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Thread: Drying Wood - confused

  1. #1
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    Question Drying Wood - confused

    Maybe this has been covered before, but I have not found it...

    The whole drying of wood concept seems a bit confusing.

    If we start with green wood (mostly what I have), we need to get it to dryness in some fashion. The easiest is to let nature take care of it and leave the wood to dry naturally outdoors. This takes a long time and usually results in cracked/checked wood that is unsuitable. Reason: Water is drawn from the wood too quickly on the ends.

    In order to reduce the cracking/checking, we try to slow down the drying process with paper bags, wax coatings, etc. This would indicate that slowing the exit of moisture from the wood ends reduces the cracks. (flawed logic perhaps?)

    Another method of removing moisture is DNA soaking. As I understand it, the DNA replaces the water in the wood. Then we are back to some method to control the rate of DNA exiting the wood with bags or coatings. The DNA drying is considerably faster than the water - but seems contradictory to slowing the removal of "liquid" from the wood to prevent cracking.

    Then we have the kiln drying aspect (this is the really confusing part...). Here we force the moisture from the wood at an accelerated rate with heat, and yet, do not have the cracking/checking that would seem to be indicated as reasons for bags and coating use for drying at home.

    Have I missed something? I guess the simple question from my long winded description is:

    Why does kiln dried (or microwaved) wood not crack during the process if moisture is being removed at an faster rate than other drying methods?

  2. #2
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    Rick - just my 2 cents - a kiln is used to dry wood that has already been cut and dimensioned. The kiln controls the humidity levels at the same time it introduces heat. The process takes quite a while to gradually reduce the humidity levels so as to prevent checking.

    The basic idea behind any method of drying wood is to slow down/control the moisture release. DNA is a faster process as it mixes with the water and aids in evaporation but you still have to wrap the rough out or things dry to quickly and it will check.
    Steve

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  3. #3
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    Hmmm...high humidity keeps the wood from cracking...

    So if I use kind of an extreme example:

    If I submerged my green wood in water, and (somehow) heated the wood to drive out the moisture quickly into the water (water = high humidity), then drained away the water (or remove the wood) - the wood would not crack?
    ...interesting.

    BTW - I know some boil wood to keep from cracking, but I think this is to rupture the cells in the wood instead of my extreme example. - or, maybe this is the same thing...

  4. #4
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    For wood that will be left outside to season, I seal the ends with heavy coatings of AnchorSeal or other wood sealer. This has worked well for me as I've cut the amount of wood loss to about 5%.

    John Jordan suggest keeping turned green wood away from air movement. I began to pack my turnings in a large box with shavings away from any source of air movement. Perhaps a bit extreme but my shop is in my basement with both warm and cold air blowing almost directly from the fan unit. Can't comment of success yet.

    The DNA process does work so give it a try.

    Burt

  5. #5
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    Thanks Burt

    I also use anchorseal for my wood - and DNA soaking as well.

    The kiln drying thing just got me thinking (yeah , I know - don't strain the brain) and I could not come to a good resolution to the seeming contradiction.

  6. #6

    Another thought

    Rick,

    Another method is the LDD (dish detergent) soak. If you read the posts from Reed Gray, it works well for him. No fire hazzard. I believe that it also helps with the sanding afterward. One more thing to think about.

    Dave

  7. #7
    It is confusing, and there are a lot of methods used to dry wood.

    First, it is difficult to dry a whole log without it cracking. Large mass, with lots of water will dry unevenly, creating more stress, and cracking.

    2 solutions here, green turn to final thickness, dry and then sand and finish, or rough turn to 10% thickness, seal, and dry. Both these methods reduce the over all mass, and help reduce and equalize the stresses that happen when wood dries. Personally I like the warped bowls.

    If you can't rough out your green wood soon, seal the ends and cover your logs with a heavy tarp, and keep them in the shade. Some people want to keep the logs off the ground. For me, the ground has more humidity than our dry summer air, and it is better here, but in other places, there are a lot of bugs in the ground, which can cause a lot of damage to the wood. Of course, spaulting and rotting are going to happen faster if the wood is on the ground.

    If you are drying too fast, the wood will crack. It can't adjust to the stresses fast enough and some thing has to give. If you dry too slow, it will mold and spalt, which isn't always a bad thing.

    When you harvest makes a difference as well. Spring trees will have a lot more water than fall and winter trees, so they will shrink a lot more. More shrinkage = more stress. Some woods have a higher water contend as well. They are all different, and you will have to experiment a lot.

    There are 2 kinds of water in wood, free water which is in the veins, or whatever the name for them is, which is how the tree transfers food from roots to leaves. There is also bound water, and this is what is in the cells. Boiling and microwaving gets this water out fairly quickly. If you just air dry the wood, the free water evaporates out first. This can be sped up if you spin the wood for a couple of hours, and throw off the water, and this is also the water than comes out when you are turning green wood). The DNA is also supposed to speed up the removal/evaporation of the free water. It may contribute to getting the bound water out. Slow drying, trys to balance the drying of both types of water.

    The ways to control and balance the drying process, so that the water can get out while the wood can adjust and warp are many. We basically control the evaporation. Sealing the end grain is one way because the free water moves out through the open pours fairly easily. Paper or plastic bagging is another way, and so is burying in shavings and refrigerator with a light bulb kilns. You keep the humidity high, and it reduces slowly. The wood does need to breathe, and keeping it out of direct sun light and wind helps. Heat and wind will speed up the evaporation a lot, and more cracking.

    As far as the soaks go, both DNA and LDD, with my thin bowls (1/4 to 1/2 inch thick walls) there is no observable or measurable difference in drying speed, crack prevention, or stability. I say this from observations made while turning maybe 8,000 bowls and using both soaks and just plain old air drying. It may make a difference on the thick turned bowls, but I have never tried that method or tested it (Any thick turners want to do a measured test of this with the 3 methods???????). The LDD (liquid dishwashing detergent) does make bowls a lot easier to sand out.

    As to the success of the DNA soakers have, I figure it could have more to do with the wrapping of the outside of the bowl than the DNA. I now wrap the outsides of my green bowls. A couple of layers of news paper, and secure it to the rim with some of the 6 inch plastic stretch film that you use to wrap boxes up on a pallet. Let about an inch of the plastic go over the rim into the inside of the bowl, and stretch it out around the rim as you wrap. Do ease or round over the edges of the rim before wrapping as a fine sharp edge will dry quicker than the solid wood, and can cause cracks. Cut out the paper on the inside of the bowl. The theory is that the inside will dry quicker than the outside, and it will pull the outside in, kind of drying the bowl in a compression mode. Since I started doing this, the only cracks I get are from ones that were already in the wood, and I didn't see them. It even works great with Pacific Madrone which is the most difficult wood to dry that I have come across. I have just wrapped the rim with the plastic and this works great as well. I use the same method for my green hollow forms too. I will start the pieces on the floor (cooler temps and higher humidity), then after a few days, up on a wire shelf. Drying time is 1 to 2 weeks, depending on season and weather. I would guess that the wrapping will help the thick turned bowls as well, and the plastic might eliminate the need for sealing the outside of the bowl, but again, it isn't some thing that I have experimented with.

    robo hippy

  8. #8
    Very nice explanation, Hippy!

  9. #9
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    thats what i was going to say...guess i dont have to now

  10. #10
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    Since this has been a topic and fuel for so much debate, I did a short and fast search a little while ago. I ran across a guy, that claims that the soaking is not required, he just "dries" his bowls with shavings and a paper bag. No DNA or sealing. He claims to have compared DNA/LDD/Anchorseal/nothing. He noticed no difference in craking/warping rates. No need to spend money on DNA or anything. He rough turns, then dries in the paper bag, then finish turns. So I'm trying 2 cannisters with paper bag and shavings only. I though it was pretty cool, that when I checked on the bowls, the paper bag was cool to the touch, and obviously "damp". No cracks after a week. I can update if anyone is really interested.

    Just wanted to add yet another option, that someone claims to work.

    As expected, there are more ways to dry wood than to skin a cat. Oops, don't want to offend cat lovers, it's just a saying.

  11. #11
    Here's a little tip for you bowl turners..

    When you rough out your green bowls, try not to leave any sharp edges- make round ones instead. Sharp edges dry out very quickly because of their small surface, and start cracks. Once a crack starts even more drying occurs at that spot, which leads to more cracking.

    I'm trying this on my latest round of bowl roughing, so we'll see how this turns out.

  12. #12
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    Rick,

    Drying methods are a matter of faith and science. While there may or may not be scientific logic behind each method, it will come down to which method you put your faith in. Try a few methods, talk to turning friends, get comfortable with which pew in the Church of Bowl Blank Drying you want to be in and just be there for a while. Along the way you'll start to experiment again and might try a different pew.

    Regardless of pew one thing that is true of all methods is that your wall thickness of the wet roughed blank must be consistant or you increase the risk of failure as the blank dries.

    Reed's logic of 'wrap only' has had me cuirous to the point of experimentation. While my documentation and my sample unit number is still limited I am getting good results skipping the DNA soak. Just as with DNA I leave the roughed blanks wrap in newspaper for 10 to 12 weeks, unwrap and feel confident to finish turn in another 4 to 6 weeks. These tests have been run on Walnut, Buckeye and hard maple on 8 to 12 bowls roughed to approximately 1" thick. DNA got me a long way so I'll never say anything bad about it but it may be time to adjust my methods.

    Frank
    'Sawdust is better than Prozac'

  13. #13
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    My method, which I've used for at least 30 years, is a bit different than most turners do. I rough out most everything that I turn, then re-turn when it's dry.

    After rough turning, I VERY liberally coat the piece on the outside with Johnson't paste wax. If the wood is very wet, I paper bag it for a few weeks before letting it dry in the open. If the wood is one that's prone to cracking, such as fruit wood, I use Anchorseal. If it's wood that's been drying for awhile, I just put on a shelf to dry. I put the date turned on the piece. It'll be dry in no more than 6 months.

    I live in hot, dry Arizona and my shop has no A/C so it gets hot. I almost never have cracks that weren't there before I started. I should mention that I never rough turn in the hot months. Most is done in the Fall after I get a fresh supply of wood from Oregon.

    I'll post a pic showing about 70 rough turned and waxed pieces in my shop in various stages of drying. NONE are cracked.

    Works for me, and has for a long time. Soaking in various solutions no doubt works, but I don't bother.

    Wally
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    Last edited by Wally Dickerman; 04-09-2009 at 3:22 PM.

  14. #14
    I haven't turned anywhere near the bowls that Wally and others have but I have been using the wrapping method. Here in MS it is very humid and I think that slows the drying. I rough turn to about 10% of the width of the bowl and them wrap in newspaper and I also put wide masking tape on the rim. I leave the inside open. After a couple of months or so, I finish turn them. I have not had a bowl crack yet.
    If the the wood is very wet, I put on some sanding sealer before the wrapping and taping.
    Last edited by Don Carter; 04-09-2009 at 6:06 PM.
    All the best.

    Don

  15. #15
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    Maybe I have a misconception about drying time.

    I thought using DNA, Boiling, LDS, Kiln, etc - all shortened the drying time while preventing cracking (as opposed to sealing with paper, plastic, wax - and letting dry very slowly to prevent cracking) Is this not true? Almost sounds like they don't (all) reduce drying time.

    I understand how letting the moisture out very slowly will give the wood a chance to stress relieve without cracking. I just don't want to wait a year before final turning a rough green bowl.

    I think using the alternate methods allows the moisture to be released more quickly - but not too quickly, so we still need to control the rate of escape with bag or coating. So now I can use DNA to be able to final turn a bowl withing 2 months instead of 12. I am guessing that the moisture escape is within the allowable "window" to keep from cracking, but closer to the maximum. There is probably some magic rate at which moisture can be released effectively for each piece of wood. Some will probably crack no matter what steps are taken.

    The kiln thing was most contradictory to my assumptions about slow moisture release, but I thought it was the fastest way to dry. Is it the fastest way?

    Still learning...

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