Topic 1 of 6 · Form 5
Wood science: structure, moisture and properties
MC (%) = (wet mass − oven-dry mass) ÷ oven-dry mass × 100.
Notes for this topic 1 file, in order
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Cell structure
- Hardwoods (angiosperms): vessels (pores) conduct water, fibres give strength, rays store food. Ring-porous and diffuse-porous woods. Examples: mukwa (Pterocarpus angolensis), Zimbabwean teak (Baikiaea plurijuga), mahogany, msasa, eucalyptus.
- Softwoods (gymnosperms): tracheids conduct water and give support; resin canals; simpler structure. Examples: pine (Pinus patula and P. elliottii, grown in the Eastern Highlands).
- Cell walls are made of cellulose, hemicellulose and lignin.
Moisture content
MC (%) = (wet mass − oven-dry mass) ÷ oven-dry mass × 100.
Worked example: A sample weighs 500 g and 400 g after oven drying. Find its moisture content.
MC = (500 − 400) ÷ 400 × 100 = 25%
- Fibre saturation point (about 25–30%): below this, wood shrinks as it dries.
- Equilibrium moisture content: the MC wood reaches in its environment; indoor furniture about 10–12% in Zimbabwe's dry highveld.
- Shrinkage is greatest tangentially, then radially, and least longitudinally – causing cupping and checking.
Properties
Density, strength (tension, compression, bending, shear), hardness, toughness, durability, workability and stability. Strength increases as moisture content falls below fibre saturation point.
Exam tip: Use the oven-dry mass (not the wet mass) as the denominator in moisture content calculations.
Check yourself
- What conducts water in hardwoods?
- Why does wood shrink more tangentially than radially?
- A sample of 240 g dries to 200 g. Find its MC.
Show answers
- Vessels (pores).
- Rays restrain radial movement, and cell wall structure causes greater tangential movement.
- 20%
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