The Chemistry of Wood

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Visit Glasgow University to find out about the chemistry of softwood (species like spruce, pine and fir). Discover what makes up the rings and why the tree changes the properties of the wood it grows as it gets older. The study of the material properties of wood is called "wood science".

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Although this might not look like a typical chemistry laboratory this is the chemistry department of Glasgow university. We’re here to find out about the chemistry of wood.

Each year a tree grows a new ring on the outside just beneath the bark. The middle part - or pith - where the rings have small diameter was made by the tree when it was young. The larger rings on the outside, were made when the tree was older.

Each ring has two parts. These parts are called early wood and late wood. The early wood is grown early in the summer. Here the cells are large and the cell walls are thin which means this part of the wood is light in colour and has low density. In late wood, the cells are smaller and have thicker cell walls meaning this part of the wood is dark in colour and is more dense.

The chemistry of the cells is important because it determines the properties of the wood and its fibres. It is the alternation of early and late wood that gives the timber its grain pattern one of the things that makes it look attractive.

This is an x-ray densitometer. It’s used to make measurements of the density of the wood in the different rings. These thin strips of wood contain the rings from the middle of the tree to the bark - which represent the whole life of the tree.

The denser wood absorbs more of the x-rays - just like bones do in a medical x-ray. In this machine the beam of x-rays is very fine - like a needle - so it can take very precise measurements of small areas.

Density changes within a ring and generally increases from the middle of the tree to the bark. The late wood parts of the ring show up as peaks in the density curve.

Wood is mainly made from cellulose which is a type of carbohydrate. Cellulose is made up of long chains of Carbon, Hydrogen and Oxygen atoms and helps to give the wood its strength.

The wood also contains lignin - a type of natural plastic - which protects the wood and also binds together the cellulose chains to give the cell walls rigidity. Understanding how the cellulose and lignin work together is important for understanding the properties of the wood.

This is an infra-red microscope which is used to study the cellulose and lignin in the different parts of the cell structure. The more vertical the chains of cellulose in the tree, the stiffer and stronger the wood. In the middle of the tree the chains go round and round in a spiral
so the middle bit of the tree is less stiff and not so strong. Towards the outside of the tree the cellulose chains are more vertical making this part of the wood stronger and stiffer.

A young tree needs to be flexible to it can bend with the wind without breaking but as it gets older and larger it must become stiffer to avoid being blown over. That's why it grows stronger and stiffer wood the older it gets.

These two pieces of wood were cut from the same tree one cut from the wood close to the middle of the tree - the pith and one cut from the wood close to the bark. The piece from the pith is more flexible than the piece from the edge. It’s easier to bend with my fingers.

The wood in the middle of the tree is less dense because it contains a lot of air and so this part of the tree is good for making insulation. The wood on the outside of the tree is good for building, because it is stronger and stiffer.

In a standing tree, the wood contains water rather than air the water it needs to stay alive. Some of the water is so tightly linked to the molecules in the cell wall that when we take the wood out of the tree and dry it - the wood shrinks - which is why this piece has split.

Different parts of the wood shrink by different amounts. When one side of the timber shrinks more than the other - the piece bends. Understanding and limiting this distortion is important for saw millers because bent pieces of wood are not so useful.

There are also some chemicals in the wood that don't have a structural function. These are particularly common in knots - which is why they are a darker colour. Here at Glasgow University they are researching into new uses for these natural chemicals such as in medicine.

These are Soxhlet extractors which are used to take the useful chemicals out of the wood so they can be studied. The general name for these chemicals is extractives.

This is a high performance liquid chromatograph which is used to separate the extractives and analyse their chemical composition.

By better understanding the chemistry of wood it will be possible for foresters to grow trees with improved properties for the various


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