Forest Utilisation Chapter 9: Physical and Mechanical Properties, Calorific Value and Defects of Wood

Forest Utilisation Notes · Chapter 9

Chapter 9: Properties and Defects of Wood

Colour, lustre, odour, specific gravity and weight classes, moisture content, grain, texture, figure, hardness, elasticity, fissility, strength, calorific value, fuelwoods, and defects due to growth, rupture, wounds, insects, marine borers and fungi.

PhysicalMechanicalFuel valueDefectsRevisionMCQ quiz

1. Physical properties

• Colour: due to infiltrated chemicals; darkens with exposure and steaming. Discolouration may mean fungal attack.
• Lustre: ability of cell walls to reflect light; spruce, boxwood, satinwood, oaks and siris are lustrous.
• Odour: stronger in heartwood; pines (resin), sandalwood, deodar, camphor and teak.

Specific gravityThe ratio of the weight of material to the weight of an equal volume of water at 4°C.

Moisture contentMoisture content is a technical term which indicates the amount of moisture in the wood expressed as percentage of its oven dry weight.

Weight class (air dry)	kg per m³	Examples
Very light	Below 300	Suji (Cryptomeria)
Light	301 to 450	Silver fir, semal
Moderately heavy	451 to 600	Siris, deodar, walnut
Heavy	601 to 800	Teak, mulberry
Very heavy	801 to 950	Babul, sissoo, dhau, sal
Extremely heavy	951 and above	Oaks, rohan

Green timber holds 50 to 200% moisture; air-dry timber 8 to 15%. Slow-grown wood is heavier; in broad-leaved trees wide rings mean heavier wood, but in conifers narrow rings mean heavier wood.

Grain

Alignment of fibres to the axis: straight, irregular, diagonal, spiral, interlocked, wavy. Interlocked grain does not reduce strength.

Texture

Relative size of cells: fine (small cells, takes polish) or coarse; even or uneven.

Figure

Pattern on longitudinal surfaces from tissues and grain.

2. Mechanical properties

HardnessIt signifies the resistance offered by a wood specimen to penetration by another body, i.e. resistance to indentation and not to cutting as is commonly understood.

FlexibilityFlexibility is the capacity of wood to bend out of shape freely without rupture.

ElasticityElasticity in wood is the property which enables it to regain its original shape after the stress has produced deformation.

FissilityThis is the capacity of wood for being split along the fibres, by means of implements such as an axe or a wedge.

Property	Examples
Extremely hard	Kusum, Mesua, sal, babul, khair, Xylia, Anogeissus, rosewood
Extremely soft	Semal, Cryptomeria
Very elastic	Grewia, Anogeissus, khair, Mesua, Dendrocalamus strictus
Extremely fissile	All Himalayan conifers, teak, yew
Very difficult to split	Rosewood, sandan
Exceptionally strong	Mesua, kusum, sal, Xylia

Kinds of strength

• Tension (tenacity): greatest along the grain.
• Crushing: needed in posts and piles.
• Shearing: resistance to fibres sliding past each other.
• Torsion: of little practical importance.
• Transverse strength: the most important; needed in beams, rafters, ladder steps and cart yokes.

Hardness rises with specific gravity, dryness, resin and soundness; heartwood is harder than sapwood. Steaming increases flexibility; climbers are the most flexible. Dryness increases elasticity.

3. Calorific value and fuelwood

Calorific valueThe calorific value indicates the number of grams of water that can be raised in temperature by one degree centigrade when one gram of wood is completely burned.

Wood	Calories
Deodar	5294
Sal	5264
Casuarina	4950
Axlewood (Anogeissus latifolia)	4948
Imli	4939
Babul	4870
Oak	3990

Heating power depends on

• Moisture: 50% of heat is lost at 45% water and 80% at 60% water.
• Specific gravity: heartwood better than sapwood; stem better than branch and root.
• Structure, size of pieces (sawdust only smoulders), resins and oils (raise heat) and soundness.

Good fuelwoods

Babul, casuarina, satinwood, jarul, oak, sal, jamun, laurel, haldu, irul

Bad fuelwoods

Maharukh, semal, salai, chomy, pula, Lyonia, mahua, mango, chir, champ. Teak is smoky for cooking.

4. Defects in wood

Defects permanently reduce the value of wood. Decay from fungi is called unsoundness.

Group	Defects
Abnormal growth	Knots (live or tight; dead or loose), twisted fibre, waviness, burr, constriction by climbers, interior bark
Rupture of tissues	Checks (not through), splits (face to face), shakes: heart and star, radial (frost crack, frost ribs), cup and ring
Wounds and injury	Pruned and broken branches, rindgalls, pith pockets, gum veins, excess resin canals, mineral streaks
Insects	Powder-post borers (Bostrychidae, Lyctidae), bark or ambrosia beetles (pinholes, shotholes), flatheaded borers (Buprestidae), longhorns (Cerambycidae), carpenter bees
Termites	About 200 species; drywood and subterranean; optimum 27 to 30°C
Marine borers	Crustaceans: Limnoria, Chelura, Sphaeroma. Molluscs (shipworms): Teredo, Bankia, Martesia
Fungi	Sap stain (sapwood only, strength not much affected) and decay (sapwood and heartwood)

Exam point: Walnut burrs from north India fetch very high prices for figured veneer. Twisted fibre is common in Hardwickia binata, Boswellia, Casuarina and chir. Powder-post borers need starch and attack only sapwood.

Memory trick: Marine molluscs are the 'TBM' shipworms: Teredo, Bankia, Martesia.

5. Quick revision

• Specific gravity: weight of wood / weight of equal volume of water at 4°C.
• MC on oven-dry weight; green 50 to 200%, air-dry 8 to 15%.
• Very heavy class 801 to 950 kg/m³ (sal, sissoo, babul).
• Hardness = resistance to indentation, not cutting. Transverse strength is the most important.
• Calorific value: deodar 5294, sal 5264, oak 3990.
• Moisture: 45% water loses half the heat.
• Shipworms: Teredo, Bankia, Martesia. Powder-post: Lyctidae, Bostrychidae.

6. Interactive MCQ Quiz

16 questions. Tap an option to see the answer and explanation, then go to the next question.

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