Forced greenwood drying

Discussion in 'Woodturning Discussion Forum' started by Gary Beasley, Nov 1, 2017.

  1. Gary Beasley

    Gary Beasley

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    I was playing with some red oak I had recently acquired and turned a greenwood bowl down nice and thin. I wanted to see how it would look on drying so I chucked it into the oven and set it on the warm cycle for several hours. That one worked pretty good with no splitting so I tried another shaped a bit different. Of course this time there were cracks and rifts showing that I had to fix with sawdust and glue. Then I got the bright idea to put the bowl in a bag I would normally use for rough turned work to dry in. The next one was thicker but even thickness and I left it on the lowest bake temp I could get my oven to display which was 170F. After leaving it overnight and the bag quit steaming up my glasses every time I checked it the bowl came out looking quite nice. Apparently the bag keeps the bowl in a steambath as the water escapes so the wood remains pliable all over as the moisture content drops, in spite of the heat and rapid drying. I'm going to try this on a rough turned poplar bowl and see if it looks any worse off than a room temp dried bowl. I'm sure this is no cure-all for drying problems and will have problems all its own but at least its another option for getting work onto the lathe in less time.
     
  2. odie

    odie

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    Interesting. I suppose the MC of the bowl would be no less than the humidity within the bag would allow? If that is so, then it may present other considerations. Keep us informed of your experiments, Gary......

    -----odie-----
     
  3. Doug Rasmussen

    Doug Rasmussen

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    I'm impatient so quick drying methods interest me.

    Please tell us more? The bag kept the bowl in a steam bath, was the bag sealed up closed on the end? Did the accumulated moisture escape through the bag surface, or did you change bags periodically? Where did the moisture go? I can understand opening the oven door and getting steamed glasses as moisture escapes, but with overnight drying presumably you didn't get up and open the oven at regular intervals. (I've never thought about it, is an oven ventilated to let excess moisture escape in the form of steam?)
     
  4. Gary Beasley

    Gary Beasley

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    I had the bag rolled and clipped closed. Before I hit the sack I checked it several times to see how fast it was drying. The bag was one of those heavyweight brown bags with handles some stores give you, maybe 30 to 50 percent thicker than the usual paper grocery bag. And yes the normal kitchen oven has vents for steam to escape from.
     
  5. charlie knighton

    charlie knighton

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    Was there any smell afterwards in the stove??????
     
  6. Richard Coers

    Richard Coers

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    No need to reinvent the wheel Gary. Folks have been doing this kind of drying for decades. Do some searches on the AAW site for past articles in the journal and save some bowls from cracking.
     
  7. Bill Boehme

    Bill Boehme Administrator Staff Member

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    There are some problems with accelerated drying. Case hardening is one. This might be of greater concern with lumber drying than it is with bowl drying. In addition to what Richard said, there are some excellent threads on this topic.
     
  8. Gary Beasley

    Gary Beasley

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    Thats good to know. My searches didn't generate any results worth looking at. As far as case hardening a greenwood bowl might benefit from that if it manages to survive the heat stress. It may be more of a thing with twice turned bowls though I've had to deal with some really tough wood on the second pass even air drying.

    No paticular smell I noticed. It might be a problem if you decided to put a finish on before drying it.
     
  9. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Case hardening is one factor that induces stress in kiln dried wood.

    Here is a thread that has a lot of information on drying defects: Homemade Drying Kiln

    In the thread, I have several links to rather lengthy sources of information, but it's worth taking the time to become fluent in this topic.
     
  10. tdrice

    tdrice

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    Water turning to steam expands by a factor of 1700 to 0ne; so it is obvious that the oven has to allow the stem to escape or you would have an explosion when you opened the door.
     
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  11. john lucas

    john lucas

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    I went through the whole rush drying things. I found the microwave to be the best Now I just rough out more bowls and there are always bowls to be finished sitting in my wood storage shed.
     
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  12. Gary Beasley

    Gary Beasley

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    So far so good, the poplar bowl dried down to about 10% in a day, no cracks. Got a bigger one drying now to see if it behaves as well. That don't mean another wood won't misbehave, but the concept of steaming the wood dry seems to hold up. I tried a mostly closed red oak bowl with an inclusion in the side the other day. Normally you would expect that to do something wierd around the inclusion but all I got was slightly raised edges that sanded down no problem.
    I'm not saying abandon tried and true techniques but this has my curiousity up and I think this is good for special chunks of wood you need to finish off pretty quick. Its good for me because I dont have a whole lot of space to store drying bowls.
     
  13. robo hippy

    robo hippy

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    Well, if I was going to do forced drying, I would try a vacuum kiln, or a solar kiln. When ripping standard kiln/kill dried wood on the table saw, you get dust. When ripping vacuum kiln dried wood, solar kiln wood, or air dried wood, you get shavings. The difference may be the case hardening that Bill was talking about. When ripping 8/4 board stock from the solar/vacuum/air dried boards, I get zero spring/cup/warping with the boards. It takes a bit longer, but you get stable wood. I don't really know what the steaming and boiling does exactly, but it does speed up the process at the cost of muddling the colors.

    Don't try persimmon in the oven. A friend had an old microwave in his shop with a twist knob timer and set it for too long. He had to stay out of the shop for a couple of days.....

    robo hippy
     
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  14. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Boiling and steaming soften the lignin ... Think of lignin as sort of like the glue that binds all the cellulose fibers together. If there are internal stresses in the wood, softening the lignin allows these stresses to be relieved without cracks developing. Some people take the boiled wood out of the water while it is still boiling hot. To me, that sounds like trading one problem for another because that can lead to cell wall collapse and case hardening. I don't boil wood, but if I did, I would leave the wood in the water until it cooled to ambient temperature. My view of boiling is that it is primarily to stabilize the wood and not a means of accelerating drying.
     
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  15. Gary Beasley

    Gary Beasley

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    Agree about muddling color with boiling, but I haven't seen evidence of that yet with the steamout. Maybe because it's not immersed in anything to transport the colors anywhere.
    Your friends microwave adventure is like using a ball peen hammer to comb your hair. I've almost set fire to a purpleheart blank in a microwave trying to change the color. Microwaves make hot spots in whatever you heat, not a well controlled heat source.
     
  16. Gary Beasley

    Gary Beasley

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    I would agree on the stabilizing, but since I'm not boiling the wood but simply heating below the boiling point to drive the water off faster I just may be getting benefits of stabilizing without as much heat stress on the cell structure. I do see some powder on the twice turned bowl I tried but soon was getting plenty of curls off it. It'll be interesting to see what I can get away with. The most telling thing is to find the woods that this fails on.
     
  17. robo hippy

    robo hippy

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    Bill, one show (Oregon Country Fair, aka Hippy Fair) I go to is out in the country, and you build a booth. On their main roads, they use lignin to keep the dust down, and it keeps the road together when it floods in the winter. I do remember hearing that the steaming and boiling process ruptures the cell walls so the entrained or bound water comes out more easily, and yes it does 'relax' wood tension. Steaming, and I guess it would depend on how it is done, is a common process on cherry, or at least it was. It evens out color in the sap wood, I think...

    robo hippy
     
  18. Owen Lowe

    Owen Lowe

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    I’ve heard about the “good old days” of the OR Country Fair… Wowsza!

    I believe commercial black walnut is also steamed to even the sap/heart wood colors. The process dulls the heart’s purple-y chocolate color that air dried walnut exhibits - in my opinion.
     
  19. Bill Boehme

    Bill Boehme Administrator Staff Member

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    The part about steaming rupturing cell walls that I've heard mentioned frequently isn't exactly what happens. I suppose that concept arose from thinking that wet heat pressurizes the cells until they rupture. Cell walls are semi permeable so moisture can pass through. Also cell walls consistent of both cellulose and lignin. Moist heat can soften lignin enough to allow it to be stretched or squeezed which is what happens with steam bending.

    Bound water isn't water inside cells that is freed by rupturing the wall. It's called bound water because it is chemically and structurally bound by Van der Waal's forces as a integral part of each cellulose molecule and therefore is actually a part of the cell wall itself. Free water is all of the rest of the water in wood that is "free", in other words exists as actual water molecules. This is why it takes so long for bound water to evaporate.

    So, are clothes still optional at the Hippie Fair?
     
  20. robo hippy

    robo hippy

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    Most fun show I have ever done, and most profitable. Cloths are pretty much optional. They come off mostly when we get rain so there are mud parades... You just never know. I plan on 20 or more years of going.... As long as I am able. Flying Karamazoff Brothers and Flying Brassiers Off Sisters juggling flaming torches in the nude at the midnight circus.... Nothing like this show any where else in the world. Not even listed in the 'How Hippy are you' things..... Almost 50 years now.... Started off selling Hacky Sacks and Juggle balls.

    robo hippy
     

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