Here is some light reading..
You suggested avoiding the center of the tree, or what is known as the "Pith". That was correct. When a log is cut into lumber, this center board (containing the pith) is called the dog board. -Reason being, the "dogs" -clamping mechanism of the head rig, latch onto the log here. Eventually the log is sliced up into boards. This dog board, is then "edged" removing the useable lumber from each side of the pith. Depending on how the log is sawn, this center board containing the pith is made into a post, or chipped down. Being that it is terribly unstable. Now the reason:
The area comprising the pith is what is known as "Juvenile wood" and/ or "Reaction" wood. The cell structure of this wood is very thin walled, and gelatinous in nature, verses normal cell structure. Without getting too complex in my explanation, this is the tree’s way of correcting directional growth in its life. The downside is that this woody material is very unstable (due to its gelatinous nature, and multitude) , shrinking and swelling way beyond that of normal wood. Thus the cracking and warping we ultimately encounter.
Tree limbs are predominantly comprised of this type of Juvenile wood. Using limbs to make a call, or projects, is just asking for trouble! A really huge limb might possibly contain enough normal wood to produce a decent product? But it’s still a gamble that a flaw will not eventually pop up. Now you see why lumber companies do not use tree limbs. Preferring to sell them as firewood.
Another tree growth response is the production of what is known as “Reaction wood”. Reaction wood (kind of a broad moniker) differs between Hardwood and Softwood species. In simple terms; Imagine a tree growing on a hillside. From a seed, the sprout emerges from the ground perpendicular to the surface of the soil. Then as it grows, it rights itself, straining to reach the sunlight overhead. So in essence, the tree is now growing at an upward angle to the hillside. –In a Hardwood species its reaction wood develops on the upper side of the tree. It is what is correctly now known as “Tension wood”. Here the tree floods the upper side of the (Hardwood) tree with a large amount of abnormal cells. These abnormal cells, “pull” the tree straight, and are under “tension”. –Think of a bow string on a bow, bending the limbs of the bow in a direction opposed to where they want to be…
In a Softwood species the reaction wood forms on the underside of the tree. It is what is known as “Compression wood”. Here the tree responds to correct the growth direction, by flooding the underside of the tree with abnormal cells. These abnormal cells act like a jack, and push the tree in the proper position. In doing so, they are under “compression”.
Keeping all this in mind… Any time a growing tree has its direction altered,[Ex. A wind storm blowing it partially over. A mudslide pushing it askew.] It is going to respond to correct this deviation using the above mentioned mechanism.
Now the fly in the ointment…We as woodworkers are drawn to the wild figure, and erratic grain patterns we find in lumber produced from such trees upon harvesting. We have certain expectations that the material will respond in a normal manner, just like the countless other calls we’ve made? But, we are not dealing with normal wood. But rather, a wood containing a cell structure that is not of the norm. When the same handling principals are applied, it responds differently! We’ll see excessive cracking, checking, warping, tear out, blotchy staining, etc. The name of the culprit? “Reaction Wood!” Some times it complies to our will? Other times not..
As far as harvesting one’s own wood? Things to remember:
Upon felling the tree, the quicker you convert it to lumber the better. Here all the ends should be suitably end coated to prevent rapid moisture loss from end grain surfaces. Wood looses moisture 10-20X faster through end grain, verses across the surface. So coat those end grain surfaces regardless if it’s in lumber, or log form! In addition, removing the bark is highly recommended if at all possible.
Get the lumber stickered, and stacked. Place dry stickers in alignment across the stack, making sure to use enough of them (proper spacing) to prevent sagging. Place stickers at the end of the load, directly up against the end of the boards in the stack. Most folks place them back too far on the ends. Here the lumber will crack right on back till it meets the sticker. –You will lose this amount of lumber! So by having the “end stickers” flush, you will not experience this degree of loss.
Thicker boards can be placed on the bottom of the stack. The weight of the thinner boards stacked on top of them will help keep them flat. As for the thinner boards. They can be weighted down (ex. Cinder blocks set on a sheet of plywood, set on a set of top stickers on the load). The thinner boards will dry quicker. So placing them on the top will allow easy access when the time comes to use them.
A covering of some sort should be placed on the “top” of the stack to keep the elements off. Do not cover the sides of the stack. We want the air to be allowed to flow across the surfaces of the stickered boards in the load. Also, make sure the load to be dried is off the ground. Use blocks, or timbers, to establish your stack upon. If it makes contact with the ground, the lumber will rot, or be exposed to termites and insects.
Record the weight /and or moisture content (moisture meter) of a few sample boards within the load at the start of drying. These can be monitored (reweighed/retested) periodically to determine how the drying is progressing?
In the end(roughly 1-2 years) you will have “Air dried” lumber. Which depending on where you live, will be around 12-20% moisture content. It is highly recommended that you bring portions of this material indoors (a dry, heated building) to allow it to further dry before using. An ideal target moisture content is 6-8% MC. This is what dry kiln operations target. The bottom line is that higher MC pieces will dry (possible cracking, warping) when worked, and placed in a drier environment. So pay attention, and know the moisture content of the material you are working with if at all possible. It’ll save you some heartache!
Dave
