak (Quercus pedunculata) have always been more expensive than their American oak (Quercus alba) counterparts, leading (or misleading) people into thinking that the former are better. The reason for the price difference is basic economics:

An average American oak tree will yield twice the number of barrels as a French oak tree of identical size. Why?

As a tree reaches maturity, it undergoes a process known as duraminisation, when heartwood is formed. During this transformation, “parenchymal cells located near the large sap-conductive vessels excrete a membranous protruberance, known as a tylose, which partially blocks the vessels1.” It is this obstruction which gives American oak its watertight characteristic, regardless of how the wood is milled, whereas the absence of tyloses in French oak result in the need to mill the wood in a more restrictive way to preserve its impermeability.

There are other differences between the two oak varieties. We will explore them in more detail in upcoming lessons.