Wood
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Linoleum
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NATwood

There is no material to which human beings have such a close relationship as to wood. For thousands of years it has been the most important and the most commonly worked material in broad areas of human life.

Wood is a living material that shows an enormous range both in its appearance and it its outstanding technical qualities.

Wood creates a pleasant atmosphere for living in, it gives warmth and comfort. Wood converts the space we occupy in our daily lives into a remarkable place to live. Each single piece of wood is completely original in colour and structure.

• Growth
• Structure
• Trunk section
• Moisture content
• Hardness (Brinell hardness)

Growth of the wood

Wood is a living material. Before a tree can be felled it has had a long life behind it (central European evergreens about 100 years, central European deciduous wood about 200 years).

A tree needs nutrients plus water, air, light and warmth in order to live. The tree gets the water and nutrients it needs from out of the ground, it takes carbon dioxide(CO2) from the atmosphere. It creates the elements it needs for growth using sunlight and the green pigment in its leaves (chlorophyll), these are chemical energy in the form of glucose, while it gives off oxygen as a by-product into the atmosphere.

In addition the tree gives off excess water into the air through its leaves adding humidity and removing dust from the atmosphere. This process is known as assimilation or photosynthesis in reference to the light that controls the process..

Since forests are of the greatest importance to human life and for the environment, it is important that we make careful use of the material wood and we must ensure that we do not use more than can be replaced by natural growth in the forests. The uncontrolled felling of the tropical forests is the issue that has put the use of tropical timber into the focus of attention. This is the case although some tropical forests are tended according to the proper principles of forestry.
 

• The bark - this protects a tree against drying out and from injury.
• The bast fibre is a tough, fibrous layer – it conducts the sap produced in the leaves to all the different areas of the tree.
• The cambium, a thin, slimy growth layer – the new wood cells are produced here: the bast fibre cells outwards and the sap wood cells inwards.
• The sapwood – this carries water and the nutrient salts it holds in solution upwards from the ground to the crown of the tree. Sapwood is often soft and not very firm.
• The core timber – no longer conducts sap in most trees and consists of cells that have become rigid and wooden.
• The annual rings – the annual ring is comprised of the early wood (light) and the late wood (dark). The age of the tree can be seen from the number of rings.
• The vascular rays (wood rays) – these transport sap inside the trunk and store the structure building materials that the tree does not need for immediate use.
• The pulp tubes – these conduct sap in the sampling tree, they dry out later.

The fine structure of the wood

Each annual ring is comprised of the early wood (spring growth) and the late wood (late summer growth). The early wood is broader than the late wood in most trees and has a loose structured tissue with thin cell walls. In the late wood growth cells with thick cell walls develop and late wood has a dense tissue structure because of this.

The difference between the early wood and the late wood is greater the more clearly the rhythm of the seasons is differentiated. It is for this reason that the rings are difficult to distinguish in tropical timber grown where there is scarcely any seasonal variation in climate. The absence of clear annual rings is thus a recognition characteristic for tropical wood.

Since the early wood is softer, it wears more quickly when it is used for floors for example, that is why the early wood areas appear to have been washed away on old plank floors. Fine-ringed wood (with narrow annual rings) is thus preferable to wood with coarse annual rings.

Every tree represents a precise record of the environmental conditions during its lifetime, because the growth of the tree is not always equal each year as a result of the differences in the climate.
 

• Radial section: This is the section running against the centre. The annual rings are shown up as fine, parallel strips. In some areas planks with vertical annual rings are referred to as comb-grained planks.
• Tangential or chordal cut: This cut runs parallel to the centre like the tangent to a circle. Since a tree is thinner the higher it grows (like a cone) the annual rings appear in hyperbola form on the cut section. These are described as chordal irregular grain and have a significant influence on the appearance of a tangential cut.

The moisture content of the wood

A living tree contains a lot of water. Both the individual cells as also the cell walls that separate them are full of water (cell sap). They are at their maximum state of expansion. The drying out process begins immediately after the tree is felled. The water content evaporates drop by drop, until a state of balance is achieved between the environmental air humidity and the moisture content of the wood. The wood cells also shrink in the process.

This slow process of drying out in the air is accelerated today by drying in special ovens. The wood is dried out to a level where the moisture content is approximately equal to the environmental humidity of the area where the wood is to be used.

Wood is hydroscopic, meaning that it is able to attract and absorb moisture from the air (to expand) and also give it out again (to shrink). A continuous balance thus exists between the air humidity and the moisture in the wood. This relative air humidity varies in accordance with the annual rhythm of the seasons. It is higher in the summer months – when the wood absorbs moisture and expands, lower in the winter months – the wood gives off moisture and shrinks.

The wood also absorbs moisture from structural parts of a building that are too wet such as the sub-floor, the walls or the like. It is thus essential that the dampness of a slab be tested before installation and that the room is well ventilated to get rid of any dampness in the room. Wood is active, it is alive. This also applies to varnished, oiled or waxed wood surfaces.

Expanding and shrinking

If we take a look at a cut section of a tree trunk, three principal directions can be allocated to this phenomenon, lengthways with the trunk, crossways radially and parallel to the tangent of the trunk circumference, the tangential direction.

The degree of the expansion and shrinkage varies in these different directions. On average the shrinkage dimensions have a 1 : 10 : 20 ratio between the lengthways, the radial and the tangential directions (crossways to the wood pile). The values vary between the different wood species. The specialist describes this behaviour of wood as being anisotropic.

The hardness of various woods

The hardness of a wood species is an important indicator of the wear and tear and the resistance to indentation it can be expected to provide.

The hardness of wood can be determined using the Brinell process. In this a 10 mm diameter steel ball is pressed against the wood with a defined level of force and for a precisely predetermined time span. The ball is moved away after the load period ends. The Brinell hardness can be calculated from the area of the indentation and the pressure force applied. The higher the Brinell hardness number measured, the harder the wood.

The hardness values given must always be seen as mean values, however, since the actual values can vary somewhat in dependence on the area where the tree grew and the method of sawing used (all wood is harder lengthways to the grain than crossways).

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