Some materials science

Everybody is familiar with trees. Trees live around us. They give us shade, fruits and clean air and, last but not least, provide wood for us.
From tree trunks you make beams, planks or strips. From the crowns and thicker branches of trees you get firewood. By digging out the stump you can make beautiful ornamental objects.

Tree, trunk

(Tree, trunk (1))

Tree stump

(Tree stump)

If you cut the trunk horizontally (cross section), in most cases you can find nicely visible rings on the surface of the cross section.

Cross section of a tree

(Cross section of a tree)


There are darker and lighter rings and one of each together make up an annual ring (7). Usually a tree grows lighter rings in spring and darker rings in summer. The former are called rings of early or spring wood and the latter rings of late or summer wood.
Looking from outside towards the inside, the outermost layer is the outer bark (6). The next one is the phloem or inner bark (5). Beneath the phloem there is the cambium (4). Every year the cambium grows an annual ring on the tree. Inside the growth rings, in the innermost part you can find the pith (1). ??
Often the colour of a tree varies in different parts of the cross section surface. It is usually darker around the pith: this is the heartwood. (2). After some time, the cells of the annual rings around the pith die, get filled with conservation and tanning materials and lose their water content: this is how heartwood is formed. The darker part is followed by a lighter one: the sapwood (3). The sapwood consists of viable cells. Nutrition from the roots to the crown is carried by the sapwood. These parts are very easy to see on the trunks of plum, cherry or acacia trees. There are trees, however, where these parts or even the annual rings are difficult or impossible to differentiate. Such trees include among others hornbeam and lime.
If you cut the trunk vertically, the growth rings appear in irregular lines making a nice pattern characteristic of the particular tree. These lines form the grain of the wood.

The sequence of growth rings

(The sequence of growth rings)

Examining a cut surface we can find tiny porous cells or pores. If the pores appear to be unarranged, we talk about diffuse porous wood. Examples for this category are lime, birch, walnut, beech, etc. In the case of acacia, oak, ash tree, etc. the pores are arranged according to the annual rings and these are thus called ring-porous wood.

Walnut pores

(Walnut pores)

Trees live in different places. The place where a tree lives influences the growth of the trunk to a great extent. It may be a windy place, a place at the edge or inside a forest, on a hillside or in a plain landscape. Accordingly, the tree may be bent, curved or crooked but it can certainly be absolutely straight as well. Growth irregularities ultimately influence the grain of the wood.
There are two categories of trees on the basis of their foliage. Trees in the category of deciduous trees like acacia, oak, lime, beech, walnut, fruit trees, etc. lose their leaves in autumn. The other category includes coniferous trees, i.e. various pines like spruces, firs, Scots pines, black pines, etc.
Trees can be categorised on the basis of the hardness of the wood, too. Soft wood is easier to work with than hard wood. Hard wood always weighs more provided the volume is the same and the wood is dry. Air dry wood has 15% water content. Considering their weight there are very light trees like the balsa tree used for building models, which weighs about 100 kg/m?, and there are heavy trees like yews, which weigh about 1000 kg/ m?.
Wood as raw material can be ranked on the basis of other characteristics as well. How well wood can be used depends to a great extent on its compressive strength, tensile strength, flexural strength and, last but not least, torsional strength.
Trees are felled in winter, after they have lost their foliage. This is when their water content is the lowest. The water content of freshly felled trees is very high, making up about half of the weight of the wood. From the trunk you make timber. Timber is piled up at airy clearings at the edges of forests and used up as fast as possible to prevent damage by wood decay fungi or insects. While being piled up there, timber also loses part of its water content. From the forest edge timber is taken to saw-mills where it is cut to the required sizes by huge power saws. The cut up timber is gathered in a specific order in an airy space, it is stacked and dried for as long as several years until it reaches the desired dryness for use.

Cherry wood drying

(Cherry wood drying)

So as to speed up the process, drying machines are used. The timber available in trade is almost always such artificially dried wood. After wood has dried out, it is able to take up moisture from the air in humid weather and evaporate the moisture in dry weather. Wood sometimes shrinks and sometimes it swells. What is more, this change of volume is not even across the whole cross section of the wood. Early wood shrinks to a greater extent than late wood and sapwood dries faster than heartwood. Therefore, changes in the volume often involve changes in the shape: wood gets twisted, bended or crooked.
Live trees grow side branches not only in their crowns but on their trunks as well. Side branches are imbedded in the material of the trunk almost like small trees. When you saw the wood, these small trunks are visible as dark, harder units resembling corks. These are called knots. If the branch was alive when the tree was felled, the knot stays inside; if it dried out before that, the knot falls out of the strip.

Falling out knot

(Falling out knot)

Knots are basically wood defects. Strips are weaker at the place of a knot and due to the hardness of the latter the parts around knots are more difficult to work with. Around knots, wood grains are thicker. On the other hand, if you find a good place for knots that stay in the timber, they can become nice decorations on the objects made. Loose or falling out knots can be removed and glued back in the wood.
It is characteristic of pines that their wood contains smaller or larger amounts of resin. Often the resin is accumulated in the wood in what are called resin nests. The sizes of resin nests may vary from very tiny to several centimetres. When you process wood, these must be removed from the timber.

Resin nests

(Resin nests)

The decay of wood is mostly caused by fungi. Wood decay fungi attack wood primarily while it is stored in wood yards but should be expected anywhere where wood has contact with moisture or the soil. The fungi first change the colour of the wood, then the wood loses its solidity and decays. The best known fungi are Lenzites abietina, Poria vaporaria and the dry rot fungus (Merulius lacrymans). Blue stain fungi (Ophiostoma species) only change the colour of the wood, while affecting the mechanical characteristics of the wood only at a minimal extent. In Scandinavian countries wood infected by blue stain fungi is very popular because there the infection is considered as an added value.
Wood decay insects feed upon wood. They make tracks all over in the timber, thereby weakening its structure. The best know species you can often find in human dwellings are bark beetles, furniture beetles and powderpost beetles.
Decay by fungi or insects can be very well prevented by well considered surface treatment.
Let us finally have an overview of the characteristic features of the wood we use.
Spruce: Spruce wood is yellowish or reddish white. Its annual rings are easy to see. Its knots are big, dark and do not fall out. It is soft; it splits easily and is easy to work with.



Spruce leaves

(Spruce leaves)

Scots pine: Scots pine is of yellowish white colour with reddish brown heartwood. Its knots are big and do not fall out. Scots pine is of medium hardness; it is heavy and splits easily. It is more difficult to work with than spruce. It has a high resin content.

Scots pine

(Scots pine)

Leaves of Scots pine 

(Leaves of Scots pine)

Lime: Lime wood is almost white and lacks heartwood. It is soft and light. It is hard to split but easy to carve.
There are several lime species: small-leaved lime, large-leaved lime, silver lime, etc.



Leaves and flowers of lime

(Leaves and flowers of lime)

Birch: Witish-reddish wood, its heartwood having no colour. It is of medium weight and is hard to split. Birch is unique for not being splintery. It is a popular raw material for plywood.




[No Title]

[No Content]