Avoiding Green Wood
Many wood species will begin splitting within a few minutes of felling a tree. The danger of checking will exist until the wood has reached moisture equilibrium with its environment. I have experienced too much wasted wood and time and decided this needed some serious study. This is a short list of what I’ve come up with. We may not completely eliminate all splitting, but here are some steps that substantially reduce this problem. First, here are some generalizations on why wood splits.
A freshly cut tree is like a kitchen sponge; its lumens (cavities between cells) and cells are filled with water. The water in green wood can weigh more than its dry weight. Squeeze green wood and water (sap) will pour out. As the wood dries water between the cells is the most mobile, but this first-to-leave water causes little change in dimension and almost no checking. When inter-cellular water is gone this state is called the fiber saturation point (FSP) and there is no water remaining in the lumens. The less mobile water bound inside the wood cells leaves more slowly. Wood does not start to warp until it has dried to its FSP. As water inside the cell leaves it shrinks, and the shrinkage is perpendicular to cell walls. The content of water in wood continues to change until it is in equilibrium with moisture in the air, a condition called equilibrium moisture content (EMC). Wood near the surface may be closer to EMC while green wood far from the surface may have lots of water and still be above the FSP. Because water moves along the grain 15 to 20 times faster than across the grain wood near ends of the log is first to dry out and check. Wood is hydroscopic. As humidity changes wood will absorb moisture or loose moisture and expand or shrink in the process. The relationship between relative humidity and average EMC is given in the following table together with the range for all species. For example this data indicates that
at 50% RH, 9% of the weight of average wood in equilibrium with its environment is water. For wood to dry the moisture content of the air must be less than that of the wood. Bowls made in equilibrium with the RH of our shop will distort according to the summer and winter relative humidity's of their final location. Boxes with lids turned to fit in the winter may not fit in the summer.
Wood dries from the outside in so that the interior is wetter than it is on the surface. Wood splits as it dries because it shrinks by different amounts in different directions. Shrinkage along the grain is negligible. On the average tangential (T) shrinkage around the circumference (parallel in direction with the rings) is twice the radial (R) change perpendicular to the rings. (The T/R ratio ranges from 1.3 for yellow birch up to 2.6 for black willow, but T/R = 2 is an average number to remember until you have reason to check a wood handbook for a more exact ratio.) This explains why V-shaped splits in the end of a log are mostly pointed toward its center because length around its circumference is decreasing twice as much as length in the radial direction. Figure 1 shows V-shaped splits that are wider toward the outside of the logs. While wood is drying some species split worse than others because a) the T/R ratio is greater, or b) the adhesion between fibers is weaker. (Some woods, such as mesquite, have serious cracks that have nothing to do with drying.)
So much for why wood splits, now lets consider how to avoid green wood splitting.
A) When you make an opportunistic discovery of
freshly felled trees, green crotches or roots be prepared temporarily to
enclose ends of logs with garbage bags tied with heavy twine. Before
bagging the ends, the threat of splitting can be reduced by removing a 1
inch or thicker slab that includes the pith (i.e. the core of the log or
first year of the tree’s growth) running down the center along the
length of the log. Leave the bolt in the largest practical length until
you are ready to turn it so that any checking that develops can be cut
away. Temporarily bag the ends and as much of the bare surface as
possible. Enclose some fresh sawdust and optionally a cup of water. If
left wrapped in a plastic bag indefinitely there could be a problem with
mold and possibly bugs although this may be only a minor issue if some
length of the end will be cut off prior to turning. If you happen to
make an overwhelming discovery of green wood consider calling in fellow
wood turners to share your find and perhaps help with transportation.
B) A more satisfactory method for long term
protection is to coat the ends of the bolt, as well as cuts, blemishes
and cut off limbs, etc. in the side with a sealer. Some turners use
several coats of left over house paint or varnish. As shown in Figure 1
above, this may not always be satisfactory. A better sealer such as
waxy Anchorseal is less water permeable. You can make your own waxy
sealer with a saturated mixture--combining approximately 6 parts paint
thinner with 1 part paraffin. (Take precautions as this mixture is
flammable.) Sealed wood should be kept under cover protected from sun
and rain. The purpose is to help wood age uniformly and reduce extreme
difference in shrinkage through out the wood. It will take years of
storage for wood of any size to reach EMC. Wood dried to a moisture
content of 10% should have no further danger of cracking. The use of a
moisture meter would be helpful in determining the condition of your
wood. I like the technology that measures moisture using electromagnetic
scanning (e.g. Wagner) based on the density of the species. Another
technology measures moisture based on the resistance between two probes
inserted into the wood. This is an inexpensive device you could make
with your ohmmeter using resistance tables of each species available on
C) Next we consider two methods for turning
green wood with reduced chance of splitting. Usually it takes too long
for wood we store to reach EMC and it is nearly impossible to buy bowl
thickness wood blanks dried to uniform EMC. Move quickly through the
following steps or tie a plastic bag around your work on the lathe if
there is any delay. Some will add fresh shavings inside the bag and
optionally some water.
a. Cut the wood down to size with a chain saw
(or hand-bucksaw). Shape it round with the band saw (or chain saw or
hand-bucksaw) to turn on the lathe without major imbalance.
b. Choose one of the following two methods:
i. Choose this method assuming you want to end
up with a symmetrically round bowl balanced on a flat bottom. a) First
rough turn the bowl from green wood to a wall thickness approximately
10% of the diameter. This would be a 1 inch rough wall thickness for a
12 inch diameter bowl, and a thinner wall thickness for a smaller bowl.
b) Put the roughed out bowl in a paper bag for a few months. I like to
use an empty dog food bag. While in the bag it will slowly dry out and
because of anisotropic shrinking it will warp from its round shape. c)
Remount your dry bowl on the lathe, remove the wall irregularity and
finish turn to your desired symmetric wall thickness. d) Remove tool
marks and apply finish to your work as desired. e) Record the species of
wood, length of time in the paper bag and if the rough wall thickness
was OK (i.e. sufficiently thin to allow distortion without splitting and
sufficiently thick to permit removing all irregularity to achieve your
desired symmetry). Use your recorded observations to base plans for your
Use this method for natural edge bowls and
for non-symmetric bowl shapes desired when you consider their
irregularity to add charm and character. This method allows almost
immediate completion with reduced risk of cracking during the lengthy
drying process above. a) Quickly turn the wall to the thinness you
ultimately desire. Do this as fast as possible before the wood has a
chance to warp. Enclose your work in a plastic bag whenever you step
away from your lathe. Once the bowl has warped out of round it may be
too thin to continue shaping and impossible to add circumferential
design on the lathe. b) Smooth the surface and apply finish to your work
as desired. c) Keep a record of the species, wall thickness and other
observations that could help you plan your future work. Thin wall
turnings uniquely shaped by natural warping dry in a matter of hours or
days rather than months or years.
c. The web and literature is rich with other methods such as accelerated drying of green wood and replacing water entrained in wood. These are briefly reviewed to let you know what some others are trying although I have inadequate experience to justify my recommendation.
i. Accelerated microwave drying while avoiding splitting requires that removal of water is uniform through out the wood. Put the wood in a paper grocery bag and apply 20 to 30 seconds of microwave energy repeated after intervals during which the heated water can move out of the interior of the wood. This will require your attention but it enables drying to be completed within hours. Ideally the weight of the wood is recorded, along with pertinent details for future planning. Drying in the microwave oven can stop when the weight no longer changes. At this point the EMC is in equilibrium with the environment of the microwave oven. Microwave drying is best suited to turners whose eagerness to dry their wood exceeds their need of time for other work.
ii. Less accelerated drying may be achieved in a chamber equipped with a means of controlling the humidity and a thermostat for holding a steady, slightly elevated temperature of about 80°F. Drying times may be reduced from months to weeks with such a construction. Unused freezer chests and insulated plywood boxes have been described in the literature.
iii. Boiling wood for 1 – 2 hours is popular with some turners. Boiling supposedly releases water bound in cells such that when water leaves the wood less distortion occurs. Boiling is reported to reduce drying time by 20% to 30%. Practitioners of boiling say they experience much reduced splitting compared to the risk of splitting by drying their rough turned bowls in a paper bag for 3 months or more. I know of no one with first hand experience.
iv. Shrinkage can be prevented by replacing cell water with a high molecular weight polymer such as poly ethylene glycol (PEG). This works best if very green wood (i.e. well above its FSP) is soaked in a 50% by weight solution of PEG and water. After soaking for some weeks water molecules in the cells are replaced by PEG molecules. Absent water, cells remain in their full size and there is little distortion of wood with aging. However do not expect to rejuvenate already dried and cracked wood. Very long soaking times are required for wood thicker than 1 inch, except along end grain where there is much greater penetration. The rate of exchange increases with elevated temperature. Treated wood turns easily and it may be glued with epoxy, but polyvinyl and aliphatic type glues do not make good joints. Although more expensive, Pentacryl wood stabilizer can be used to treat wood similarly to PEG and it accepts most glues and finishes. Soaking in water with liquid dish detergent has been reported to substantially reduce splitting, but there are fewer turners using this method.
For a symmetric bowl:
1. Quickly rough turn to a wall thickness 10% of the diameter of the bowl.
2. Enclose your work in a plastic bag when temporarily stepping away from your lathe.
3. Wrap your rough turned bowl in a heavy paper (dog food) bag and let dry for some months.
4. When dry, remount, remove distortion and turn to the desired wall thickness.
5. Smooth the surface and apply finish.
For a natural edge, or non-symmetric bowl:
1. Quickly turn the bowl to the ultimately desired wall thinness.
2. Enclose your work in a plastic bag when temporarily stepping away from your lathe.
3. Smooth the surface and apply finish.
1. Dry quickly by heating the interior of the rough turned wood in a microwave. Stop microwaving after the weight of the wood stabilizes.
2. Replace moisture while the wood is very wet by soaking with PEG or Pentacryl. Dry.
© Hal Mahon 2006