Wood Defects and Preservation: Protecting Timber for Stronger Building

Subject: Basic Technology

Class: JSS 3
Term: First Term
Week: 1 (Second Period)
Age: 13 years

Topic: Wood Defects and Preservation Treatment of Timber
Sub-topic: Types of Wood Defects and Methods of Preservation

Duration: 40 minutes

Behavioural Objectives:
By the end of the lesson, students should be able to:

1. Define wood defects and explain how they affect timber.
2. Identify and describe the types of wood defects.
3. List the methods of preserving timber.
4. Explain the importance of timber preservation.

Keywords: Wood defects, Preservation, Natural defects, Artificial defects, Warping

Set Induction:
Show students a sample of a deformed or defective wood and ask if it can be used to make a strong table or chair. Then, explain how wood defects affect the quality and value of timber.

Entry Behaviour:
Students have seen wood used for different purposes and may have noticed wood that looks damaged or deformed.

Learning Resources and Materials:

• Samples of defective wood
• Pictures of wood defects (e.g., twisted, bowed, or split wood)
• Chemicals used for wood preservation

Building Background / Connection to Prior Knowledge:
Students have learned about the stages of wood processing and parts of a tree in the previous lesson.

Embedded Core Skills:

• Critical thinking
• Observation
• Problem-solving

Learning Materials:

• Lagos State Scheme of Work
• Textbooks on Basic Technology
• Wood samples showing defects

Reference Books:

• Lagos State Basic Technology Scheme of Work

Instructional Materials:

• Samples of defective wood
• Charts showing different types of wood defects

Content

Definition of Wood Defects
Wood defects are faults or deformations in wood that reduce its value and strength. Defects can happen because of poor seasoning, bad storage, insect attack, or during the tree’s growth.

Types of Wood Defects

1. Natural Defects:
   These defects happen naturally during the growth of the tree. Examples include:
   • Heart shake: A crack that starts from the center of the tree and moves outward.
2. Artificial Defects:
   These defects happen after the tree has been cut down, often due to poor handling, seasoning, or insect attacks. Examples include:
   • Warping: This occurs when wood twists or bends.
   • Shrinking: The wood loses its size due to poor seasoning.
   • Cupping: The wood becomes hollow or curved like a bowl.
   • Bow: The wood bends along its length.

Preservation of Timber
Preservation is the process of applying chemicals to timber to prevent damage from insects, fungi, or decay.

Importance of Preservation

1. Prevents Insect Attack: Preserved timber is safe from termites and other insects.
2. Increases Durability: Timber lasts longer when it is treated.
3. Improves Appearance: The chemicals used can make the wood look more beautiful.
4. Enhances Strength: Preserved wood becomes stronger and more suitable for construction.

Types of Timber Preservation

1. Oil Preservatives: Chemicals like creosote oil and coal tar are used to protect timber from insects and moisture.
2. Tar Preservatives: Coal tar is applied to timber to prevent decay.
3. Salignum: A chemical that protects wood from insects and fungi.

Evaluation

1. Wood defects are __________ that reduce the value of wood.
   a) Features
   b) Faults
   c) Designs
   d) Structures
2. __________ is a crack that starts from the center of the tree.
   a) Cupping
   b) Bow
   c) Heart shake
   d) Warp
3. Warping occurs when the wood __________.
   a) Bends
   b) Breaks
   c) Shrinks
   d) Fades
4. A common cause of artificial defects is __________.
   a) Tree growth
   b) Poor seasoning
   c) Photosynthesis
   d) Proper storage
5. __________ is the process of applying chemicals to protect wood.
   a) Seasoning
   b) Conversion
   c) Preservation
   d) Felling
6. One method of timber preservation is the use of __________.
   a) Water
   b) Salt
   c) Coal tar
   d) Fire
7. The chemical __________ is used to prevent decay and insect attack.
   a) Salignum
   b) Water
   c) Vinegar
   d) Oil
8. The process of wood bending along its length is called __________.
   a) Cupping
   b) Shrinking
   c) Bowing
   d) Twisting
9. __________ helps timber to last longer.
   a) Felling
   b) Sanding
   c) Preservation
   d) Seasoning

(Continue with more questions using a similar format.)

Class Activity Discussion:

1. What is a wood defect?
   A wood defect is a fault or problem in the wood that makes it less valuable or strong.
2. How do natural defects happen?
   Natural defects happen while the tree is still growing, like heart shakes.
3. What causes artificial defects?
   Artificial defects are caused by bad handling after the tree is cut, such as poor seasoning.
4. What is warping in wood?
   Warping occurs when the wood twists or bends out of shape.
5. Why is timber preserved?
   Timber is preserved to protect it from insects, fungi, and moisture.
6. What is cupping in wood?
   Cupping is when the wood curves like a bowl, creating a hollow surface.
7. What are the types of wood preservatives?
   The common types include oil preservatives, coal tar, and salignum.
8. How does preservation make timber stronger?
   Chemicals used in preservation make the wood more durable and resistant to damage.
9. What is heart shake?
   Heart shake is a crack that starts from the center of the tree and moves outward.
10. Why is seasoning important?
    Seasoning helps to remove moisture from the wood, preventing defects like shrinking.
11. What happens if timber is not preserved?
    If timber is not preserved, it can be attacked by insects, fungi, or decay quickly.
12. What does shrinking do to the wood?
    Shrinking makes the wood lose its size, often due to bad seasoning.
13. What is salignum used for?
    Salignum is a chemical used to protect wood from insects and fungi.
14. Can defective wood still be used?
    Defective wood is weaker and may not be suitable for strong furniture or construction.
15. How does coal tar help timber?
    Coal tar helps protect timber from moisture and decay.

Presentation

Step 1: The teacher revises the previous topic on the stages of wood processing.

Step 2: The teacher introduces wood defects and explains the types (natural and artificial) with examples.

Step 3: Students share their experiences of seeing defective wood, and the teacher corrects their contributions.

Activities

Teacher’s Activities:

• Display pictures and samples of defective wood.
• Explain the causes of natural and artificial wood defects.
• Discuss the importance of timber preservation.

Learner’s Activities:

• Observe the samples of defective wood.
• Participate in the discussion on defects and preservation.
• Answer questions on the importance of timber preservation.

Assessment

Evaluation Questions:

1. Define wood defects.
2. List two examples of natural wood defects.
3. What causes artificial wood defects?
4. Explain the importance of preserving timber.
5. What is warping in wood?
6. How does preservation increase the lifespan of timber?
7. Name two chemicals used in preserving timber.
8. What is heart shake in wood?
9. Why is bad seasoning dangerous to wood?
10. What happens during the shrinking of wood?

Conclusion:

The teacher will move around the class, check students’ work, and provide necessary corrections. The lesson will be concluded with a recap of the key points.