Class: Senior Secondary School 1 (SS1 / SSS 1)
Term: Second Term
Week: 9
Age: 15 years
Duration: 45 minutes
Subject: Furniture Making
Curriculum Theme: Trade
Previous Lesson: Methods Of Seasoning Timber: Air And Kiln Seasoning.
Topic: Defect In Timber
Subject Matter: Growth defects, Seasoning defects, Insect caused defects, Defect classification

Specific Objectives

By the end of the lesson, pupils should be able to:

Cognitive Domain:

Define timber defects.
Classify timber defects into main categories.
Explain different types of growth, seasoning, and insect-caused defects.

Affective Domain:

Appreciate the importance of identifying defects in timber.
Recognize the impact of defects on the quality and strength of timber.

Psychomotor Domain:

Identify common growth defects in timber samples.
Identify common seasoning defects in timber samples.
Identify common insect-caused defects in timber samples.

Discuss with peers the implications of using defective timber in furniture making.

Reference Materials

The following resources were used in planning this lesson:

9 Years Basic Education Curriculum for Technology.
State Unified Scheme of Work for Furniture Making SSS 1.
Evans, B. (2010). Woodwork Technology for Senior Secondary Schools. Macmillan Nigeria Publishers Ltd.
https://www.wood-database.com/wood-articles/wood-defects-and-blemishes/
https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190.pdf

Instructional Materials

The teacher will teach this lesson with the aid of:

Samples of cracked wood.
Samples of insect damaged timber.
Charts showing various timber defects.
Pictures or diagrams of different timber defects.

Rationale for the Lesson

Understanding timber defects helps pupils select good quality wood for furniture and construction projects. It also enables them to identify problems that can affect the strength, durability, and appearance of wood products, leading to better craftsmanship.

Prerequisite/Previous Knowledge

Pupils have basic knowledge of timber as a material and its various applications in furniture making.

Lesson Content/Board Summary

Defects in Timber

Definition of Timber Defects

Timber defects are imperfections or irregularities that reduce the quality, strength, durability, or appearance of wood. These defects can arise during the growth of the tree, during the seasoning process, or due to external factors like insect attacks.

Classification of Timber Defects

Timber defects are generally classified based on their origin. The main classifications are:

Growth defects
Seasoning defects
Insect-caused defects

Types of Timber Defects

Growth Defects

Growth defects are imperfections that occur in the timber while the tree is still growing. They are natural occurrences in the wood structure.

The following are common growth defects:

Knots: These are portions of branches that are embedded in the timber. They can be live (firmly attached) or dead (loose).
Shakes: These are separations of wood fibers along the grain, often due to strong winds or uneven growth. Types include:
- Ring shakes (separation between annual rings)
- Heart shakes (separation radiating from the pith)
- Star shakes (cracks radiating from the bark towards the center)
Wane: This is the presence of bark or rounded edges on a piece of timber where it should be square.
Twisted fibres (Spiral grain): This occurs when the wood fibres grow in a spiral direction around the tree trunk instead of growing straight.
Reaction wood: This is abnormal wood formed in leaning stems or branches, causing unusual shrinkage or swelling. It includes compression wood (in conifers) and tension wood (in hardwoods).

Seasoning Defects

Seasoning defects occur during the drying or seasoning process of timber, often due to improper drying conditions or rapid moisture loss.

The following are common seasoning defects:

Checks: These are small cracks or splits that appear on the surface of the timber, usually at the ends, due to rapid drying.
Splits: These are larger and deeper cracks that extend from one face to another, often an advanced stage of checks.
Warps: These are distortions in the shape of the timber. Types include:
- Bow: Curvature along the length of the timber.
- Cup: Curvature across the width of the timber.
- Twist: Warping in which the ends of the timber twist in opposite directions.
- Spring: Curvature along the edge of the timber.
Collapse: This is an abnormal and excessive shrinkage of timber, leading to a corrugated or sunken appearance on the surface.
Case hardening: This occurs when the surface of the timber dries too quickly, becoming hard and stressed, while the core remains wet.
Honeycombing: This is an internal cracking of the timber, often caused by severe case hardening.

Insect-Caused Defects

Insect-caused defects are damages inflicted on timber by various wood-boring insects, which can weaken the wood structure and reduce its aesthetic value.

The following are common insect-caused defects:

Termite attack: Termites bore tunnels and chambers within the timber, consuming the cellulose and leaving behind dust and hollowed-out sections.
Powder-post beetle attack: These beetles bore small, round holes and produce fine, flour-like dust (frass).
Woodworm attack: Similar to powder-post beetles, woodworms create small holes and tunnels, often in older furniture.
Carpenter ant damage: Carpenter ants excavate galleries in wood to nest, but they do not consume the wood. They push out sawdust-like material.

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher greets the pupils, reviews the previous lesson on timber types, and asks pupils to mention some qualities of good timber. The teacher then introduces the topic “Defects in Timber” by displaying samples of defective timber (cracked wood, insect-damaged wood) and asking pupils what they observe.
Pupils’ Activity: Pupils respond to questions, observe the timber samples, and listen attentively to the introduction of the new topic.
Learning Point: Pupils recall previous knowledge and are prepared for the new lesson.

Step 2: Definition of Timber Defects

Time: 7 minutes
Teaching Skill: Explanation
Teacher’s Activity: The teacher explains what timber defects are, emphasizing that they are imperfections that reduce the quality, strength, or appearance of wood. The teacher writes the definition on the board.
Pupils’ Activity: Pupils listen, take notes, and ask questions for clarification.
Learning Point: Pupils understand the meaning of timber defects.

Step 3: Classification of Timber Defects

Time: 5 minutes
Teaching Skill: Classification
Teacher’s Activity: The teacher explains that timber defects are classified based on their origin: growth defects, seasoning defects, and insect-caused defects. The teacher lists these classifications on the board.
Pupils’ Activity: Pupils listen, note the classifications, and prepare for detailed explanations.
Learning Point: Pupils know the main categories of timber defects.

Step 4: Growth Defects

Time: 8 minutes
Teaching Skill: Demonstration/Explanation
Teacher’s Activity: The teacher explains growth defects, providing examples such as knots, shakes (ring, heart, star), wane, and twisted fibres. The teacher uses charts and diagrams to illustrate these defects and encourages pupils to identify them.
Pupils’ Activity: Pupils observe the charts and diagrams, identify the defects, and take notes.
Learning Point: Pupils can identify and explain common growth defects.

Step 5: Seasoning Defects

Time: 8 minutes
Teaching Skill: Explanation/Demonstration
Teacher’s Activity: The teacher explains seasoning defects, defining checks, splits, and warps (bow, cup, twist, spring). The teacher uses samples of cracked wood and diagrams to show how these defects occur during drying.
Pupils’ Activity: Pupils observe the samples and diagrams, ask questions, and record important points.
Learning Point: Pupils can identify and explain common seasoning defects.

Step 6: Insect-Caused Defects

Time: 7 minutes
Teaching Skill: Explanation/Demonstration
Teacher’s Activity: The teacher explains insect-caused defects, focusing on damage from termites, powder-post beetles, and carpenter ants. The teacher uses samples of insect-damaged timber and pictures to illustrate the different types of damage.
Pupils’ Activity: Pupils observe the samples, identify the types of insect damage, and note the characteristics.
Learning Point: Pupils can identify and explain common insect-caused defects.

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

Define timber defects.
Mention two main classifications of timber defects.
List three examples of growth defects.
State two types of seasoning defects and explain one.
Name two insects that cause defects in timber.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 5 minutes
Teaching Skill: Summarization
Teacher’s Activity: The teacher summarizes the key points of the lesson by briefly reiterating the definition, classifications, and types of timber defects. The teacher then assigns homework: “Draw and label any two growth defects and any two seasoning defects you have learned today.”
Pupils’ Activity: Pupils listen to the summary, ask any final questions, and copy the homework assignment.
Learning Point: Pupils consolidate their understanding of the lesson and have an activity to reinforce learning.

Lesson Keywords

Defect – An imperfection or fault in timber.
Timber – Wood prepared for use in construction or carpentry.
Growth defects – Imperfections arising during the tree’s growth.
Seasoning defects – Imperfections arising during the drying of timber.
Insect defects – Damage caused by wood-boring insects.
Knot – A portion of a branch embedded in timber.
Shake – A separation of wood fibres along the grain.
Warp – Distortion in the shape of timber (e.g., bow, cup, twist).
Check – A small crack on the surface of timber, especially at the ends.
Termite – An insect that bores into and consumes wood.

Differentiation

For visual learners, the teacher will use a variety of charts, diagrams, and actual timber samples to illustrate different defects. For pupils who may struggle, the teacher will provide simplified definitions and focus on identifying the most common defects. Advanced learners will be encouraged to research specific causes of defects and their prevention methods, or to discuss the economic impact of defects on the timber industry.

Note for teachers using this lesson plan

Ensure that actual samples of defective timber are available for demonstration, as this enhances pupils’ understanding and ability to identify defects in real-world scenarios. Encourage pupils to bring their own examples if possible. Pay attention to safety when handling timber samples with splinters or sharp edges. Allocate sufficient time for pupils to observe and interact with the instructional materials.