General

The main characteristics of wood

Posted on by Artamin

Wood, together with stone, are the first materials that human beings have ever known and worked with: the first tools and homes were built using it, and with the discovery of fire, man was able to warm himself.
Even today, contemporary civilization uses it for the same purposes.

The main characteristics of wood: knowing it to work it better

Each wood has its own characteristics and, although many have similar colors and shades, each plant is different even if it belongs to the same species; each point of the same plant differs from the others and this is its first important characteristic: heterogeneity and, even more specifically, anisotropy, i.e. its diversity along a direction of the plane (cross, radial, tangential).

cross section

radial section

tangential section

From a botanical point of view, trees can be divided into two macro species: coniferous woods and broad-leaved trees.
To the first group belong the trees that have needles instead of broad leaves and that, for the most part, do not fall during the winter, with characteristic cone-shaped fruits: among the most common are the pine, the fir, the yew and cypress.
 
The broad-leaved trees, on the other hand, have wider leaves which usually lose during the colder season if they grow in areas with a temperate climate, such as oak or chestnut.

Soft and hard woods

Commercially, the timber from coniferous is considered softwood while that from broad-leaved is hardwood.
However, this is an improper and potentially misleading distinction if we consider the hardness of the wood: some conifers, such as the yew, are harder than other broad-leaved species and therefore should not be considered as an exclusive parameter for the choice of wood.

 

 
 
Also from a commercial point of view, broad-leaved wood is considered coarse, while fruit wood (cherry, olive, pear, maple, etc.) is called fine.
 
Softwoods are made up of very short cells called tracheids: water and nutrients flow through their walls.
Hardwoods, on the other hand, consist of long tubular vessels that allow the water to rise from the roots, and medullary rays that distribute the nourishment in a radial direction (from the center towards the outside).

Physical properties

 
The resistance of wood is its ability to fight the forces due to external, mechanical or atmospheric stresses.
The most resistant part of the trunk is its radial section, the one that exposes the grain along the widest surface of the board, the weaker one, on the contrary, is on the side perpendicular to the grain.
 
Density expresses the mass / volume ratio of each wood species: sometimes it is also expressed as compactness, and is a very important parameter when choosing a type of wood that must be subjected to significant structural loads, which usually should not be less than 600 Kg / mc.
 
Hardness is the ability of a material to be penetrated by an external body, in the case of wood it translates into the ability to be cut and worked.
Very hard woods are more resistant to compression than soft ones.
 
Finally, rigidity expresses the ability of the wood to be curved.
One might apparently think that a soft wood is easier to bend than a hard one, however the most important parameter is represented by the distribution of the fibers: the more these are rectilinear and parallel the greater its ability to be curved.
 
The factors that most influence the physical properties of wood are environmental ones, in particular humidity.

Seasoning

 
Among the main characteristics of wood is hygroscopy, that is, its ability to release or acquire moisture from the outside through its pores.
 
A living plant can contain a degree of humidity that is 100% higher than its weight (dry), both in the form of vapor and liquid transported through its cellular system.
When it is cut down, the plant gradually releases moisture to the outside, with a consequent loss of weight but not of volume.
Around the 30% threshold, called the fiber saturation point, a slow but progressive loss of volume will also begin, a phenomenon of considerable importance called shrinkage.

 

 
The seasoning of the wood starts from the already sawn timber and can take place by means of specific dryers that are able to bring the humidity value contained to very low values (6%), or exposed to natural air, where each board is stacked one on top of the other and interposed by some strips to allow a uniform seasoning on both surfaces.
The natural curing process in contact with the air is very slow: sometimes it takes a couple of years to reach a balance of humidity with the external environment with values of about 12-15%.
 
A dried wood is generally more resistant than green wood, it deteriorates less, is more stable and, for the reasons indicated above, also lighter.
For these reasons, when choosing our timber for the construction of any object, it is always preferable to start from the best possible condition.

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