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”. 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 color. Reasons for differences in wood figure and color. Cutting and drying of wood – moisture content, wood stability, and ‘How to read a board”. 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 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 % Lignin % 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 thick- ness and cell size are the main factors which determine the ‘latewood’ from the ‘earlywood’ in an annual growth ring.

Spring Wood (large vessels) Summer Wood (small vessels) One Annual Ring Parenchymal Ray

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

$ $$$$$ 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 α 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

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 α FACE End Grain View 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 WaterFree Water “Green Wood” Moisture Content, % ADKDOD 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 either put under tension or compression, resulting in ring thickness differences.

Reasons for Developing Wood Figure Differences in seasonal growth rates – “early wood” vs. “late wood” (tree rings). (Ring-porous vs. diffuse porous). Differences in seasonal growth rates – “early wood” vs. “late wood” (tree rings). (Ring-porous vs. 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 bands’, “streaks”. 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! 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. Physical stresses during growth; branches, inclusions. Abnormal cell division – burls. 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. 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. 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. 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 Red Oak Maple Walnut Cherry

bocote jarrah burl leopardwood wenge zebrawood padauk H. mahogany snakewood ziricote narra pink ivory myrtle pearwood lacewood

Summary The structure of wood is highly complex. The structure of wood is highly complex. Cell divisions, maturation, and death of the mature xylem cells produce wood. 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. 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. 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. 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. Zebrawood Microberlinia brazavillensis Honey Locust Gleditsia triacanthos Pernambuco Caesalpinia echinata

Similar appearances….. Cocobolo Dalbergia retusa Fabaceae Bocote Cordia eleagnoides Boraginaceae