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Different pieces of wood cut from a large tree may differ decidedly, particularly if the tree is big and mature. In some trees, the wood laid on late in the life of a tree is softer, lighter, weaker, and more even-textured than that produced earlier, but in other trees, the reverse applies. This may or may not correspond to heartwood and sapwood. In a large log the sapwood, because of the time in the life of the tree when it was grown, may be inferior in hardness, strength, and toughness to equally sound heartwood from the same log. In a smaller tree, the reverse may be true.
Reasons for splits and cracks during timber drying and their control
The chief difficulty experienced in the drying of timber is the tendency of its outer layers to dry out more rapidly than the interior ones. If these layers are allowed to dry much below the fibre saturation point while the interior is still saturated, stresses (called drying stresses) are set up because the shrinkage of the outer layers is restricted by the wet interior (Keey et al., 2000). Rupture in the wood tissues occurs, and consequently splits and cracks occur if these stresses across the grain exceed the strength across the grain (fibre to fibre bonding).
The successful control of drying defects in a drying process consists in maintaining a balance between the rate of evaporation of moisture from the surface and the rate of outward movement of moisture from the interior of the wood. The way in which drying can be controlled will now be explained. One of the most successful ways of wood drying or seasoning would be kiln drying, where the wood is placed into a kiln compartment in stacks and dried by steaming, and releasing the steam slowly.
Steve Martin wrote:Greetings! Yes, I mis-stated the date and the name (location) of the wood I was referring to. Went back and looked at photos again and noticed that, what I erroneously called heartwood but meant pith, in several photos on April 14 and May 4,13,20 and 27. As I remember, it is a small dark circle about 1/3 down from top edge on inside of bowl. You are correct that eliminating the heartwood would not leave much to work with. I apologize for the temporary (I hope) brain fart.
Steve Martin wrote:After thinking more about it, I would like to add the following. My understanding of the splitting phenomenon is that splitting is primarily due to loss of moisture by cells toward the outside of the piece, away from the pith, before/faster than the cells closer to the pith losing their moisture. As the cells in the outer growth rings lose moisture they occupy less space than the growth rings toward the pith which haven't lost as much moisture. Because they occupy less space than they used to occupy, something must give, and it does give, ergo! a split which increases because the split encourages more loss of moisture from the cells along the split. I realize that other factors, including growth stresses, lack of or surplus of moisture during specific growth periods, how the wood is handled after the tree is cut before it gets to the bowl maker/carver, exposure to sun and wind, etc. all can influence splitting.
Steve Martin wrote:When carving spoons out of native cherry that grows here in North Carolina, I have had the piece split while I was carving, usually overnight when I couldn't finish the spoon in one sitting. Later, after finishing the spoon, I put it in the microwave and finished drying it. The splits closed up with no gluing or other encouragement on my part. This has also happened with native maple. My attempt to explain this is that when the moisture is evened out throughout the piece so that the outer growth ring wood RELATIVELY takes up the same space/area/volume compared to the wood underneath it while they were both filled with water/sap, toward the pith, there no longer needs to be a split in the outer wood. Have a great day!
In 1998, a German company, Tecnaro, developed a process for turning lignin into a substance, called Arboform, which behaves identically to plastic for injection molding.
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