Thursday, April 5, 2007

Drying Out


One day while walking through my woods, I found something seemingly out of place amongst my ash, cherry and maples. The canopy in the woods must be about 70 feet high but growing only about 25 feet high is this little ironwood tree. You'll have to forgive my poor taxonomy skills, it may actually be something more specific, but to me ironwood is just right. It is as though this little tree is carved out of stone. It takes all of my weight to get a 2" branch to move at all. Tapping on the trunk is like tapping granite. It's like walking into a dog kennel and finding an elephant!
I've become very fond of this tree and visit it often (if you stop by, it's likely I'll drag you out to see it too).
When I told my mother that I was going to make chairs from green wood, she replied "Are you allowed to do that?" This sense of mystery surrounding moisture and wood runs deep even in the woodworking community. For an amazing account of the process and it's scientific side I highly recommend Bruce Hoadley's Understanding Wood. Indispensable. If you are drying planks, you will benefit greatly from the solid info there. For my needs in the shop, I rely on experience (having made many mistakes) and a lowtech understanding of how wood dries. Basically, I am always treading the line between rotting the wood and cracking it up.
The rule of thumb for drying planks is one year for each inch of thickness. This is a gross simplification, but it is enough to deter anyone interested in working with trees. Who wants to wait 2 years to get started. Half the reason we love woodworking is the instant gratification of results. This is enough to guide anyone down the path of kiln dried wood and the tools and techniques that follow. I think this is a shame because a slightly different approach can grant the spontaneity that we are seeking. Once the tree is cut down, the process of it equalizing moisture content with the air around it begins. If the process is too slow, it will rot. If the process is too fast, it will crack. One of the great reasons for working with a rot resistant wood like white oak is that I can keep it relatively moist for years and still have usable wood. Of course, sealing the end grain immediately will prevent excess moisture loss and checking on the ends of the log or boards or splits. By splitting the wood into small pieces, the moisture loss will occur relatively evenly and quickly, making the challenges of drying seem simple.
I like to think of the wood like bread. It only takes exposure to the air for a few hours for the outer layers of bread to begin to harden. Leave it overnight and it is downright stale, through and through. Now imagine taking the same piece of fresh bread and making toast. High heat will harden the outer layers dramatically and leave the inside soft. With wood, I want to make it stale, not toasted. This means that I try to err towards patience. Chair parts have a ratio of surface area to mass that almost eliminates worries about rotting and checking during air drying. I simply leave my pieces, once shaped and bent, sitting around the shop as long as I can. I don't rush green wood into the kiln, courting disaster. About 2 or 3 days sitting in the shop is enough to prep a green piece for the kiln. Then it is into the kiln at 140 degrees until the piece stops losing weight. Honestly, I never weigh the pieces. I simply judge by how well they hold the bends, and how light they feel. Extra time in the kiln is the rule, not the exception. My kiln is a simple affair and I will post its details tomorrow.

3 comments:

Rooch said...

Thanks for the blog and the email response yesterday.
True Ironwood grows in the Mexico region around Hermosillo, and more of a bush than a tree. I wonder what kind of variation the tree is.

Anyway- thanks and have a great day
Scott E.

Doug said...

I believe the ironwood tree in the picture is a hornbeam tree. I've had great success making tool handles from this species.

Peter Galbert said...

Doug,
I think that you are right about the hornbeam. I plan on checking it out further once the leaves bloom. Thanks for the tip