Density is a ratio of the mass to volume of a quantity of substance. It is expressed in terms of a mass per unit of volume. Since wood swells or shrinks, depending on how much water is absorbed in the structure, density for wood can be determined in several ways. The manner in which the density is determined should be recorded next to the units. The following definitions are given by the TAPPI TEST Methods 1991.

As-Received density: Simply determine the mass and volume of the material as received

Green density: Take the maximum swollen weight and divide it by the maximum swollen volume (Presoak the wood before measuring either the mass or volume)

Basic Density: Oven dried weight divided by the swollen maximum volume

Oven-dry density: Determine both the mass and volume from the oven-dried matter

Bulk density: Take the mass and the volume of the material at specified temperature and moisture content conditions

Swollen: Refers to the woody material being submerged in water for at least an hour and usually overnight.

Oven Dried: The stems or green wood are placed in an oven at 105 degrees Celsius until the mass is constant.

Note: 1 cm3 of water is equivalent to 1gram of water. When the wood disk or stem is submerged a certain amount of water is displaced. The amount of water displaced is recorded in grams by the triple beam balance. The conversion to volume is easily completed because 1 cm3 of displaced water is equivalent to 1 gram mass. The set up measures the mass of water displacement and not the mass of the wooden dowel submerged.

What value is there in knowing the density of wood?
The density of wood help determine the amount of energy a certain volume of wood will provide. The denser the willow clone the less volume of wood needed to provide a certain amount of energy. The lower the bulk need, the lower the cost for storage or transport etc. This information can help guide the process in selecting willow clones that are denser and have higher energy yields per volume of material used.

The chart below provides density values for several species of wood. You can check your student's results from lab 3 with these values. For an extended lab activity give each student team a copy of the chart below and have them calculate their percent error using their results.