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”Wood Defects” Knots, Spiral Grain, Juvenile Wood, and Reaction Wood

FW1035Lecture 6

Bowyer et al, Chapter 2, pp. 40-43, Chapter 6, pp. 109-129

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Grain Orientation in Wood

• Grain direction - direction of the long axis of longitudinal wood cells

• Spiral grain caused by anticlinal division with new primary cell wall formation in one direction only

• Interlocked grain• genetically controlled• common in elms• may cause warping in lumber

upon drying• may exhibit nice figure in

veneers. “ribbon figure”

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Ribbon Figure

Quarter-sawn lumber from tree stem with interlocked grain.

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Knots are Branch Stubs

• Tight knots - incorporation of living branches into the stem• knots are integral part of the

surrounding wood

• = “intergrown knots”

• Loose knots – stem growth encases a dead branch• may fall out upon lumber drying

• = “encased knots”

• “Grain” of wood deviates around knots – weak point

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Juvenile Wood • Short fibered xylem with high microfibril angles and low specific gravity.

• Wood produced during the first 5-15 years of growth

• As tree grows, the SG increases and the fibers lengthen.

• Gradual transition from juvenile wood to mature wood

• Caused by effects of hormones from apical meristems on cambium

• As cambium in stem becomes farther from and less influenced by the apical meristem, transition to mature wood

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Physical Characteristics of Juvenile Wood that Affect its Use

• Cells are shorter than mature wood

• Thin cell walls and less latewood • Leads to lower density

and strength

• More spiral grain

For Softwoods in Particular:• Density

• 10-15% lower than mature wood

• Strength• 15-50% lower than mature

wood

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Transition from Juvenile Wood to Mature Wood is Gradual!

Not the same as transition from sapwood to heartwood, which is abrupt.

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Juvenile Wood in Solid Wood Products

• Greater tendency for spiral grain• Shrinkage

• often shows up to 10 times the longitudinal shrinkage of mature wood due to the greater S2 microfibril angle

• Trees with high juvenile wood content may yield only 20-50% as much high grade dimension lumber as older trees

• Sawmill loss may be reduced when planning specifically for cutting juvenile wood

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Reaction Wood

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Compression Wood and Tension Wood

• Reaction wood is called compression or tension wood in response to where it forms in a stem• Softwoods form

compression wood• Hardwoods form tension

wood to correct growth irregularity in a stem

• Reaction wood forms in branches of most trees

Tension

Compression

Reaction Wood in Softwoods and Hardwoods

• Compression Wood• Softwoods• Underside of branches or leaning stem• Commonly in juvenile wood• Appearance is similar in most species

• Tension Wood• Hardwoods• Top of branches or leaning stem• Common in juvenile wood also• Appearance and microanatomy is less consistent than for

compression wood

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General Appearance of Compression Wood

• Eccentric growth rings that appear to contain an abnormally large proportion of latewood in the widest portions

• Non-centrally located pith in stem

• Often darker color (red/brown)

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Microanatomical Characteristics of Compression Wood

• No S3 layer• Larger S2 microfibril

angle, ~45°• Spiral cavities in S2

layer• Longitudinal tracheids

are 10-40% shorter• Tips of tracheids are

distorted

• Latewood longitudinal tracheids are most affected

• Rounded cross-section, rather than prismatic

• Intercellular spaces are present

• Greater cell wall thickness

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Effects of Compression Wood on Utilization

• Large longitudinal shrinkage (1-2%) causes warping and bending of boards upon drying

• Higher lignin content (average of 38% versus 29% in normal wood) gives lower pulp yields

• Lower strength properties than density would lead you to predict

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General Appearance of Tension Wood

• Cut surfaces have “wooly” or fibrous appearance• causes overheating and

dulling of saws• difficult to sand and finish

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Appearance of Tension Wood in Stem

Microanatomy of Tension Wood

• Cell modifications are usually in the earlywood

• Most commonly affects fiber cells• more numerous fibers,

fewer rays, vessels, etc.• secondary cell wall is

significantly different

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Microanatomical Differences

• Changes in the secondary cell wall• almost entirely made of

cellulose• forms a floppy layer that

is loosely attached to the primary wall

• gelatinous layer called the “G” layer

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Appearance of the Gelatinous Layer

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Utilization of Tension Wood

• Can produce good paper properties if pulping conditions are modified

• Good for “dissolving pulps”• cellulose source for making cellophane, rayon,

and nitrocellulose

• Solid wood products have lower quality• tendency to ‘collapse’ upon drying• higher longitudinal shrinkage 1-5x normal wood)

may lead to warp and bending• lower strength properties