A recurring question from both advanced and novice turners is how to turn green wood successfully into a bowl or wood sculpture. Turning green wood is an absolute pleasure, as the soft wet wood cuts very easily. Aside from dodging sap flinging off the lathe, the moisture in the wood also keeps the cutting tools cooler which translates to longer edge life, eliminates virtually all saw dust, and rarely exhibits any end-grain tear out.
Given all the bennies of turning green wood, what’s the problem? Green wood shrinks. Green wood shrinks unevenly. As the wood seasons, the cells holding the moisture shrink. The shrinkage is greatest across the grain as compared to along the grain, with a difference as high as 15-25%. This uneven shrinking usually causes cracking and checks unless precautions are taken to prevent it.
However, this shrinkage can yield some very interesting results, even warping the bowl into what appears to be a hand-carved sculpture. A precisely circular bowl turned from green wood rapidly shrinks into an unpredictable shape. What is desired is to be able to complete one these creations without the wood cracking or checking, relegating the piece to hold faceplate screws in the shop.
Aside for allowing a many months (or longer) for a cut log to fully season prior to turning, there are two methods for turning green wood:
- Rough-shaping: Turn a rough-shaped bowl with uniform and thick sidewalls, then allow the turning to fully cure before final shaping. By leaving adequate side thickness to compensate for warpage, the final bowl will be cut to circular shape and finished like any seasoned wood. By rough-shaping the bowl, considerable wood is removed reducing the thickness, which will cure fairly rapidly when compared to a 10″ thick block of wood. The wall should be thick enough to allow for the differential shrinkage (cross-grain vs. in-grain), leaving enough wood to be able to shape the final wall thickness, but not so thick that internal stresses cause cracking and checks.
- Finished Shaping: Turn the bowl to finished wall thickness and seal with multiple coats of penetrating oil over the next few days after finishing the piece. However, as the wood is green, it is much softer than seasoned wood. Thus, extreme care must be taken to support thin walls as the cut is made to insure integrity of the sidewall. It is extremely important to maintain uniform wall thickness from sidewall to bottom. I find a maximum of 1/2″ wall thickness to be ideal for freshly-felled wood, although I prefer considerably less.
As green wood is turned, the internal stresses of the wood are released without the usual cracking and checking. The wood is thin enough to allow movement, resulting in crack-free finished pieces.
There are a number of articles available describing methods to speed seasoning a turning blank, ranging from microwaving, boiling, plastic bags, and so on. If you’re in that big of a hurry, go ahead and mount the blank, and either rough-turn or finish-turn a bowl as soon as possible after cutting the wood. Failure rates found with these methods is very low and is consistent with failure rates in turning seasoned wood.