CTECHJHS 1 • Term 1Topic 2Free Trial Lesson
Compliant and Resistant Materials: Wood & Timber
Classification of wood into hardwoods and softwoods, Ghanaian timber species, timber conversion methods, seasoning, and common defects.
Curated Video Lesson
Visual explanation and practical step-by-step walk-through
Comprehensive Study Notes
Aligned with Ghana NaCCA & WAEC BECE syllabus standards
Topic Introduction & Real-World Context:
Wood is one of Ghana's most valuable renewable natural resources. Understanding timber anatomy, the distinction between hardwoods and softwoods, timber conversion methods, and the seasoning process allows designers to select the right wood species for durable building and furniture projects.
What You Will Master in This Lesson (NaCCA Objectives):
Classify timber into hardwoods and softwoods with specific Ghanaian examples.
Identify the anatomical cross-section features of a tree trunk (pith, heartwood, sapwood, cambium, bark).
Explain the principles and merits of plain sawing versus quarter sawing.
Distinguish between natural air seasoning and artificial kiln seasoning.
1. Hardwoods versus Softwoods
The botanical distinction between hardwoods and softwoods is based on tree reproduction and cell anatomy:
Hardwoods (Angiosperms): Broad-leaved trees producing covered seeds within a fruit or nut. Broad cellular pores (vessels) provide high density and strength. Examples in Ghana: Odum (Milicia excelsa), Mahogany (Khaya senegalensis), Wawa, Sapele, Teak, Asanfina, Dahoma.
Softwoods (Gymnosperms): Needle-leaved evergreen conifers producing naked seeds in cones. Non-porous tracheid cellular structure. Examples: Pine, Cedar, Spruce, Fir (largely imported into Ghana for packaging and crate work).
Note: 'Hardness' is botanical, not purely physical; balsa is technically a botanical hardwood despite being feather-light.
Key Takeaway: Hardwoods have broad leaves and covered seeds with vessels; softwoods have needles and cones with tracheids.
Real-World Application: Choosing mahogany for a high-end dining table provides rich reddish grain that polishes to an exquisite deep luster.
2. Cross-Section Anatomy of a Tree Trunk
A radial cut across a mature timber log reveals distinct concentric biological zones:
Pith (Medulla): The soft, spongy primary tissue at the exact center of the trunk.
Heartwood: The dead, inactive central timber cylinder. Pores are plugged with gums, tannins, and resins, making it dark, dense, strong, and highly resistant to termites and fungal decay.
Sapwood: The outer living ring that carries moisture and mineral sap up to the crown. Lighter in color, vulnerable to wood-borer attack.
Cambium Layer: The microscopic living layer between sapwood and inner bark where new cells divide annually.
Bark (Inner and Outer): Shields the tree trunk from physical abrasion, wild bushfires, and insect attacks.
Annual Rings: Concentric rings recording seasonal growth (one light early-wood ring + one dark late-wood ring = one year).
Key Takeaway: Only seasoned heartwood should be used for permanent structural furniture, as sapwood attracts beetles and rots quickly.
Real-World Application: Observing 25 distinct rings on a felled teak log confirms the tree grew for a quarter of a century.
3. Timber Conversion and Seasoning
Logs felled in the forest must be mechanically sawed into commercial lumber and properly dried:
Conversion Methods:
- Plain / Through-and-Through Sawing: Parallel longitudinal cuts through the log. Cheapest, quickest, least waste, but boards warp easily.
- Quarter Sawing: Log divided into four quarters, then sliced radially. Produces decorative figure, minimal shrinkage across width, but produces more sawdust waste.
Seasoning (Drying Wood):
- Air Seasoning: Stacking planks horizontally with spacer sticks (stickers) in ventilated outdoor sheds. Takes 6–12 months. Economical but slow.
- Kiln Seasoning: Drying timber inside an insulated heated oven with steam injection. Takes 2–3 weeks, kills insects, achieves exact target moisture (10–12%).
Key Takeaway: Unseasoned 'green' timber shrinks, cups, bows, and checks as it loses moisture, ruining completed furniture joints.
Real-World Application: Stacking planks with 25 mm stickers allows tropical breezes to circulate evenly across all board faces during air seasoning.
Common Mistakes Students Make in BECE Examinations:
⚠️Assuming all hardwoods are physically hard and all softwoods are soft (e.g. balsa is a botanical hardwood).
⚠️Using green, wet timber for table tops, which inevitably causes shrinkage splits within months.
Teacher's BECE Exam Pro-Tips:
⭐Be prepared to sketch and label the tree cross-section (pith, heartwood, sapwood, cambium, bark, annual rings).
⭐List at least four indigenous Ghanaian hardwood species: Odum, Mahogany, Wawa, Sapele.
Quick Revision Summary Checklist:
I know the botanical differences between hardwoods and softwoods.
I can draw and label the 6 parts of a tree trunk cross-section.
I can compare plain sawing with quarter sawing.
I understand the necessity of seasoning timber before use.
Step-by-Step Worked Examples (2)
Real BECE exam-standard problems with complete solution steps
Example 1: Distinguishing Hardwood from Softwood
Problem StatementA carpenter selects Odum for outdoor window frames rather than unseasoned Pine. Why is Odum suitable?
Step-by-Step Solution:
Step 1: Odum is a tropical dense hardwood rich in natural toxic extractives (chlorophorin).
Step 2: These extractives give Odum high natural resistance against termites, fungi, and rot.
Step 3: Softwoods like unseasoned pine absorb moisture rapidly and decay when exposed to tropical rains.
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Key Takeaway / Exam Rule: Ghanaian dense hardwoods like Odum and Dahoma are naturally suited for weather-exposed joinery.
Example 2: Calculating Tree Age from Growth Rings
Problem StatementA freshly cut cross-section of a teak trunk reveals 18 distinct concentric annual rings. What is the tree's approximate age?
Step-by-Step Solution:
Step 1: Recall that in temperate and marked wet/dry tropical seasons, each concentric ring corresponds to one growth year.
Step 2: Count 18 rings from the pith to the cambium layer.
Step 3: The tree is approximately 18 years old.
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Key Takeaway / Exam Rule: Annual rings visually record both the chronological age and historical climatic growth conditions.
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