WOODWORKING
Chapter 7PHYSICAL PROPERTIES OF WOOD
Introduction:
Lumber comes from trees – no surprise there – and the type of tree it comes from determines whether the
lumber is hardwood or softwood, which, you will remember, has nothing to do with the hardness of the
wood but the color, grain pattern, and cost. Whether you choose to use hardwood or softwood, it is
important to know that the physical properties of all wood are the same. In every tree trunk and branch there
is sapwood and heartwood. Sapwood is the soft part of the tree’s trunk used for transferring sap and water to
the branches and leaves. When sapwood is retired (i.e. it is no longer working for the tree), it hardens and
new sapwood is formed. This hardened sapwood is called heartwood. To better understand how this works,
you must understand how a tree lives and grows.
The outer skin of a tree is called bark. It protects the tree from insects, disease, storms, and/or extreme
temperatures. When milling a tree for lumber, this outer skin, or armor, is removed and discarded. Inside the
tree, the wood is made of long cells. These cells are made of cellulose (the same material used for making a
household sponge). These cells are bounded together by lignin. To better understand this, think of a bundle of
straws (representing cells) attached together with glue (representing lignin). They may be easy to peel apart,
but not so easy to break. Wood is much the same. Long cells, like straws, carry food and water up and down
the tree.^1 This all takes place in the sapwood. So, you can imagine, lumber formed from this part of the tree is
softer and weaker. The heartwood, however, is hard and strong. It is formed from old layers of sapwood, and
has dried, darkened, and hardened with age. Finally, the pith is located in the center of the tree. When the tree
was young, the pith was the first layer of sapwood, and it carried and stored food. As future layers of
sapwood formed, the pith was no longer used, so it hardened and became part of the heartwood. Being the
oldest element of the tree, the pith is used to form the strongest lumber.
Harvesting Trees for lumber:
When trees are harvested for lumber, both the heartwood and the softwood are used. Following are the
characteristics of both:
Sapwood:
Is less durable and more permeable than heartwood.
Many times, sapwood is graded a higher quality than heartwood because it is smoother and less
prone to knots and blemishes formed by encased knots (see above right illustration). The highest-
grade sapwood is virtually free of any imperfections.
Coloration of sapwood ranges from white to light yellow tones.
Sapwood is best used for planks, siding, partition wall studs, and other building components not
subject to lots of stress.
Unless treated sapwood is more susceptible to decay
You can identify lumber made of sapwood simply by looking at the end grain of the board. If the
grain pattern is virtually straight, chances are it has been cut from the outer sapwood layer where
the growth rings of the tree are a greater diameter.
(^1) Note: These, cells, by the way, form the lines we call grain. The reason we have to work extra hard on board ends is because these open cells
must be closed properly to prevent stain from soaking into the straw like material and discoloring our wood.
Unit II: Understanding Wood