Warping on purpose???

[Dana Berenson]

I've seen some bowls that are oval shaped (view from the top) and someone told me they were turned green and let warp on purpose.
Can someone tell me what the process is to accomplish that without cracking? I assume a specific grain orientation is recommended. Are there certain kinds of wood that work better for this kind of application?

Thanks!

[robert hainstock]

Dana, wood warps because it loses or gains moisture content. In my expirience, no matter how good it is sealed, it will always gain or lose moisture. When this happens warping or splitting, or cracking occur. Wood can ONLY gain or lose moisture thru the end grain, (pores). And as it does the wood either swells, or shrinks. This movement, (always cross grain, NEVER end grain) is what causes warpage. A piece of wood cut to a specific length will never get longer or shorter. It will however try to get wider or narrower. Thus it will warp across the grain. As to what woods do this more readily, I suspect the open grain woods to be more susceptible. Woods like oak, ash, walnut,elm. Good luck with your projectted idea, and remember that It will be very hard for you to control warpage.
Bob

[Jim Becker]

"Warping on purpose" is quite common with folks who like to turn woods like apple and madrone wet and very thin. (Both close grained, but less stable species) These all do need to be hand sanded and finished after they dry, but by turning thin and even thickness all through, cracking is minimized. In general, a "faceplate" orientation will often warp more than an end-grain orientation, but like in anything, nature is what it is and you never know "exactly" what you're going to get until it happens. Wood is like that.

[Jim Underwood]

Robert,
I respectfully disagree with a couple of your statements. Mostly it's a matter of degree.. For most practical purposes, your position is correct.

But since I'm a stickler for accuracy, I submit the following:

First of all, wood can lose moisture from side grain. It's just that moisture loss from end grain is much quicker because endgrain exposes the pores of the wood to the air, and the moisture can escape more readily. It also varies with the species of wood. Some woods if sealed up on the ends while green, will eventually rot because the wood cannot lose moisture.

Second of all, wood does move along the axis of the tree. Wood moves in all three dimensions. It's just that axial movement is much less than tangential and radial movement. If I remember correctly, tangential movement of wood is the greatest of the three. Please correct me if I'm wrong.

A first hand example of axial movement of wood: A friend of mine made a dock on a lake near here. When he put the treated wood down for the decking it was still quite wet from the treatment. He snapped a line on the edge, cut the ends of the decking off flush. A month or two later, it looked like he'd cut them all off in different lengths.

The key to drying wood is to control it, not stop it completely.

To partially answer the question first asked, I think one of the keys to preventing cracks when you turn something very green, is to turn it to a consistent thickness. This way the whole thing dries more or less consistently. Also keeping it in a cool dry place helps. I'm not sure if you want drastic wood movement whether you'd wrap it in newspaper or not, or leave it off, and just let it dry as quickly as possible.

Grain orientation? If you want an oval shaped bowl, then you should turn the bowl the traditional way, ie. side grain. Why? Simply because it shrinks more tangentially, causing the bowl to be "oval" shaped when its dried. If you turn it end grain, then mostly it will just shrink instead of warp.... At least, that's what my last Hollow Form did......although... those guys who do those really wild warpy objects may have turned those things endgrain...... I'm trying to find an image of those things on Google, but I can't find it.

[Richard Madison]

Does anyone out there ever steam a dry woodturning in order to re-shape it? Think I read that this is done with hat turnings, which are typically turned VERY thin.

[scott schmidt grasshopper]

thanks jim for getting the facts corrected. . I turn all green wood and it moves all three ways . somewhere there is a list of which wood shrinks how much . that tells you how much to expect( not counting the variables of burl and feather.) as for drying. here is what I do, turn the bowl to final thickness (must be uniform) then soak in LDD for a couple days to dry . during low humidity and high temp( summer) I store them in paper sack . others wrap the OUTSIDE with newspaper or saran wrap ( this slows down the drying so the inside of the bowl can shrink first) the way I understand it . bowls crack cause they have something to push against so if you turn it thinner it will warp and flex instead. dry the bowl slowly to avoid cracking.
second option throw it in the microwave and cook it . that steams it ( but is another thread)

[Raymond Overman]

Dana,

I think there are a couple of different ways to look at your question.

Do people let things warp on purpose? Yes, looking at Christian Burchard's madrone baskets is a good example of turning something and knowing that it's going to warp. Then working with the warped piece to make what I consider one of my favorite series of work.

Do people warp things on purpose? Yes. Look at JoHannes Michelson's hats for an example. He purposely binds them in a series of molds with industrial rubber bands which controls the warping and creates a certain effect.

Do people tell you they let it warp on purpose when really, they just weren't patient enough to use the turn and re-turn technique to make a round bowl? Yes, and it makes a nice looking bowl with a "how'd he do that?" factor. I've done it myself and think it's ok for some pieces. Especially natural edge pieces where the bowl looks elongated already. The key to doing this is to have a consistent thickness throughout the whole piece, preferably as thin as possible to prevent cracking. Sometimes the cracks can't be helped though.

[Frank Kobilsek]

Is Dana describing a natural edge bowl as oval?
Frank

[Brian Brown]

Dana,

Get a copy of this book. Turning green wood by Michael O'Donnel. It has great information about your questions and more. It has some charts that give the expected shrinkage for all three directions for various common woods. A bit scientific for my feeble brain, but still lots of good information.

[Reed Gray]

I am some one who prefers to turn green and then let them warp. I love the shapes. To me it is 'why turn it twice?' I don't consider myself artistic, but the warping makes it seem so.

As to the process, it is simple, turn to final thickness, of maybe less than 1/4 inch to maybe 1/2 inch thick, and let it dry. Then sand and finish. The drying is the tricky part. All woods are different, and some like maple and walnut are easy to dry, and some like the fruit woods are more difficult. Madrone is in a class of its own. The soaking methods (DNA/alcohol and LDD/dish soap) when applied here do nothing to stabilize the wood or reduce drying time. The DNA makes the wood harder (I don't know how or why, but it does) and the LDD makes the wood much easier to sand. I used to dry the blanks by just putting them on the floor for a few days, and then up on a wire rack to finish drying. This works for most woods. I have tried paper bags on the madrone, and get better success than I did with just air drying. I have put them in shavings, but have had the shavings color the wood, so I don't do that. You can use plastic bags for drying, but have to turn the bags inside out every day or two. I now (borrowing from the DNA method) wrap the outside of the bowl in a couple layers of newspaper, and wrap the rim with plastic film (the stuff, 6 inch, you wrap around pallets of boxes to keep them from falling off) and cut out the paper on the inside of the bowl. While I have only dried about 100 bowls this way, I have been able to leave the madrone thicker than I normally would, and it doesn't seem to crack. I can reuse the paper wrappings as well on similar sized bowls. You can also wrap the outside totally in the plastic. The theory is that the inside dries first, pulling the outside in, so that the bowl dries in a compression mode. With the plastic on the rim, this presents a bit of compression as well. Do also take care to round over the rim of the bowl slightly. A sharp edge is more prone to splitting, as well as cutting you while it is turning. You can also boil your bowls to stabilize them, but I don't do this because of the trouble, and because the boiling seems to muddle the colors some. Drying time is minimum 1 week, to 2 weeks. General rule here is that if you dry it too fast, it will crack, and if you dry it too slow, it will mold. This will vary depending on your local weather, temperature, and humidity. If you wanted to, I think you could apply some finish oil to help seal it, but I wouldn't apply any end grain sealer as it would have to be removed before sanding and applying finish.

Sanding is a bit of a problem. I use a recess to mount the bowl, and with the drying, the recess also warps. I use a set of extended jaws on a smaller chuck to expand into the recess. You do not need the death grip here that you need when turning, because sanding is done at slow speeds. If you rotate the bowl while mounting, you can get all 4 jaws in contact with the wood. My older PM 3520A model would go down to 10 rpm or less before turning off. My Robust wouldn't do that when I got it (turned off at about 50 rpm like the PM B models), but I called Brent, and he walked me through the process so I could have the really slow speeds for sanding. I would imagine that any of the frequency modulated lathes can do this, but you need a technician to tell you how. Most turners never have a use for these slower speeds. I do power sand the bowls this way. I have a piece of cork under the trigger to limit my high speed to around 600 rpm or less. You don't need high speed to sand, and the sanding goes as fast, or faster, you don't lose any time, and the drill lasts longer. Some of the bowls are so warped that I could never turn off the bottoms so I don't worry about that. If you use a tenon, I would imagine that you could do the same thing, but the tenon leaves a thicker part of the wood, which could cause cracks, and would have to be turned off.

I turned a bowl yesterday from some dry wood. First time in a while. Turning green wood is so much more pleasant.

robo hippy

[Leo Van Der Loo]

It looks like there's a bit of misunderstanding of the wood movement, so I will put this link here for all to load and save, so there doesn't need to be wrong or misunderstandings being bandied about.
Also a small excerpt to give some of the essence of this, HTH

http://pods.dasnr.okstate.edu/docush...EM-5009web.pdf

[Leo Van Der Loo]

Dana, yes you can predict the wood movement of the wood and influence it also, though wood has often all kinds of conditions that make this very difficult, like knots or uneven heart or sapwood, figured wood caused by tension and also in long limbs that grow more horizontal, and or the combination of these.
If you take a plank-wise blank that is mostly sapwood nice and even and just a little heartwood and turn it into an even thickness walled bowl, the sapwood will shrink around the heartwood, making it look like the heartwood is pushed-up, though that is not the case, by shrinking the rim gets bend down as the center is where the most shrinkage is (longest length), and by doing so it will pull the ends towards each other.
You can make even more of a change if you turn the walls very thin and heat in the microwave, then twist or bent the walls and let cool and dry in that position.
You can experiment with this, just get green wood and keep it wet while turning and turn the walls thin then do the above, lots of fun to be had.
Oh here's a picture also of a Black Walnut bowl that dried into this form, with no help of me ;-))

[lynn smith]

The wood doesn't necessarily have to be green, just needs to have a relatively high moisture content.
Also certain woods lend themselves to warping or distorting.
I think it helps to have part of the heart wood as the top edge of the bowl.
It has been my experience that fruit woods tend to move more, at least wood here on the left coast.
Here is an example of a NW native species of wild cherry.
This was not done deliberately, but knowing what I know now I have an idea of what to start with, however with wood there is never a guaranty.

Sorry not the best photo.

The fun is in the experimentation.