Wood Structure and Wood Structure and ProcessingProcessing

Robert S. WallaceRobert S. Wallace

Department of Ecology, Evolution andDepartment of Ecology, Evolution andOrganismal BiologyOrganismal Biology

Iowa State UniversityIowa State University

Wood Structure and Wood Structure and ProcessingProcessing

Some topics to be discussed in this presentation:Some topics to be discussed in this presentation: • The structure of wood – “cells to boards”. The structure of wood – “cells to boards”. • How wood is made by the tree.How wood is made by the tree.• Reasons for differences in wood figure and Reasons for differences in wood figure and

color.color.• Cutting and drying of wood – moisture Cutting and drying of wood – moisture

content, wood stability, and ‘How to read a content, wood stability, and ‘How to read a board”.board”.• Wood shrinkage and movement – importance Wood shrinkage and movement – importance

of knowing why this happens.of knowing why this happens.

Wood is one of the major building materialsused by humans throughout our existence.

Wood has outstanding physicalcharacteristics which include bothphysical strength and resiliency.

Wood has inherent aesthetic beauty andvery desirable working properties...

Chemical Components of Wood:

Cellulose - hemicellulose ---------------- 70%Lignin ------------------------------------------- 25%Extractives – tannins, starch, oils, fats, resins, waxes, etc. ---------------- ~ 5%“Ash” – minerals, crystals --------------- < 1%

If we could unroll the tissues of a log…..If we could unroll the tissues of a log…..

Wood’s production Wood’s production starts with cellstarts with celldivision of the division of the

cambium layer…..cambium layer…..

Wood’s production starts with cellWood’s production starts with celldivision of the cambium layer…..division of the cambium layer…..

Ring-porous versus Diffuse-porous Woods

Tyloses

Red oak – Quercus rubra White oak – Quercus alba

Were it not for tyloses, barrelswould leak their liquid contentsthrough the open cells of woodsthat lack them. For this reason, “white oak”species are used for cooperage.

Annual Growth

Differences incell wall thick-ness and cellsize are the mainfactors which determine the‘latewood’ fromthe ‘earlywood’in an annualgrowth ring.

Spring Wood(large vessels)

Summer Wood(small vessels)

One AnnualRing

Parenchymal Ray

Cross Section of a Four-year-old Tree Stem

MatureBark and

SecondaryXylem

The bark is a protective layer ofcells that contains awaterproof material(suberin) which resists water flow andreduces damage tothe actively-dividingcambial layer of cellsjust inside the “softbark”, or phloemlayer.

Heartwood vs. SapwoodThe darker-coloredheartwood is the result of additionalextractives beingdeposited in the oldercells of the xylem.

As wood accumulatesaround the outside ofthe (dead) secondaryxylem, the normalmaturation processdarkens the wood withthese chemicals.

Yew (Taxus baccata)

Gymnosperm Woods“Softwoods”

- Gymnosperms are non-flowering plants which produce their seeds in cone structures.

- Examples include pine, spruce, fir, cedar, larch, ginkgo,

- The wood of gymnosperms can be characterized by the lack of vessels

- In many species, resin ducts are found.

-The wood usually has a “piney” aroma, due to the presence of terpenoids.

Angiosperm Woods“Hardwoods”

- Angiosperms are flowering plants which produce their seeds in fruits.

- Examples include cherry, maple, ash, walnut, oak, teak, and many exotic species.

- The wood of angiosperms is often characterized by the presence of vessels

- Woods can show a considerable range of color and figure characteristics.

- Hardness varies from extremely soft -(e.g. balsa) to extremely hard.

Milling Lumber

$

$$$$$

Maximum Use of Log

Range of different cuts

Economics of Log Use

Plain-sawn boards – rings at 0º to 30º to face

Rift-sawn boards – rings at 30º to 60º to face

Quarter-sawn boards – rings at 60º to 90º to face

αFACE

End Grain View

Wood does not move equally in all directions!!Fully dependent upon the position of the

piece where it is cut from the log.

T

R

Distortion due to shrinkagedepends on location within the log…

Difference in Wood Movement inPlain-sawn versus Quarter-sawn Boards

Plain-sawn boards – rings at 0º to 30º to face

Rift-sawn boards – rings at 30º to 60º to face

Quarter-sawn boards – rings at 60º to 90º to face

αFACE

End Grain View

Read your lumber!

Drying WoodBefore wood is used, the water contained in its cells must be removed under controlled conditions to minimize distortionand maintain quality of the finished product. Both ‘free’ water andmost ‘bound’ water must be removed during the drying process.

Stickered boards are air-dried to about 30% MC to remove free water.

Boards are then kiln-dried to remove bound water to the target EMC level.

Bound Water Free Water

“Green Wood”

Moisture Content, %15 30 50

ADKDOD

100105

FSP

Range of Shrinking and Swelling Uniform, fully swollen dimension

EMC in this range

0

Equilibrium Moisture Content Ranges by Geographic Region

Relationship between equilibrium moisture content (EMC %) and relative humidity

Reaction Wood

Compression Wood Tension Wood

As the tree leans,the wood is eitherput under tensionor compression, resulting in ringthickness differences.

Reasons for Developing Wood FigureReasons for Developing Wood Figure• Differences in seasonal growth rates – “early wood” Differences in seasonal growth rates – “early wood” vs.vs. “late “late

wood” (tree rings). (Ring-porous wood” (tree rings). (Ring-porous vs.vs. diffuse porous). diffuse porous).

• Differences in deposition of extractives (tannins, etc.).Differences in deposition of extractives (tannins, etc.).

• Presence of parenchymal rays in the wood – “fleck”, ‘ray Presence of parenchymal rays in the wood – “fleck”, ‘ray bands’, “streaks”.bands’, “streaks”.

• Unusual growth patterns in xylem formation: “birdseye”, Unusual growth patterns in xylem formation: “birdseye”,

“quilting”, “ribbon”; charcterized as “figured” lumber (at a “quilting”, “ribbon”; charcterized as “figured” lumber (at a cost premium!) – cost premium!) – All wood has figure!All wood has figure!

• Physical stresses during growth; branches, inclusions.Physical stresses during growth; branches, inclusions.

• Abnormal cell division – burls.Abnormal cell division – burls.

Wood ColorWood Color• The major components of wood, celluloses and lignin, The major components of wood, celluloses and lignin,

are pale materials that do not contribute significantly are pale materials that do not contribute significantly to color in wood. to color in wood.

• During growth and maturation of xylem, the depositing During growth and maturation of xylem, the depositing of of extractivesextractives (tannins, resins, etc.) in the cell matrix (tannins, resins, etc.) in the cell matrix provides color to the otherwise “neutral” background provides color to the otherwise “neutral” background of the wood cells.of the wood cells.

• Some chemicals found in the array of extractives are Some chemicals found in the array of extractives are photosensitivephotosensitive, reacting with light to change or deepen , reacting with light to change or deepen in color. Other chemicals will oxidize over time, and in color. Other chemicals will oxidize over time, and shift color (chromatic) value as the wood ages.shift color (chromatic) value as the wood ages.

The Diversity of Wood

Commonly Used HardwoodsRed Oak

MapleWalnut

Cherry

bocote

jarrah burl

leopardwood

wenge

zebrawood

padaukH. mahogany

snakewood

ziricote

narrapink ivory

myrtlepearwood

lacewood

Summary• The structure of wood is highly complex. The structure of wood is highly complex.

• Cell divisions, maturation, and death of the mature Cell divisions, maturation, and death of the mature xylem cells produce wood.xylem cells produce wood.

• The figure and color of the wood are influenced by The figure and color of the wood are influenced by the different growth characteristics of the plant, as the different growth characteristics of the plant, as well as the kinds of chemicals deposited there. well as the kinds of chemicals deposited there.

• How the wood is cut from the log and how it is How the wood is cut from the log and how it is dried determine how stable the wood wil be in dried determine how stable the wood wil be in service, as well as influencing the figure of the service, as well as influencing the figure of the wood.wood.

• There are thousands of species of trees that have There are thousands of species of trees that have uses or potential uses as sources of woodworking uses or potential uses as sources of woodworking lumber.lumber.

Questions??

Woods From the same Family (Fabaceae)

CocoboloDalbergia retusa

WengeMillettia laurentii

PurpleheartPeltogyne sp.

ZebrawoodMicroberlinia

brazavillensis

Honey LocustGleditsia triacanthos

PernambucoCaesalpinia echinata

Similar appearances…..

CocoboloDalbergia retusa

Fabaceae

BocoteCordia eleagnoides

Boraginaceae