Wood Structure and Processing
Robert S. Wallace
Department of Ecology, Evolution and
Organismal Biology
Iowa State University
Wood Structure and Processing
Some topics to be discussed in this presentation:
• The structure of wood – “cells to boards”.
• How wood is made by the tree.
• Reasons for differences in wood figure and
•
•
color.
Cutting and drying of wood – moisture
content, wood stability, and ‘How to read a
board”.
Wood shrinkage and movement – importance
of knowing why this happens.
Wood is one of the major building materials
used by humans throughout our existence.
Wood has outstanding physical
characteristics which include both
physical strength and resiliency.
Wood has inherent aesthetic beauty and
very 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…..
Wood’s production
starts with cell
division of the
cambium layer…..
Wood’s production starts with cell
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, barrels
would leak their liquid contents
through the open cells of woods
that lack them.
For this reason, “white oak”
species are used for cooperage.
Annual Growth
Differences in
cell wall thickness and cell
size are the main
factors which
determine the
‘latewood’ from
the ‘earlywood’
in an annual
growth ring.
Parenchymal Ray
Summer Wood
(small vessels)
Spring Wood
(large vessels)
One Annual
Ring
Cross Section of a Four-year-old Tree Stem
Mature
Bark and
Secondary
Xylem
The bark is a
protective layer of
cells that contains a
waterproof material
(suberin) which
resists water flow and
reduces damage to
the actively-dividing
cambial layer of cells
just inside the “soft
bark”, or phloem
layer.
Heartwood vs. Sapwood
The darker-colored
heartwood is the
result of additional
extractives being
deposited in the older
cells of the xylem.
As wood accumulates
around the outside of
the (dead) secondary
xylem, the normal
maturation process
darkens the wood with
these 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
Economics of Log Use
$
Maximum Use of Log
Range of different cuts
$$$
$$
FACE
α
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
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 shrinkage
depends on location within the log…
Difference in Wood Movement in
Plain-sawn versus Quarter-sawn Boards
FACE
α
End Grain View
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
Read your lumber!
Drying Wood
Before wood is used, the water contained in its cells must be
removed under controlled conditions to minimize distortion
and maintain quality of the finished product. Both ‘free’ water and
most ‘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
Moisture Content, %
0
15
5
10
OD
KD
AD
30
50
100
FSP
EMC in this range
Range of Shrinking and Swelling
“Green Wood”
Uniform, fully swollen
dimension
Equilibrium Moisture Content Ranges
by Geographic Region
Relationship between equilibrium moisture
content (EMC %) and relative humidity
Reaction Wood
As the tree leans,
the wood is either
Compression Wood
put under tension
or compression,
resulting in ring
thickness differences.
Tension Wood
Reasons for Developing Wood Figure
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Differences in seasonal growth rates – “early wood” vs.
“late wood” (tree rings). (Ring-porous vs. diffuse porous).
Differences in deposition of extractives (tannins, etc.).
•
Presence of parenchymal rays in the wood – “fleck”, ‘ray
bands’, “streaks”.
•
Unusual growth patterns in xylem formation: “birdseye”,
“quilting”, “ribbon”; charcterized as “figured” lumber (at
a cost premium!) – All wood has figure!
•
Physical stresses during growth; branches, inclusions.
•
Abnormal cell division – burls.
Wood Color
•
•
•
The major components of wood, celluloses and lignin,
are pale materials that do not contribute significantly
to color in wood.
During growth and maturation of xylem, the depositing
of extractives (tannins, resins, etc.) in the cell matrix
provides color to the otherwise “neutral” background
of the wood cells.
Some chemicals found in the array of extractives are
photosensitive, reacting with light to change or deepen
in color. Other chemicals will oxidize over time, and
shift color (chromatic) value as the wood ages.
The Diversity of Wood
Commonly Used Hardwoods
Cherry
Red Oak
Walnut
Maple
myrtle
pearwood
ziricote
bocote
snakewood
lacewood
H. mahogany
padauk
wenge
leopardwood
zebrawood
pink ivory
jarrah burl
narra
Summary
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•
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The structure of wood is highly complex.
Cell divisions, maturation, and death of the mature
xylem cells produce wood.
The figure and color of the wood are influenced by
the different growth characteristics of the plant, as
well as the kinds of chemicals deposited there.
How the wood is cut from the log and how it is
dried determine how stable the wood wil be in
service, as well as influencing the figure of the
wood.
There are thousands of species of trees that have
uses or potential uses as sources of woodworking
lumber.
Questions??
Woods From the same Family (Fabaceae)
Cocobolo
Dalbergia retusa
Wenge
Millettia laurentii
Purpleheart
Peltogyne sp.
Honey Locust
Gleditsia triacanthos
Pernambuco
Caesalpinia echinata
Zebrawood
Microberlinia
brazavillensis
Similar appearances…..
Cocobolo
Dalbergia retusa
Fabaceae
Bocote
Cordia eleagnoides
Boraginaceae