You may have heard the terms heartwood and sapwood, but do you know the difference? More importantly, do you know which one you should use for your woodworking projects?
Junk in the Trunk:
To understand the difference between heartwood and sapwood, we need to learn a bit about how a tree grows. If you were to crosscut the trunk of a mature hardwood tree and remove the bark and outer cambium layer (which eventually becomes new bark), you'd notice two distinct sections of the trunk.
The outer, lighter colored wood is the sapwood. This is the "working" part of the tree, as water and sap will flow through the sapwood much like blood through your arteries, veins and capillaries. While this part of the trunk is vital to the tree when it is living, it doesn't make for very good stock for woodworking. Sapwood contains a lot of moisture, will shrink considerably when dried, and is much more susceptible to fungus.
The inner, darker section of the trunk is the heartwood. Heartwood is formed from old, "retired" sapwood, and becomes the strong spine of the tree. Heartwood is preferred for woodworking, as it is far less susceptible to fungus and doesn't contain nearly as much moisture as sapwood, which means it will shrink less when dried. Many experienced woodworkers will actually remove the sapwood and use only heartwood on their furniture projects.
Once the tree has "promoted" some of it's sapwood to heartwood status, the sap will stop flowing through that part of the wood and the converting material essentially dies. As part of the conversion process, the pores will begin to plug up with organic matter which causes the cell walls to change color due to the presence of chemicals called extractives. The extractives are responsible for the rich character and colors found in heartwoods.
Should Sapwood be Discarded?:
Just because you end up with some excess sapwood after trimming it from the hardwood when you begin a furniture project, does that mean that the sapwood is "junk?"
My answer is no. While the sapwood will never be as strong, rich or beautiful as the heartwood, it still has its uses. Just be certain to dry the sapwood thoroughly, and use it in projects where a little bit of movement will not cause problems, and where it will be thoroughly sealed (with paint or polyurethane) when finished.