Preparing green wood
From WikiWOOD
Wood for turning projects can be obtained commercially, or it can be found in your community.
When a turner has obtained wood from the community it is probably not dry enough for successful turning. It may be freshly cut and very green. This article addresses the approaches and techniques for preparing wood that is not at moisture equilibrium for storage until it can be turned. This type of wood is usually referred to as Green Wood.
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Green Wood
When you cut a piece of wood, some of the forces internal to the wood are released and the wood moves. Look at a piece of wood that grew as a branch. Usually the growth rings are larger on the bottom of the branch, to equalize gravity. Wind may also cause this effect in the trunk of a tree. This is usually referred to as reaction wood. When cut, it will react, by moving or cracking. Many turners avoid reaction wood because of this unpredictable behavior.
Wood that is freshly cut will have a high moisture content, perhaps 50 percent. Kiln dried wood is usually in the 6 - 12 percent range. The rule of thumb for drying Green Wood is "One year for each inch of thickness." The goal of the Turner is to dry wood to "moisture equilibrium" for successful turning. Successful turning means no cracks. The wood can warp, since the warp can be successfully turned away.
Consider the structure of wood. Growth occurs in rings, annually. There is usually early wood and late wood each year. How much depends on the species. The trunk and branches of the tree allow the water and nutrients from the root system to move up and out to the leaves. They also enable the products from the leaves to move elsewhere in the tree. Think of the tubes these move up and down the tree as straws. The end of the straw is end grain. The side of the straw is face grain.
When wood dries, there is moisture loss through the end grain at a faster than through the side grain. (Just like a end of a straw.) One part of successfully drying wood is to equalize the loss rates.
The Cutting Process
Branch Blank Preparation
Branch wood can be used for spindle turning items. Spindle turning covers small items used in the kitchen, called Treen, to long spindle and/or newel posts for a porch or staircase. Branch wood might be appropriate for treen and other small items. It would not be appropriate for long thin spindles. Branch wood contains reaction wood. The process is similar to preparing blanks for bowls discussed below. Some of the cuts would be different.
The branch wood is cut to length and trimmed. The change from the image on the left to the image on the right shows the trimmed result. A turner can turn the untrimmed branch shown on the left, the roughing is much smoother, when stub branches are trimmed.
A branch would be rough trimmed in the field. At the shop, it would be further cut, probably using a band saw or hand saw for turning between centers. This trimming would eliminate the stub twigs and branches that take a lot of time and cutting air to remove on the lathe. The turner also would square up the ends for a more secure hold between centers.
The piece would then be mounted between centers for turning.
Bowl Blank Preparation
- See also: Cutting logs for bowl turning
To process logs into turning blanks the turner will utilize a chain saw or band saw, and appropriate safety equipment for the saw. The choice of saw depends on the size of the log and the size or type of saw available to the turner.
| Image:50px-Stop hand.png | Please read carefully. The safe use of a chain saw requires several items of safety equipment, and training in the safe use of the saw. Safety Equipment for the Chain Saw and proper use may be found elsewhere in Wikiwood. |
Start by cutting the logs into sections. If you have a 20 inch diameter log, cut the sections about 24 inches long. If you have a 10 inch diameter log, cut them to about 12-13 inches.. Using this approach the turner has some length on each end to trim if some checks or cracks occur. The log is the optimum size and shape for a bowl or other turned object.
When the turner encounters that part of the log containing a fork, called a crotch, the approach is slightly changed. Cut the crotch piece into a "Y". From the top of the tops of the crotch, allow 2 inches above the place they actually separate and cut these off. Determine the diameter of the log about 4 inches below these cuts. Add 2 - 4 inches to this diameter and measure the length of your log to make the bottom cut.
The very first part of the log, called the pith, is very unstable. The twig that originally starting this log is now the pith. You can see this on the end of the log as the starting place for the annual growth rings. The turner must remove the pith from of the log as the first step in avoiding cracks. Examining a piece of wood that has not had the pith removed one will observe most cracks starting from the pith.
- One approach is to cut the log into two pieces through the pith. This cut is a challenge.
- A second approach is to cut the log into three pieces. A slice about 2-4 inches wide containing the pith, and the two outside pieces Next cut the middle slice into two pieces with the square in the middle containing the pith. The turner discards this waste and use the two outside pieces for spindle turnings.
- A crotch piece will have one pith on the lower end of the log, as it was growing in the tree. It will have two piths, on the top. The turner will cut the crotch to eliminate all of these parts of the pith. Either of the approaches above will work.
The blanks are now shaped like a capital letter D. If you attempt to mount these between centers, the tail stock will be holding bark. That is not the most stable for holding a blank.
- One approach involves trimming the blanks once more. The arc full of bark is trimmed off the outer edge. Now your blank will look more like a rectangle with the outer side shorter than the inner side. If you use a wood stove or fireplace, this approach results in an additional piece for the wood pile. It also decreases the amount of time the turner will spend turning waste from the blank.
- Another approach is to remove the bark from the place the tail stock will be placed. This is best done just prior to mounting the blank on the lathe. A large drill bit, a spade type or forstner type is used to remove this part of the bark.
The blank may be further trimmed to a rounded shape at this time. The Turner may continue with the chain saw or may switch to a band saw. The rounded shape may be a rough circle or it may be a much more exact circle. This choice of square blank, rough circle or exact circle, is more of a personal choice of the individual turner.
The turner may rough turn the blank at this time. The blank can also be finished turned at this time. Typically, there will be a number of blanks from a log, thus some may be further prepared for storage.
Proper Preparation for Storage
The blanks should have their end grain sealed with some type end grain sealer for successful storage. This may be a commercial preparation, of water based wax emulsion sealer, available from your choice of turning supply retailers. It can be wax, such as candle wax or paraffin, heated to a liquid. It can be paint. The paint can be left over from a project or obtained from a paint outlet from a wrong color mix for an inexpensive price. The turner should keep in mind that a properly formulated house paint will allow the house to breathe. This property, valued in a paint, may not accomplish the seal required of an end grain sealer.
Apply a good coat, of your chosen type of sealer, on the end grain. After the seal dries the turner makes sure there is a good coat of seal on all end grain. The turner then stores the sealed blank out of direct sunlight and protected, so rain does not wash the sealer off the end grain. Allowance for some air circulation should be made.
The wood will dry. The rule of thumb for drying is "One year for each inch of thickness." Thus a 4 inch thick bowl blank will be at moisture equilibrium in 4 years.
Rough Turning the Blank
If the turner rough turns this 4 inch thick blank into a 8 - 10 inch diameter bowl, it should be left with a wall thickness of 10 percent. A 10 inch diameter bowl would have a rough turned wall thickness of 1 inch. This thickness would allow any warping during the ongoing drying process to be turned away on final turning. The 8 - 10 inch diameter of the bowl or vessel is what would be left from a 14 inch diameter log minus 2-3 inches taking the pith out, and 1-2 inches of bark removal leaving about 8 - 9 - 10 inches of diameter.
Now, the turner applies the end grain sealer to the end grain of the bowl (4 sides). Some of the fresh shavings are placed in the bowl, and it all goes in a brown paper grocery sack. This 4 inch bowl blank is now a roughed out bowl that will reach moisture equilibrium in 9 - 12 months. Alternatively, roughed bowls are stored without the paper bag.
Some turners have turned a freshly cut blank into a roughed shape and store in a plastic bag. This method does not utilize the breathing quality of the paper bag. The workpiece will build moisture inside the plastic bag. The turner should take the work from the bag frequently to keep the moisture from forming mold. This method can be used to induce spalting in many species of wood.
Moisture Equilibrium
Green Wood can start with a moisture content as high as 50 percent. Kiln dried lumber has a moisture content of 6 - 12 percent. The turner is not necessarily interested in waiting until the wood is dried to the level of kiln dried wood. The process of drying wood is to eliminate the cracking of the finished turning. Moisture Equilibrium is that point where the piece of wood is not loosing or gaining moisture into or from the atmosphere. A turner in Texas, USA will find the moisture content at equilibrium to be a different number than a turner in Lincolnshire, UK.
Some turners choose to use a moisture meter to determine the moisture content and thus moisture equilibrium. Some turners choose to use weight to determine moisture equilibrium. The weight of the blank or rough turned piece is recorded. The piece is weighed from time to time. When there is no change in weight over a period of a week or two, the turner can final turn the piece with some degree of assurance the piece will not crack or warp excessively.
Sources of Community Wood
When you get some wood given to you, make something from the wood for them in return.
Many local turning groups organize wood distributions or wood sales. Members will bring wood they are not able to turn or store.
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