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ScienceWise - Jul/Aug 2007

A Tale of Three Timbers

Article Illustration
The corner of a block of balsa wood. Note how thin-walled the cells are.
Article Illustration
Magnified view of toilet paper. The ‘fibre’ which can be readily identified as a softwood tracheid due to the presence of four bordered pits.
Article Illustration
A longitudinal view of nine tracheids on the edge of a wood block of pine wood showing the many bordered pits.
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A tangential longitudinal view of a walnut wood block showing silica particles in the rays.

The amazing structure of wood revealed by electron microscopy

During his professional life as a microscopist at the ANU Electron Microscope Unit Dr Roger Heady recorded thousands of scanning electron micrographs on a diverse array of topics. Now he’s a Visiting Fellow at the Fenner School of Environment and Society and sorting through his vast collection of images to catalogue them and pick out some of his more interesting images for anyone interested.

While he’s captured aspects of everything from beer bubbles to mineral crystals under the scanning electron microscope, Dr Heady’s first love has always been the structure of wood and timber. So, as an example of some of his work, here are three stories and images of very different types of wood.

The timber from the tree Ochroma lagopus is light, easily cut and glued. You’re more likely to know it as balsa wood. “The word ‘balsa’ is Spanish,” says Dr Heady. “It means ‘raft’, and it refers to its excellent floatation qualities. Although one of the lightest woods in the world, balsa is technically a ‘hardwood’ since the trees belong to the angiosperm, not gymnosperm, group. Balsa wood has an air dry density of only 120 Kg/m3 whereas oak is 700 Kg/m3.

“Balsa wood’s cells are big and very thin-walled, so that the ratio of solid matter to open space is very small. Only about 40% of the volume of the wood is solid substance. Balsa trees grow in the humid rain forests of Central and South America. To give the balsa tree the strength to stand, each cell of the wood is full of water, thus making them rigid – like a car tyre full of air. Green balsa wood contains five times as much water weight as it has actual wood substance, whereas most hardwoods contain little water in relation to wood substance. Green balsa wood must therefore be carefully kiln-dried to remove most of the water before it can be sold. Most Balsa wood sold in Australia is now grown in New Guinea.”

The timber of Red Turpentine is used in shipbuilding and wharf decking because of its resistance to marine borers. It’s said this resistance due to the presence of silica particles in its wood. Wood of the Queensland walnut also contains silica particles which tend to blunt saws, planes, and other hand-tools used to cut them.

“By imaging Queensland walnut in the ‘back-scattered-electron’ (BSE) mode, it’s easy to pick out these silica particles,” explains Dr Heady. “The BSE mode shows differences in the elemental composition of the sample. Elements with a higher atomic number appear brighter because they have bigger atomic nuclei and so reflect more electrons. Silicon has a higher atomic number than the carbon and oxygen of the wood so it appears bright and easily seen against the wood background.”

Toilet paper consists of a mat of randomly inter-woven fibres that are formed into a thin flat sheet. The ‘fibres’ are almost always obtained from wood pulp, and the type of wood used is softwood (wood from a conifer tree) – most likely Pinus radiata or other plantation-grown softwood.

In relation to toilet paper making, “When it comes to toilet paper, the term ‘fibre’ is a misnomer since fibres only occur in hardwoods, not in softwoods,” comments Dr Heady. “Softwood toilet paper fibres are actually tracheids – tubes 20 µm in diameter and about 2-3 mm long that comprise the xylem of the conifer tree. They conduct water up the tree.

“Tracheids are interconnected to each other by apertures called bordered pits. In the process of paper-making tracheids are separated from each other and flattened into a sheet. However, the bordered pits present in the treated tracheids can often still be observed in the SEM image.”

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