wood structure and processing robert s. wallace department of ecology, evolution and organismal...
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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. Sapwood
The 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