Lesson Notes By Weeks and Term v3 - Junior Secondary 3
Processing of Materials (a) Timber
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Processing of Materials (a) Timber
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Subject: Basic Technology
Class: Junior Secondary 3
Term: 1st Term
Week: 1
Theme: Materials And Processing
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define key terms and concepts related to timber processing; describe the properties of good timber; identify common timber defects; explain treatments and preservation methods for timber; state the importance of timber treatments.
This section provides in-depth explanations of the core concepts related to timber processing.
A. Definition of Key Terms Timber: This refers to wood as a raw material, either standing trees suitable for felling, or felled trees and logs before conversion. In a broader sense, it also refers to wood that has been processed into planks and beams for building or carpentry purposes.
Example: A large Iroko tree in the forest is timber. Logs cut from a felled tree are also timber.
Lumber: This term is often used interchangeably with "timber" but specifically refers to wood that has been sawn into planks, boards, or other structural forms ready for use.
Example: The planks sold at a sawmill are lumber.
Wood Processing: This is a comprehensive term that describes the entire sequence of operations involved in converting timber from a felled tree into a useful product. It typically includes felling, debarking, sawing, seasoning, and preservation.
Seasoning: The controlled process of removing moisture from freshly cut timber. This is done to reduce its water content to an acceptable level, making it more stable and durable.
Example: Allowing planks to air-dry under a shed for several months.
Preservation: The application of chemical or physical treatments to timber to protect it from decay (fungi), insect attacks (termites, borers), and weathering, thereby extending its service life.
Example: Painting a wooden fence with creosote or applying anti-termite chemicals to roof timbers. B. Properties of Good Timber The quality of timber is determined by several desirable properties that influence its suitability for different applications.
1. Strength and Toughness: Good timber should be strong enough to withstand loads (compressive, tensile, shear) and tough enough to resist sudden impacts or shocks without breaking. Nigerian
Example: Iroko wood is highly valued for its strength in structural applications like beams and pillars.
2. Durability (Resistance to Decay): It should naturally resist attack from fungi, insects (like termites and wood borers), and adverse weather conditions. Naturally durable timbers last longer without extensive treatment. Nigerian
Example: Teak and Mahogany are known for their natural durability against pests and decay.
3. Workability: Good timber should be relatively easy to cut, saw, plane, chisel, and nail without excessive effort or splintering. Nigerian
Example: Obeche is prized for its excellent workability, making it suitable for furniture and carvings.
4. Appearance: The grain, colour, and texture contribute to the aesthetic appeal, especially for furniture and decorative uses. Nigerian
Example: Padauk has an attractive reddish-orange colour, making it desirable for decorative paneling.
5. Density: Dense timbers are generally stronger and harder but can be heavier and harder to work. Less dense timbers are lighter and easier to work. The appropriate density depends on the intended use.
6. Low Moisture Content (after seasoning): Properly seasoned timber has a stable moisture content, which prevents warping, cracking, and decay.
7. Resistance to Fire: While all wood burns, some species or treated woods have better fire resistance than others.
8. Freedom from Defects: Good timber should ideally be free from common defects that compromise its strength, durability, or appearance.
C. Common Timber Defects Timber defects can be classified into two main categories: natural defects and seasoning defects. These defects can reduce the strength, durability, and aesthetic value of the timber.
1. Natural Defects (occur during tree growth): Knots: These are sections of branches that have become embedded in the main trunk as the tree grows. They are usually harder than the surrounding wood and can weaken the timber, making it difficult to work and affecting its appearance.
Shakes: These are longitudinal cracks or separations in the wood fibers, often following the growth rings. They can be Ring Shakes (separation along an annual ring) or Heart Shakes (splits radiating from the pith). Shakes reduce the shear strength of timber.
Wane: This refers to the presence of bark or rounded wood at the edge or corner of a sawn timber piece. It occurs when the saw cut does not fully remove the outer curvature of the log, indicating an un-sawn portion. * Sap Stain: A discoloration (blue, grey, or black) caused Preservatives often used: CCA (Chromated Copper Arsenate): A highly effective, long-lasting preservative, though its use for residential applications is restricted in some countries due to arsenic content. Still used for exterior applications like utility poles and marine piling in some regions. Tanalith (Alkaline Copper Quat - ACQ, Copper Azole - CA): Newer generation, arsenic-free alternatives to CCA, offering good protection against fungi and insects.
Nigerian Context: Used for railway sleepers, electricity poles, structural timbers exposed to harsh weather or ground contact, common in industrial timber yards.
Creosoting: Applying creosote (a coal-tar derivative) by hot or cold dipping, brushing, or pressure treatment. It's a dark, oily substance highly effective against decay and insects, especially for timber in ground contact or marine environments.
Nigerian Context: Primarily used for railway sleepers, fence posts, and marine timbers. * Oil-borne Preservatives: Solutions of fungicides and insecticides dissolved in petroleum oils. Examples include pentachlorophenol (PCP), though its use is also restricted in some places. E. Importance of Timber Treatments Treatments and preservation methods are vital for maximizing the utility and lifespan of timber.
1. Increased Durability: Treated timber can resist decay, insect attack (termites, borers), and fungi, significantly extending its service life. This is crucial in tropical climates like Nigeria where these threats are prevalent.
2. Enhanced Strength and Stability: Seasoning removes moisture, reducing swelling, shrinking, warping, and cracking, making the timber dimensionally stable and stronger for structural applications.
3. Cost-Effectiveness: While treatments add initial cost, they prevent premature replacement of timber structures and products, leading to long-term savings.
4. Improved Appearance: Treatments like varnishing and painting protect the surface, enhance the natural grain, and allow for aesthetic finishes, making the timber more attractive for furniture and decorative uses.
5. Safety and Health: Preserving timber in structural applications (e.g., roof trusses) prevents weakening and structural failure, ensuring safety. Proper treatment can also prevent fungal growth that might impact indoor air quality.
6. Wider Range of Applications: Untreated timber is limited in its uses. Treated timber can be used in harsh environments (ground contact, marine, high humidity) where untreated timber would quickly deteriorate. These are longitudinal cracks or separations in the wood fibers, often following the growth rings. They can be Ring Shakes (separation along an annual ring) or Heart Shakes (splits radiating from the pith). Shakes reduce the shear strength of timber.
Wane: This refers to the presence of bark or rounded wood at the edge or corner of a sawn timber piece. It occurs when the saw cut does not fully remove the outer curvature of the log, indicating an un-sawn portion.
Sap Stain: A discoloration (blue, grey, or black) caused by fungi feeding on sapwood. It doesn't usually affect strength but degrades appearance.
Decay/Rot: Caused by fungi attack while the tree is standing or after felling due to high moisture content. It leads to the decomposition of wood fibers, significantly weakening the timber.
2. Seasoning Defects (occur during improper drying): Warping: A general distortion of the timber from its true shape.
It can manifest as: Bowing: A curve along the length of the timber.
Cupping: A curve across the width of the timber, making it concave on one face.
Twisting: A distortion in which the ends of the timber rotate in opposite directions.
Checking: Small cracks that appear on the surface of the timber, running across the grain. They are superficial but can deepen.
Splitting: Deeper cracks that extend from one face of the timber to another, often occurring at the ends.
Honeycombing: Internal cracks or separations that develop within the timber, not visible from the outside. Caused by rapid drying of the outer layers while the core remains wet.
Case Hardening: The outer layers of timber dry and shrink, becoming rigid and stressed, while the inner core remains moist. This can lead to internal stresses and subsequent warping or checking during further processing. D. Treatments and Preservation Methods for Timber Timber treatment and preservation aim to enhance its durability and stability.
1. Seasoning (Moisture Removal): This is the most fundamental treatment.
Purpose: To reduce the moisture content to an equilibrium state, preventing shrinkage, warping, checking, decay, and making it suitable for finishing.
Methods: Air Seasoning (Natural Seasoning): Timber is stacked in open-sided sheds or well-ventilated areas, protected from direct sunlight and rain. Air circulates freely, gradually drying the wood. This method is slow (months to years depending on thickness) but cost-effective and produces timber with minimal internal stress.
Nigerian Context: Commonly used by small-scale sawmills and local carpenters due to low cost and reliance on natural weather conditions.
Kiln Seasoning (Artificial Seasoning): Timber is placed in large, insulated chambers (kilns) where temperature, humidity, and airflow are precisely controlled. This method is much faster (days to weeks), allows for specific moisture content targets, and kills insects/fungi present in the wood.
However, it is more expensive due to energy consumption and equipment cost.
Nigerian Context: Used by larger timber processing companies for specific applications requiring precise moisture content, such as furniture exports.
2. Preservation (Protection from Pests and Decay): Purpose: To protect seasoned timber from biological attack (termites, borers, fungi) and weathering.
Methods: Surface Application: Applying preservatives by brushing, spraying, or dipping. This provides superficial protection. Nigerian
Example: Applying wood paint, varnish, or insecticide solutions (e.g., termiticide) to wooden doors, windows, or furniture. Pressure Impregnation (e.g., CCA, Tanalith): This is a highly effective method where timber is placed in a sealed cylinder, and a vacuum is applied to remove air from the wood cells. Preservative solution is then introduced under high pressure, forcing it deep into the timber pores.
Preservatives often used: CCA (Chromated Copper Arsenate): A highly effective, long-lasting preservative, though its use for residential applications is restricted in some countries due to arsenic content. Still used for exterior applications like utility poles and marine piling in some regions. Tanalith (Alkaline Copper Quat - ACQ, Copper Azole - CA): Newer generation, arsenic-free alternatives to CCA, offering good protection against fungi and insects.
Nigerian Context: Used for railway sleepers, electricity poles, structural timbers exposed to harsh weather or ground contact, common in industrial timber yards. * Creosoting: This section outlines practical activities for both teachers and students to facilitate understanding.
Teacher Activities: Introduction (10 mins): Begin by asking students about common wooden items they see daily (doors, tables, chairs, roof trusses, canoes, mortar and pestle). Lead a brief discussion on where these materials come from and how they become usable.
Introduce the topic: "Processing of Materials (a) Timber" and state the learning objectives clearly. Concept Explanation and Demonstration (30 mins): Definitions: Explain key terms (timber, lumber, seasoning, preservation) using simple language and relate them to local examples.
Properties of Good Timber: Discuss each property. If possible, show samples of different Nigerian timbers (e.g., a piece of Iroko for strength, Obeche for workability, a plank with an attractive grain). Ask students to feel, observe, and describe differences.
Timber Defects: Use diagrams, pictures, or actual timber off-cuts (if available) to show examples of knots, shakes, wane, warping, checking, and splitting. Explain how each defect affects the timber's quality.
Treatments and Preservation: Explain seasoning methods (air vs. kiln) with diagrams or descriptions of local practices. Explain preservation methods (surface application, pressure treatment, creosoting). Show examples of wood products treated with paint/varnish. Discuss the use of treated electricity poles or railway sleepers as examples of pressure treatment/creosoting.
Importance of Treatments: Summarize the benefits, relating them to the longevity of wooden structures in the local community.
Interactive Q&A and Discussion (15 mins): Facilitate a class discussion, encouraging students to ask questions and share their observations from their homes or communities regarding timber use and maintenance.
Pose questions like: "Why do some wooden doors last longer than others?" or "What do carpenters do to make wood smooth?" Activity Guidance (5 mins): Assign a short group activity or class exercise to reinforce understanding.
Student Activities: Active Listening and Note-taking: Students listen attentively to explanations and take concise notes on key definitions, properties, defects, and treatment methods.
Observation and Identification: If samples are available, students observe and handle different timber pieces, noting their characteristics (colour, grain, density, workability if tools are safe to use under supervision). Identify visual examples of timber defects presented by the teacher.
Participation in Discussions: Students actively participate in Q&A sessions, sharing their understanding, asking clarifying questions, and contributing local examples. Group Work/Paired Activity (e.g., "Think-Pair-Share"): Students could discuss in pairs or small groups: "List three uses of timber in your home and how they are protected." "Discuss why a carpenter would prefer seasoned wood over fresh wood." Drawing/Sketching: Students could sketch simple diagrams of different timber defects or methods of air seasoning as presented by the teacher.
Construction and Housing: The knowledge of timber processing is directly applicable to the Nigerian construction industry. Students can understand why specific types of wood (e.g., Iroko for strength, Mahogany for durability) are chosen for roof trusses, doors, window frames, and flooring in Nigerian homes. Understanding defects helps in inspecting the quality of timber used in building projects. The use of treated timber for structures exposed to weather or ground (like fence posts or electricity poles common in rural areas) highlights the importance of preservation against termites and rot.
Furniture and Craftsmanship: Nigeria has a vibrant furniture-making industry and rich tradition of wood carving. Students can connect the properties of good timber (workability, appearance, durability) to the production of high-quality chairs, tables, beds, and decorative carvings. They will appreciate why seasoned and well-preserved timber results in long-lasting and aesthetically pleasing furniture, distinguishing between local untreated pieces and commercially treated ones.
Environmental and Economic Impact: This topic integrates with environmental awareness by discussing responsible sourcing of timber and the importance of sustainable forestry (though not explicitly covered in this lesson, it's a natural extension). Economically, understanding timber quality and preservation impacts local timber businesses, sawmills, and carpenters, affecting the cost and lifespan of wooden products available in Nigerian markets. For instance, the demand for properly seasoned and treated wood drives the growth of modern timber processing facilities.