The most important commercially used species in Finland are spruce, pine and birch.
The characteristics of a species are defined by the age, speed of growth, growth area and the climate. Each tree has furthermore individual different properties, from the pith to the sapwood and also lengthwise from top to bottom.
When a cross section of a log is examined one can note that the outer ring, sapwood, has living cells, whereas the inner part – heartwood – is made of dead cells.
The material situated between the bark and sapwood is called cambium, whose cells split mostly inwards resulting in the growth in the thickness of the tree. Growth – or annual rings are visible because spring wood is made of larger cells and summer wood is made of smaller cells.
- The cross-sectional picture illustrates the structure and terms of the tree.
Source: Puuinfo Oy, © Antti Saikkonen
The most important properties of the raw material:
- possible defects
As the density grows the strength properties of wood also improve. There are different densities between species, also the growth rate shown in the proportion of spring and summer wood have an effect on the density.
The quality, number and position of knots have a major impact especially on timber grades, because they weaken the strength properties, result in changes during the kilning and moreover they have an effect on visual grades of timber.
As for sawn logs, the main undesired properties are the excessive gradient, bow, compression wood (cells in trees which have bent etc in the forest) and rot.
Bow and gradient lower the production yield in the sawing. Compression wood may result in splits and shakes.
Rot in turn weakens the mechanical strength, results in discolouration and also reduces the yields and reduces brightness in the pulp production.
Excessive resin content can have an effect on surface treatments of a finished product, but on the other hand because of resin especially the heart wood is more durable for instance against the elements of weather.
The spring wood trakeid cells are bigger and they conduct the water supply into the tree. The summer wood trakeids cells have thicker walls and they strengthen the tree trunk mechanically. So called parenchyma cells are responsible for the growth, they store the growth materials and also alters them. Cell properties, such as the length, have an effect on its processing properties.
Spruce is light in colour and both the sapwood and heartwood are of the same colour. The density of spruce is about 400 kg/m3. Actual wood material is made 95% of fibres (or trakeids), which are on average 3.5–4.0 mm long.
In pine the sapwood and heartwood are easily differentiated. The sapwood is light in colour, whereas the heartwood is darker. The average density of pine is 390–420 kg/m3. The wood fibre consists 93% of trakeids, with an average length of 2.9 mm.