10 Tips on drying your own wood and wood movement

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10 Tips on drying your own wood and wood movement
User: lauralynn
Date: 9/22/2015 10:59 pm
Views: 1483
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History

Period pieces or even ancient furniture were built out of wood that was sawn to as near the shape as possible for it’s use and then dried in the air till it was ready to be used for the project.  Since logs, boards, and even later veneers were still sawn by hand, wood was an expensive material.  It was very important to not waste this material, so stable wood to work  could make or break your reputation and your pieces.  Depending on the trees that were available in the region and the climate, wood could be used almost immediately after sawing or take several months of drying before it could be used.

Why dry your own

Drying your own wood can be both rewarding and profitable.  Since woodworkers come in all shapes and sizes I’ll group drying into small craft wood and large lumber sizes.  Small craft wood is best cut up from trees with a chainsaw and left to dry slowly.  Small pieces also allow you to get highly figured lumber from almost anywhere on the tree.  Thick pieces should be air dried slowly, or roughed to shape at 1” thickness and left to air dry further.  Very small pieces like pen blanks can be dried quite easily in a household kitchen.  Lumber is usually stickered and allowed to air dry for a few weeks to a year.  Air circulating around the piles costs no money and will do much of the drying for you.  After that a short trip to the kiln and you are ready to make furniture.

1. Harvesting your wood

Trees come down for a variety of reasons and the “green” movement makes many recyclers offer their trees to anyone who can use them instead of pushing them into a landfill.  Being good with a chainsaw and having a friend with a truck to haul your finds can be a very rewarding experience aside from saving you a lot of money.  Commercially, tree companies will save the best logs which are considered to be the first cutting above the ground and sell them directly to sawmills.  These logs are also available in small quantities from urban areas if you are lucky enough to have a sawmill.  Urban trees should always considered to have metal or debris in them and treated accordingly.  Wood cut directly from the tree is called “green”.  Wood that is between 10-20% moisture is call “air dried”, and finally lumber from 6-9% is kiln dried lumber when speaking of hardwoods.  In construction timbers like pine, local home centers sell pine that has been kiln dried for one day to set the pitch in the wood so sap doesn’t continue to ooze out.  It will be stamped kiln dried, but has a moisture content near 15%.

2. Cutting it to size

It is the lucky person who knows what the wood will yield before cutting if from the tree.  It is even more lucky to be able to cut just what you need for a project so you have little wasted time in the milling and drying steps.  As a woodworker I sometimes use lumber and craft sized wood in the same pieces.  Jewelry boxes for example can almost always be made from highly figured boards that are cut with a chainsaw oversized and dried at home.  When I need lumber and have to buy commercially I start to look at the grade of the wood and use my cut list to mark out where on the board I would take my pieces.  Having your cut list when you are buying your wood can save you a bundle of money and wasted time.

3. Stickering

Stickering varies based on the thickness and overall dryness of the wood.  Thinner and also wetter wood requires more frequent spacing of the stickers to avoid warping during air drying.  Make sure your sticks are stacked directly above each other to keep even pressure throughout the boards.  A good number to look at is 12” apart for 1” lumber when green.  Air dried lumber at 1” might be 18-24” inches apart for stickers.  2” green lumber just doubles what it is for 1” and ½” lumber makes the stickering twice as much.

Sticks that are longer than your pile is wide work well placed diagonally, but still require you to place them above each other.    On expensive woods that you are trying to avoid any distortion strap clamps are frequently bound around the piles to make them into a tight bundle so they can’t warp.

4. Final Drying

Final drying is easily done after air drying is completed.  Lumber can be brought into a climate controlled area like a home with central air or a heated shop and left to acclimate.  I can typically take wood 1” thick down about 1% in moisture for each 2 days in my home shop during the winter when the humidity is low.  Use of a dehumidifier in the summer should give you the same results.  I use 2 different kinds of moisture meters to guarantee my readings.  Once I’m under 10% moisture I don’t minding cutting the boards apart to rough oversized shape for my project.  I like to leave (when I can spare it) about 2” extra in length and 1” extra in width for all parts.  The added air flow to my smaller boards increases their drying speed dramatically.  Commercially lumber would be loaded into a kiln on stickers and dried for 1-2 weeks to bring the wood to 6%.

5. Kiln Drying

Kiln drying is often synonymous with final drying.  Getting the wood to 6% moisture is done commercially, but isn’t needed if the environment for the project doesn’t routinely get to very low humidity.  Coastal areas typically run about 10% moisture in the wood during the summer in a home, while desert areas can be near 6% for most of the year.  The important thing to remember is that it is “Relatively Dry” which is a term I’ve invented meaning it is as dry as it needs to be for either the project or the future location.  For example, wood that is turned into large bowls is never kiln dried commercially nor is it measured with a moisture meter for dryness.  The artist would turn it roughly to shape and then let it air dry in the shop till it acclimated to the environment of the shop.  Final drying would occur in the resting place of the piece in a home!  Kiln dried pine for homes doesn’t dry all the way until long after the walls are up and the heat is on. 

6. Drying Defects

Wood is very much like a tightly packed bundle of straws.  When the water moves from inside the wood where there is a high concentration,  to a low water concentration  outside the wood it takes the path of least resistance out the ends of the straws.  Water trying to get out is much like traffic leaving town for the weekend in a busy city.  There are accidents at major intersections as people (water) seeks the easiest path to escape.  When this happens you get cracking or checking at the ends of the wood.  Commercially lumber that is sold has 3” added to length to account for this defect which is very common.  When the wood is highly figured and has grain changes in multiple directions the water can escape through other parts of the board and the wood can literally tear itself apart.  Some woods are known for their instability or stability based on how much shrinkage occurs from green to final dryness.  Catalpa and mesquite are domestic species that are very stable and really can be used without any drying.  Madrone and myrtle burl which grow near the coasts are very unstable and have to be boiled to stabilize the interior before they can be dried.  Much of the drying defects occur because wood near the outside of the tree dries faster that wood near the interior.  If you look at a round cut from a tree you can see the crack is much wider on the outside than when it reaches the pith or center.

7. Moisture meters

Moisture meters come in 2 basic versions.  Pinned and pinless.  There are advantages and disadvantages to both.  Pinned meters can check thicker lumber by driving a nail or screw to the core of the wood and then seeing the moisture in the center which is the most important.  Remember that if the wood in the center is dry then the outside is even dryer.  They do suffer one large flaw in that when the wood is hot as in a kiln they will give a falsely lower reading than the wood actually is.  It is best to have a sample board that you can cut a piece from and check outside the kiln after it is cooled to room temp for the most accuracy.  Pinless meters work well at any temperature and can be used in multiple spots on the board without producing pinholes, but don’t give an accurate reading on wood thicker that 1.5”.  The meter will say the wood is dry because it is measuring the outside dryer layer.  All meters work on the principle of resistance and measures the resistance within the wood.  The amount of water varies the amount of resistance which is what the meter shows us.

8. Movement after drying (Springback and seasonal)

All wood moves all the time.  There is a large change from “green” off the tree to “kiln dried”.  By kiln drying our wood we are making it more stable so that we can plan for how much it will move.  In general terms fast growing trees are less stable and will move more when drying and after dried.  The second consideration is the count of the growth rings.  Hi growth count lumber means the wood is very hard as the wood fibers are compacted compared to a large growth count tree.  You can remember it by thinking that a slow growing tree is slow to lose water when drying, slow to take on moisture after drying, and slow to change shape or distort during or after the drying process.

Wood shrinkage is like that of a balloon filled with water.  A balloon gets very wide when filled with water, but the length does not vary as much as the width from empty to full.  A piece of lumber does the same thing by shrinking in width compared to what it does in length.  As humidity changes from the seasons where the final project is sitting the wood will continue to take on and release small amounts of water making it shrink or swell across its width (across the grain).  If wood averages 8% shrinkage in width from green to dry, it probably only changes 1-2% from seasonal changes while moisture changes might range from a low of 4% to a high of 16% annually in some climates.

9. Keeping it dry

Wood should be stored in a climate controlled environment after you have it to the moisture content you want it to reach.  Ideally the climate will be the same that the shop and final resting place of the piece.  If you can keep wood movement to a minimum after drying warping should be greatly minimized.  I like to take mine from the kiln and let it acclimate to my shop for about 2 weeks.  I also always make sure I know the location of where the piece will stay.  If it is sitting in the open air in coastal California I know 10-12% moisture is fine, but if I’m looking indoors in Arizona I’m looking at 6%.  Try and keep your wood as close as possible to this as you work with it.  I’ve wrapped mine in plastic between sessions of working on it and applied weight to keep it from distorting.  Nothing is more disturbing that coming out to the shop the following day after you reached a final dimension on a piece and finding it slightly warped!

10. Planning for movement in your projects

Some woods like Mesquite, Catalpa, and Cocobolo have very little shrinkage and therefore require less drying and less care when planning.  Most woods however need to be planned for in advance.  Since most wood movement is across the width of the grain that is where the planning has to occur.  Quartersawn boards will expand and contract in thickness and can be a great way to counteract the effects of shrinkage when planning out a piece.  The fact that so many craftsman pieces are around today in quartersawn white oak testifies to the stability of the pieces after more than 100 years.  A simple rule of thumb is to plan for at least 1/8th inch of wood shrinkage and expansion for each 8 inches of width in a board.  Many species will be more or less, but that number will catch most problems that you might run into.  That means plan to elongate screw holes that much, loosen the internal tightness of tenons where needed, and don’t make your drawers fight tight as a tac.

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