Timber Seasoning Techniques Quiz

Timber and Wood-Based Products

• Wood is a hard, fibrous substance forming the trunk and branches of trees.
• Timber is wood retaining its natural structure, suitable for engineering.
• Lumber is roughly sawn and cut wood, ready for use.

Introduction to Trees

• Trees consist of roots, trunk, and crown.
• In autumn, sap descends and forms cambium layers.
• Cambium layers harden to form concentric rings.
• Cambium cells (fibers) are cemented by lignin.
• Arrangement of cells and fibers determines wood grain.

Tree Anatomy

• Annual Rings: Visible rings formed by cambium growth layers.
• Pith: The central core of a tree trunk.
• Heartwood: The central, older, and often darker wood.
• Sapwood: The outer, lighter wood that transports water.
• Cambium: Layer between heartwood and sapwood where new cells are produced.
• Bark: Outer protective layer of the tree.

Tree Grain

• Rapid growth: Wide annual rings result in coarse-grained wood.
• Slow growth: Narrow annual rings lead to fine-grained wood.
• Straight-grained: Wood elements run parallel to the pith.
• Cross-grained: Elements do not run parallel to the pith axis

Timber Classification

• Position based:

  • Standing timber: Living tree.
  • Rough timber: Part of a felled tree.
  • Converted timber: Sawed logs (lumber).

• Elasticity Modulus based:

  • Group A: Modulus above 12.5 kN/mm².
  • Group B: Modulus above 9.8 kN/mm² & below 12.5 kN/mm².
  • Group C: Modulus above 5.6 kN/mm² & below 9.8 kN/mm².

• Seasoning Characteristics:

  • Highly refractory (Class A): Slow, difficult to season, no surface defects.
  • Moderately refractory (Class B): Seasonable, no surface defects.
  • Non-refractory (Class C): Rapid seasoning, no defects.

• Treatability:

  • Classifies resistance of heartwood to preservatives.
    • Easily treatable
    • Treatable but complete preservation is difficult
    • Partially treatable
    • Very refractory to treatment

Characteristics of Good Timber

• Narrow annual rings
• Compact medullary rays
• Dark color
• Uniform texture
• Sweet smell, shining fresh cut surface
• Sonorous sound on impact
• Free from defects
• Heavy weight
• No woolliness at fresh cut surface

Wood Properties

• Physical: Density, specific weight, bulk density, moisture movement, shrinkage, swelling, heat conductivity, sound conductivity, resistance to acids and alkalis.
• Mechanical: Compressive strength, tensile strength, bending strength, stiffness, toughness, cleavability, hardness.

Testing of Timber

• Methods: Specific Gravity, Volume Shrinkage, Radial and Tangential Shrinkage, Static Bending strength, Impact Bending, Compressive Strength, Tensile Strength, Cleavage, Brittleness, Torsional, Moisture Content Tests

Timber Seasoning

• Seasoning: Process to reduce timber moisture content for preventing fermentation and suitability for use.

• Objectives: Reducing shrinkage and warping, increasing strength and durability, reducing tendency for splitting, making suitable for painting, reducing weight.

• Natural/Air Seasoning: Logs stacked in rows in a shed, slow drying rate.

• Artificial Seasoning:

  • Water Seasoning: Soaking logs in water to leach out sap, followed by air drying.
  • Boiling: Quick, expensive method to remove sap, followed by drying.
  • Kiln Seasoning: Large-scale rapid drying in heated chambers using circulating air.
  • McNeill's Process: Timber stacked, combusted fuel in the fire place, requires 15-60 days.
  • Chemical or Salt Seasoning: Aqueous chemical solutions with lower vapor pressure than pure water.

Timber Preservation

• Preservation: Adds extra protection to seasoned timber using preservatives to prevent decay, water absorption, or attacks from fungi, water, insects etc.
• Methods:
  • Surface application: Dipping, spraying, or brushing preservative onto debarked timber.
  • Soaking treatment: Submerging debarked timber in preservative solution.
  • Hot and Cold Process: Two baths (hot water and preservative) are used for sterilization against insects and fungi.
  • Boucherie process: Logs attached via hose pipes to a reservoir with preservatives to replace sap through hydrostatic pressure.

Defects in Timber

• Defects often occur during the growing, conversion or seasoning process.
• Abnormal Growth Defects:
  • Checks: Longitudinal cracks perpendicular to the rings.
  • Shakes: Longitudinal separations between annual rings. (e.g., heart shake, cup shake, star shake)
• Other Defects:
  • Rind gall: Swelling caused by sap over wounds.
  • Knots: Bases of twigs or branches embedded in the stem.
  • End splits: Splits at the ends of logs.
  • Twisted fibres: Fibres twisted by constant wind during growth.
  • Upsets: Crushing of fibers during growth or felling.
  • Foxiness: Yellow or red discoloration due to decay.
  • Rupture: Damage caused by injury or impact.
  • Defects from conversion practice: Defects due to poor milling or attempts to economize.
  • Defects from seasoning: Errors in moisture content from uneven or inadequate drying.
• Common seasoning defects: Checks, splittings, warpage (e.g., cupping, twisting, bowing)

Decay in Timber

• Timbers are affected by fungi, insects, rodents, weather, and chemicals.
• Sapwood is more vulnerable to decay than heartwood.
• Fungal/Bacterial attack: Fungi produce spores that create mycelia that destroy wood tissue. Require moisture.
• Insect attack: Varying levels and types of insects can affect timber.
• Rodent damage: Rodents can penetrate both wood and concrete.

Wood Products

• Wood-based products are developed to use timber efficiently.
• Manufactured under controlled conditions, with desired shapes, dimensions, strength, and durability.
• Raw materials come from timber waste or unused tree parts.
• Gluing material influences product durability

Veneer (IS 14315)

• Primary process in manufacturing wood-based products.
• Veneer thickness ranges from 0.4mm - 0.6mm.
• Logs stored in cold storage to avoid splitting
• Barks removed and cut into veneers
• Two types: Rotary and Sliced veneer
• Defectives based on surface quality (A and B grades) based on discoloration, dotes, etc.

Plywood (general)

• Layers of veneer glued together in odd numbers (3,5,7).
• Improves dimensional stability. , stiffness and strength
• Outer layers are known as faces
• Interior layers are known as the cores.
• Plywood categories are based on appearance, bond strength, moisture resistance

Plywood Manufacturing Process

• Veneers are dried to specific moisture content,
• Cooled, to regulate air temp,
• Veneers with moisture issues will be redried,
• Veneer edges may be glued to create larger sheets (composing process).
• Veneers may undergo a finishing process with edges trimmed, and surfaces sanded.

Plywood Merits

• Good strength and stability.
• Reduces shrinkage and swelling.
• Better resistance to splitting.
• Superior strength-to-weight ratio to many materials

Block Boards (IS 1659)

• Solid core plywood boards made from wood strips or slabs.
• Veneers glued between wood pieces.
• Veneers can be rotary or sliced.
• Thickness of cross-bands range from 1 - 3mm.
• Different grades based on construction and intended use (interior or exterior applications).

Particle Boards (IS 3129, 12406, 3478)

• Made from wood chips, or other cellulose materials,
• Materials pressed together, using organic binder.
• Created from wood waste.