Glossary

Navigating the world of woodworking science, wood materials, and wood behavior can be confusing. Wood Work Science has compiled this comprehensive A–Z glossary to help beginners and experts alike understand the terminology used in our guides, tests, and reference articles.

A

Anisotropy: Definition: The property of wood where mechanical and physical behaviors vary with direction (longitudinal, radial, tangential). This directional dependence drives different shrinkage, strength, and permeability values.
See also: Grain (Direction), Radial Shrinkage, Tangential Shrinkage.
Annual Ring: Definition: The growth ring formed in one growing season, typically showing earlywood and latewood zones. Ring patterns inform species identification and strength/density variation.
Context: Critical in reading grain for sawing patterns and dimensional stability.
Air-Drying: Definition: Natural drying of lumber under ambient conditions to reduce moisture content before further processing. Rate and final moisture depend on climate, airflow, thickness, and species.
See also: Kiln Drying, EMC, Case-Hardening.

B

Board Foot (BF): Definition: A volume measure equal to 1 in × 12 in × 12 in of lumber. Used in pricing, yield calculations, and mill tally.
See also: Quarters (Thickness), Yield (Lumber Recovery).
Brashness: Definition: Abnormal brittleness of wood characterized by sudden failure with little plastic deformation. Often associated with defects, over-drying, or certain growth characteristics.
Context: Important in impact and shock-loading applications.
Bound Water: Definition: Water held within the cell wall matrix by hydrogen bonding; removal causes shrinkage below fiber saturation point (FSP). Distinct from free water in lumens.
See also: FSP, Moisture Content (MC), EMC.

C

Case-Hardening: Definition: Residual stress pattern from improper drying where the shell is in tension and the core in compression (or vice versa), leading to cutting and machining distortions. Detected by prong tests and relieved by conditioning.
See also: Kiln Drying, Moisture Gradient, Cupping.
Cupping: Definition: A form of warp where a board curves across its width, typically high on the edges and low in the center. Driven by tangential vs radial shrinkage differences and moisture gradients.
Context: Common in flat-sawn stock and moisture-imbalanced installations.
Cellulose: Definition: The primary structural polysaccharide of wood cell walls forming microfibrils that confer tensile strength. Comprises roughly 40–50% of wood by mass in many species.
Synonyms: β-1,4-glucan (polymer backbone).

D

Density (Oven-dry): Definition: Mass of oven-dried wood per unit volume, typically expressed at a reference moisture basis. Strong predictor of mechanical properties and energy content.
See also: Specific Gravity, Moisture Content (MC).
Diffusion (Moisture Diffusion): Definition: The movement of bound water within wood cell walls driven by concentration gradients. Governs drying rates below FSP.
Context: Influenced by temperature, species, and microstructure.
Delignification: Definition: Chemical removal or modification of lignin, as in pulping or bleaching processes. Excess delignification reduces stiffness, hardness, and durability.
See also: Lignin, Extractives.

E

Equilibrium Moisture Content (EMC): Definition: The moisture content at which wood is in hygroscopic equilibrium with ambient temperature and relative humidity. Determines in-service dimensional stability.
See also: MC, FSP, Hygroscopicity.
End Grain: Definition: The surface exposed by cutting across the grain, showing growth rings and pores. Highly permeable and prone to rapid moisture exchange and end-checking.
Context: Requires sealing to control drying defects.
Extractives: Definition: Low-molecular-weight compounds (oils, resins, tannins) not part of the structural cell wall. Affect color, odor, natural durability, gluing, and finishing.
See also: Natural Durability, VOCs.

F

Fiber Saturation Point (FSP): Definition: The moisture content at which cell walls are saturated with bound water but lumens contain no free water (typically ~30% MC). Below FSP, wood shrinks and strength changes with MC.
See also: Bound Water, EMC.
Figure: Definition: The visual pattern on sawn surfaces arising from grain orientation, rays, and cellular structures (e.g., curl, fiddleback, ray fleck). Independent of color or stain.
Context: Enhanced by quarter-sawing and finishing techniques.
Flat-sawn (Plain-sawn): Definition: Lumber sawn with growth rings roughly tangent to the wide face, yielding wider boards and prominent cathedral grain. More prone to cupping and higher tangential movement.
See also: Quarter-sawn, Radial/Tangential Shrinkage.

G

Grain (Direction): Definition: The orientation of wood fibers relative to a reference axis; properties vary strongly parallel vs perpendicular to grain. Determines machining behavior and strength.
Synonyms: With the grain, cross-grain.
Green Wood: Definition: Wood in a freshly cut state with MC above FSP. Heavier, weaker in some modes, and susceptible to biological attack.
See also: MC, FSP, Kiln Drying.
Glue Line: Definition: The thin adhesive layer bonding wood surfaces in an assembly. Its strength depends on surface preparation, adhesive chemistry, pressure, and curing conditions.
Context: Critical in engineered wood products and joinery.

H

Hardwood: Definition: Wood from angiosperm trees (broadleaves), often with vessels and diverse cell types. Not necessarily harder than softwoods; properties vary widely by species.
See also: Softwood, Vessel Elements, Quercus (Oak).
Hygroscopicity: Definition: The tendency of wood to absorb and desorb moisture from air until EMC is reached. Drives dimensional change and many serviceability issues.
See also: EMC, MC, Shrinkage.
Heartwood: Definition: The non-living, inner wood often enriched with extractives, typically darker and more decay-resistant than sapwood. Lower permeability and different finishing behavior.
Context: Influences durability, color matching, and adhesive choice.

I

Interlocked Grain: Definition: A grain pattern where fiber direction alternates in successive growth increments, leading to ribbon figure on quartered faces. Difficult to plane; prone to tear-out.
See also: Figure, Quarter-sawn.
Impact Bending: Definition: Resistance of wood to sudden loads or shocks, often measured by pendulum tests. Relates to toughness and brashness.
Context: Important for tool handles, sports equipment, and pallets.
Incising: Definition: Pre-treatment process of making small slits in wood surfaces to improve preservative penetration. Common for refractory species before pressure treatment.
See also: Permeability, Preservative Treatment.

J

Juvenile Wood: Definition: Wood formed near the pith during early growth, typically with lower density, higher longitudinal shrinkage, and reduced strength. Increases warp risk.
See also: Pith, Longitudinal Shrinkage.
Janka Hardness: Definition: A standardized measure of surface hardness based on the force to embed a steel ball to half its diameter. Useful for flooring wear resistance comparisons.
Context: Correlates with density but varies by species and orientation.
Joinery (Mechanical Performance): Definition: The science-informed design of wood joints considering anisotropy, glue-line behavior, and fastener withdrawal. Proper moisture conditioning is essential to minimize stress.
See also: Glue Line, Grain (Direction), MC.

K

Kerf: Definition: The width of material removed by a saw blade. Impacts yield and heat generation during cutting.
Context: Narrow-kerf blades improve recovery but may deflect in dense species.
Kiln Drying: Definition: Controlled drying using heat, humidity, and airflow schedules to reach target MC and relieve stresses. Faster and more uniform than air-drying when properly managed.
See also: EMC, Case-Hardening, Conditioning.
Knot: Definition: The basal portion of a branch embedded in the trunk, appearing as a localized defect. Reduces clear wood area and can concentrate stresses or cause machining issues.
Synonyms: Sound knot, loose knot (by integrity).

L

Lignin: Definition: A complex aromatic polymer that cements cell wall components, conferring stiffness, compressive strength, and decay resistance. Absorbs UV and affects color stability.
See also: Cellulose, Hemicelluloses (Xylan).
Longitudinal Shrinkage: Definition: Dimensional change along the grain; typically very small but can be significant in juvenile and reaction wood. Influences end-checking and lengthwise warp.
See also: Reaction Wood, Juvenile Wood.
Laminated Veneer Lumber (LVL): Definition: An engineered wood product made by bonding thin veneers with parallel grain alignment for high strength and stiffness. Offers predictable properties and efficient resource use.
Context: Common in beams, headers, and rim boards.

M

Modulus of Elasticity (MOE): Definition: A measure of stiffness (slope of stress–strain in elastic region), often reported parallel to grain. Predicts deflection under load.
See also: MOR, Young’s Modulus (Dynamic).
Modulus of Rupture (MOR): Definition: An estimate of maximum bending strength derived from flexural tests. Sensitive to defects, MC, and density.
Context: Used in design values and grading.
Moisture Content (MC): Definition: The mass of water in wood expressed as a percentage of oven-dry mass. Central to dimensional stability, strength, and bonding performance.
See also: EMC, FSP, Hygroscopicity.

N

Nominal Size: Definition: The stated dimensions of lumber prior to surfacing (e.g., 2×4), not the actual finished size. Standards define the actual dimensions by region.
Context: Important for layout, load calculations, and purchasing.
Natural Durability: Definition: Inherent resistance to decay and insects due to extractives and anatomical factors. Varies widely; heartwood typically more durable than sapwood.
See also: Extractives, Preservative Treatment.
Neutral Axis: Definition: The plane within a bending member where fiber stress is zero during pure bending. Its position shifts with asymmetry and composite layups.
Context: Relevant in beam design and laminated members.

O

Oven-dry Weight: Definition: The mass of wood after drying at 103±2°C until constant weight, used as the reference for MC and specific gravity calculations. Essential for accurate material property comparisons.
See also: MC, Specific Gravity.
Oriented Strand Board (OSB): Definition: Engineered panel made from oriented wood strands bonded with adhesives under heat and pressure. Exhibits directional stiffness and strength.
Context: Common in sheathing and subfloors.
Oil Finish (Penetrating): Definition: Finishes such as linseed, tung, or polymerized oils that penetrate cell walls and lumens to enhance figure and provide moderate protection. Film build and VOC profile vary by formulation.
See also: VOCs, Uptake.

P

Permeability: Definition: The ease with which fluids move through wood, governed by pores, pits, and ray pathways. Sapwood is typically more permeable than heartwood.
See also: Vessel Elements, Tyloses, Preservative Treatment.
Preservative Treatment: Definition: Processes (e.g., pressure impregnation) that introduce biocides to improve decay and insect resistance. Effectiveness depends on retention, penetration, and species treatability.
Context: Often preceded by incising for refractory woods.
Pith: Definition: The central core of the stem consisting of early tissue; often associated with juvenile wood. Can be a site of checking and weakness in small-diameter material.
See also: Juvenile Wood.

Q

Quarter-sawn: Definition: Lumber sawn so growth rings are roughly perpendicular (60–90°) to the wide face, improving dimensional stability and wear. Highlights ray fleck in species like oak.
See also: Flat-sawn, Radial Shrinkage, Figure.
Quercus (Oak): Definition: A genus characterized by prominent rays and, in white oaks, tyloses that reduce permeability. Known for high tannin content and pronounced ray fleck on quartered faces.
Context: Species-specific properties affect finishing and bonding.
Quarters (Thickness Designation): Definition: Lumber thickness reporting in quarter inches (e.g., 4/4 = 1 in rough). Affects drying schedules and machining parameters.
See also: Board Foot, Kiln Drying.

R

Radial Shrinkage: Definition: Dimensional change from the pith outward (perpendicular to growth rings), typically less than tangential shrinkage. Ratio to tangential shrinkage predicts warp tendencies.
See also: Tangential Shrinkage, Anisotropy.
Reaction Wood: Definition: Abnormal wood formed to reorient leaning stems or branches: compression wood in softwoods and tension wood in hardwoods. Exhibits altered density, shrinkage, and machining behavior.
Context: Increases warp and processing challenges.
Resin Canal: Definition: Tubular intercellular spaces in some softwoods (e.g., pine) that contain resin. Impact permeability, pitch bleed, and finishing.
Synonyms: Resin ducts.

S

Specific Gravity (SG): Definition: The ratio of oven-dry wood density to water density, often standardized for MC and volume basis. Correlates strongly with strength and hardness.
See also: Density (Oven-dry), Janka Hardness.
Shear Strength: Definition: Resistance to sliding failure along a plane within the wood, measured parallel or perpendicular to grain. Critical for joints and fastener performance.
Context: Influenced by MC and defects.
Sapwood: Definition: The outer, living wood that conducts sap; typically lighter and more permeable than heartwood. Generally less durable to decay.
See also: Heartwood, Permeability.

T

Tangential Shrinkage: Definition: Dimensional change along the tangent to growth rings; typically the largest shrinkage component. Drives cupping in flat-sawn boards.
See also: Radial Shrinkage, Anisotropy.
Tyloses: Definition: Balloon-like outgrowths from ray parenchyma that block vessel elements, reducing permeability. Contribute to white oak’s liquid-tightness.
Context: Affects finishing, drying, and treatment.
Thermal Modification: Definition: Heat treatment of wood (typically 160–220°C in limited oxygen) to improve dimensional stability and biological resistance. Can reduce strength and darken color.
See also: Extractives, Hygroscopicity.

U

Urea-Formaldehyde (UF) Resin: Definition: A thermosetting adhesive widely used in interior panels; low cost and good cure speed but limited moisture resistance. Formaldehyde emissions are regulated.
See also: VOCs, Glue Line.
Uptake: Definition: The absorption of finishes or preservatives into wood, dependent on permeability, viscosity, and surface preparation. End grain shows highest uptake.
Context: Influences coating performance and retention.
Ultrasonic Testing: Definition: Non-destructive evaluation using acoustic wave velocity to estimate stiffness and detect defects or density variation. Useful for sorting and QA in engineered products.
See also: MOE, X-ray Densitometry.

V

Veneer: Definition: Thin sheets of wood produced by slicing or rotary peeling, used for plywood, LVL, and decorative faces. Veneer quality affects bond and panel performance.
See also: LVL, OSB (contrast in strand vs veneer).
Volatile Organic Compounds (VOCs): Definition: Organic chemicals that evaporate at room temperature, originating from wood extractives and finishes/adhesives. Impact indoor air quality and regulatory compliance.
Context: Low-VOC formulations mitigate emissions.
Vessel Elements: Definition: Tube-like cells in hardwoods that conduct water; diameter and distribution influence permeability and appearance. Visible as pores on end grain.
See also: Tyloses, Hardwood.

W

Warp: Definition: Any deviation from a true plane, including cup, bow, crook, and twist. Caused by anisotropic shrinkage, growth stresses, and moisture gradients.
See also: Cupping, Reaction Wood.
Wane: Definition: The presence of bark or lack of wood on the edge or corner of a board. Affects grading and yield but not inherent material properties.
Context: Common in live-sawn edges.
Weathering: Definition: Surface degradation from sunlight, moisture cycling, and temperature, leading to graying, checking, and erosion. UV primarily degrades lignin at the surface.
See also: Lignin, Finishes, Thermal Modification.

X

Xylem: Definition: The woody tissue of trees responsible for water conduction and structural support; the material we call wood. Composed of tracheids, vessel elements, fibers, and parenchyma.
Context: Distinct from phloem (inner bark).
Xylan (Hemicellulose): Definition: A major hemicellulose component of hardwoods influencing moisture sorption and thermal degradation. More susceptible to hydrolysis than cellulose.
See also: Cellulose, Lignin.
X-ray Densitometry: Definition: Non-destructive measurement of density profiles using X-rays, often in panels and growth ring analysis. Correlates with strength and stiffness variations.
Context: Used in QA and research.

Y

Young’s Modulus (Dynamic): Definition: Elastic modulus estimated from vibration or acoustic methods, reflecting stiffness along the grain. Useful for rapid sorting of lumber and instruments.
See also: MOE, Ultrasonic Testing.
Yield (Lumber Recovery): Definition: The proportion of usable lumber obtained from logs after sawing and processing. Influenced by kerf, sawing pattern, defects, and target sizes.
Context: A primary mill efficiency metric.
Yellowing (Finish): Definition: Color shift of coatings or wood surfaces toward yellow due to UV exposure, oxidation, or binder chemistry. More pronounced in oil-based finishes.
See also: Lignin, VOCs, Weathering.

Z

Zone Lines (Spalting): Definition: Dark demarcation lines formed by fungal interactions in wood, prized for decorative effect. Indicate incipient decay and may reduce strength.
Context: Managed use in turnery and veneer with caution.
Zinc Borate: Definition: A boron-based preservative and flame-retardant used in composites and treated wood for decay and insect resistance. Offers low leachability in many matrices.
See also: Preservative Treatment, Natural Durability.
Z-direction (Through-Thickness) Properties: Definition: Mechanical behavior perpendicular to the plane of boards or panels, often the weakest orientation (e.g., rolling shear in panels). Critical for delamination and fastener withdrawal.
See also: Anisotropy, Shear Strength.

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