Learning Wood

by | Mar 6, 2019 | 0 comments

I love learning about trees and wood.  Wood is both one of human’s simplest and most complex raw materials; anyone with an ax can ‘mine’ wood, yet wood’s chemical structure is so complex that there are many mysteries that still defy science.     

I have been reading Forest Products and Wood Science: An Introduction,  484 pages of extremely dull reading.  Page 46 for example:  “Fujita and Harada (1991) describe cellulose microfibrils in a very straightforward manner, describing them as consisting of a ‘core crystalline region of cellulose surrounded by the paracrystalline [less highly ordered] cellulose and short chain hemicellulose.'”  Yikes!

Here’s a few things I did find interesting:

Trees are 20-35% lignin.  Lignin provides stiffness; trees wouldn’t stand upright without it. Wood is about half cellulose (the cotton in your tee-shirt is 99% cellulose)  So when we heat wood to plasticize lignin, we are trying to turn trees into tee shirts….

Softwood fibers are 3-4 times longer than hardwood fibers.  I had always heard that fiber length partially determined wood strength, but clearly that’s wrong!  Turns out it’s the number of fibers, not their length, that matters.  And since fibers are heavy, you can get a good idea of how strong a piece of wood is by measuring it’s density.

Dry white oak is half air.  Sugar pine is three-fourths air.  What a bunch of fluff we work with!

Heavy woods tend to shrink more than lighter woods as they dry.  However, extractives decrease shrinkage, so walnut’s heartwood will shrink less than it’s sapwood.  I’ve seen it happen – now I know why!

Ring-porous woods (oak, ash, hickory….)  are stronger the faster they grow…. to a point.  Growth rings more than 1/4″ wide can be weaker than wood with growth rings closer to 3/16″, which seems to be the happy medium.   I knew it to be true, but now I’m proven by science.

Pines tend to make stronger wood the older they get, not the faster they grow, like I’ve always read.  I’ve been spreading around out-of-date information for ten years or more.  How have my students survived?!?







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