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Many materials are described in terms of relative density or specific weight. This is the ratio of the mass of a certain volume of that substance to the mass of the same volume of water. However, this quantity is not suitable for wood because of the large variation in moisture content, with the result that the mass of wood varies and the volume of wood is not always constant. With wood, we prefer to speak of volumetric mass.
Wood Moisture Content
This depends on the moisture content. When moisture content changes below the fiber saturation point, both the volume and the mass change. The volumetric mass also changes as a result. At moisture contents above the fiber saturation point, only the mass changes, the dimensions have become maximum and thus the volume. The density is in practice given at a specific moisture content of, for example, 10%. The density of wood differs greatly between the wood species and within wood species: significant differences can be observed even within one tree.
Year Rings and Volumetric Mass
An old view that is still almost common within the European timber trade is that for European wood species narrow annual rings indicate relatively heavy wood for coniferous wood species and that broad rings indicate heavy wood for hardwood species. However, there are many exceptions to the rule, so that the growth ring width cannot be regarded as an absolute measure of the density, and thus of the strength.
Influence of Volumetric Mass
The density is influenced by the age at which the wood was formed in the tree and the location of the wood in the trunk can also have some influence on this. Especially with coniferous wood, the ratio in the amount of relatively thin-walled spring wood to the relatively thick-walled summer wood that is formed plays an important role. Finally, heredity and environmental factors can have a disruptive effect on this relationship. In tropical wood types there are usually no growth rings, so that such indications of the density are completely lacking.