Message Thread:
40 inch diameter log in solar kiln
6/15/14
Website: http://www.heinrichspillmann.com
Does anybody have experience or can comment or has heard of somebody kiln drying large ( approx 40 inch diameter 9 feet long) timber. I am carving a two sided bench (see attached picture incl. scale model ) out of red oak log. I am planning to build a solar kiln ( Virginia Tech model) for it. I am hoping to reduce the weight but also to somewhat stabilize it and hopefully prevent huge excessive cracking. I pretty much used only the lower part of the log and will cut out the center. I have posted here before and Gene, Henco and Dave commented on the potential process of solar kilns. I am curious if anybody can comment on the issue of temperature and humidity which probably also translates into the ratio of the size of the solar collector to the size of the drying chamber with only one log in it. I expect the finished still green piece to be approximately 1000 lbs. but essentially only have the surface of one solid log and not linear board feet to dissipate the moisture.
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6/15/14 #2: 40 inch diameter log in solar kiln ...
Heinrich,
I would NOT put your carved bench into a kiln. Kilns are designed and meant to dry dimensional lumber that is generally 2" in thickness or less. Red oak has a high specific gravity and higher than average radial to tangential shrinkage ratio (in other words it is highly prone to drying stresses) resulting in warpage and cracking. These drying stresses are greatly increased with increases in the woods thickness.
The challenge for you will be to SLOW down the drying rate, otherwise the wood will crack and distort as it will try to dry too quickly if left unprotected.
One approach for you to consider is to apply a coat or two of clear shellac sealer to your piece which will help seal the wood and reduce the chances of drying defects. Clear shellac is cheap, can be applied easily and won't change the color of the piece much. I would place the piece in the shade outside and give it some loose overhead cover to protect from the elements.
Best of luck.
6/15/14 #3: 40 inch diameter log in solar kiln ...
Thank you Mark, I was afraid of the no kiln response, because actually my biggest issue is that I would like to be able to show this piece indoors. I have experienced the destructive effects of a heated environment on "green" pieces. with this one of course the question is how long will it untill it is safe to bring it in to gallery space if I can not accelerate the drying process? It sounds like it will be a looooong time, years probably.
6/15/14 #4: 40 inch diameter log in solar kiln ...
Heinrich,
One possibility to accelerate the dryinig process and to reduce cracking is to cut a series of deep parallel slots (say 1/2" wide and 3" on center) on the underside of the piece (out of view) that will allow the wood to distort rather than crack as it drys. Remember, the wood wants to warp away from the core of the log, so slots on the underside (outside) of the log should allow the wood to shrink towards the slot, closing it up as it moves.
So, in summary dry slowly by sealing the wood with several coats of shellac (especially the end grain and heavily figured areas) and provide spaces (slots) so that when the wood distorts, the piece can move a bit without cracking.
Another thought- Be sure that the wood thicknesses are not too different from each other. What I mean by this is do not have thin cross-sections project from thick areas as these thickness differences will cause the wood to dry very quickly in the thinner section causing stress at the interface with the thick areas. Transition the thicknesses gradually for best drying results..
The bench should be a beautiful and unique piece worth the added hassle. These types of challenges are what make me loose sleep at night, but keep me excited about woodworking.
6/16/14 #5: 40 inch diameter log in solar kiln ...
Mark,
As you can see from the attached scale model, of course I complicated the issue considerably by planning thin cross section protruding from thicker areas, trying to remove as much wood as possible, to reduce the weight of the finished piece. As you point out though, it looks like I need to review that too. I like your suggestion of ventilation and stress reducing deep grooves on the underside of the piece. I have also thought of cutting the piece into four quarters as indicated by the pencil lines on the model and join them again after considerable drying as I do with my carved chairs. But also as I progress with carving the top of this bench I question again the wisdom of shaping the under side as shown in the model, but rather keep the mass of whole the log undisturbed and just deal with the weight. I agree about the sleep issue, but a beautiful challenge is so much better then wasting my time sleeping. I always thought I can sleep when I am old but apparently that's not the way it works either. Go figure
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6/18/14 #6: 40 inch diameter log in solar kiln ...
Looking at the scale model, it seems a modification in design may make a large difference in drying capabilities. Looking at where a person will sit, there is a lot of mass that could be removed under and behind the seat area; a person would need to keep a "leg" at the rear there for balance, opposite the "leg" that appears in front of a settees feet. I love the design as it is, and am not saying you need to change it; I just think it would dry more reliably if you did.
6/18/14 #7: 40 inch diameter log in solar kiln ...
Website: http://www.heinrichspillmann.com
Brian
I very much appreciate your input. But unfurtunatly it is the mass of these tree trunks that fascinate me, that us why I would like to keep the pieces as voluminous as possible.
A new thing occurred to me; what would happen if I would slab the whole bench into 2 inch slabs, dry them in a kiln and glue them back together. I am curious if anybody can talk to this. The horizontal surface of the bench is approximately 4 inches below the plinth. What are my chances of ending with somewhat straight boards ready to be glued back up after the proper mc is reached? I am planning to build a solar kiln (Virginia tech model) to do the job.
6/18/14 #8: 40 inch diameter log in solar kiln ...
Your design is YOUR design... stick with it.. get it done,
see what happens to it as it dries and learn from it...
That is how folks with designs that vary from the normal learn and hopefully will become famous because of it...
Good luck in your ventures...
6/19/14 #9: 40 inch diameter log in solar kiln ...
If you really want to do this, with the least headaches, you should go at it while it is green, then cut in from the bottom, hollowing it out, so you are only drying wood which is about an inch or two thick. This will allow the wood to dry faster, but also it will relieve a lot of the stresses, by being able to deform a little.
While you are working on it, you can spray it down to keep it wet, which will prevent minor checks from starting, plus green wood is only half as hard as after it dries, and the dust it less.
If you go off to lunch or at night, cover it with a drop cloth, or plastic to prevent drying until you have it thinner.
If this is not a commissioned piece, you will stand a better chance of selling it, if it doesn't require a forklift to get it into someone's home.
As for drying it, after doing as I stated above, rather than trying to dry it fast, I would set it up on sawhorses, or a stand to allow air under it, but then find a big cardboard box that you can set over it, and remove off and on throughout the day. This will keep the MC more even throughout the thickness, which will reduce checking. Slow is good.
6/20/14 #10: 40 inch diameter log in solar kiln ...
I love the organic shape of the bench. I hope it has straight grain with few knots (or none). The idea of the relief cuts and shellac are good, but you (or the customer) will have to accept that it will likely crack over the next five years or so. Then you can go back and fill the cracks with an epoxy/turquoise mix to make the cracks part of the design. Let us know what you do and how it works out.
6/21/14 #11: 40 inch diameter log in solar kiln ...
I very much appreciate all your input. Ec Aumiller I' am all with you on getting it done, it is all about trying to find ways to realize the vision. Still there is a lot of experience out there and some of that might apply or be usefull, that is what I appreciate on this forum. I don't like the idea of hollowing the piece out from the bottom, labor for one but i seems it would also creat wired stresses and with that cause cracking. Nobody commented on the idea though to saw the carved piece up into boards, kiln dry them and glue them back up. I am thinking of 12/4 boards, the widest approx 35 inches and then finish-carve and sand the surfaces after they are glued up again. I am thinking of turning the piece side ways and cutting plainsawn boards. That way I would end up with about 13 or 14 boards of various length. I could even cut out the center portion of each board and end up with a hollowed out piece and much lighter piece at the end once all the boards are glued up again. Any thoughts, any body.
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6/21/14 #12: 40 inch diameter log in solar kiln ...
Heinrich,
Sawing the piece into boards, then reassembling has its own issues. When you saw lumber from a log, there is a range of quality in the resulting lumber. Some boards will warp or twist because of grain run out or knots etc. Drying 12/4" red oak is a slow process and most kiln operators would air dry lumber this thick for a full year or more before even attempting to dry in the kiln. Once in the kiln, plan on at least a couple of months or more and count on loosing some of the boards to defect....
So, in theory your idea should work, but in practice it is fraught with difficulties.
Protect the log from drying too fast and keep scratching your head. You will come to terms on how to approach this piece with both patience (but don't wait too long) and creative thought..
6/21/14 #13: 40 inch diameter log in solar kiln ...
I am starting to think that it might be best to find a client with a covered pergola type outside lounge area for this piece. There it can sit in the shade and dry. It just might not be feasible to carve some thing like this for heated interior spaces ever.
I still would like to explore the saw mill route but probably with an other piece and also maybe with a wood species that is easier to kiln dry then oak. Any suggestions? I am moving my operation into the yard of a consortium of different tree service guys and might have easier access to different spicies.
6/21/14 #14: 40 inch diameter log in solar kiln ...
Heinrich,
Any of the softwoods or hardwoods with lower specific gravities (say around 0.5 or lower) are the easiest to dry. Species with high specific gravities, interlocking and/or open grain are harder to dry without defect.
For a list of tree species specific gravities see the link.
http://www.wagnersg.com/
6/21/14 #15: 40 inch diameter log in solar kiln ...
Thanks Mark, that makes sense and I'll look into that the next time around. My intention all along wasbto build a small solar kiln. In the mean time I actually have a great time transforming this log.
7/2/14 #16: 40 inch diameter log in solar kiln ...
Vergil Leih dries turned tree trunks. He uses a micro wave oven he built. Basically put the wood in a plastic bag with extra water, heat it to vaporize the water( you would have to hold it there till the temperature penetrated the piece), then draw down the moisture.
Maybe transport your piece to him in northern Minnesota and dry it there.
Also research drying with denatured alcohol.
good luck
7/2/14 #17: 40 inch diameter log in solar kiln ...
Website: http://www.heinrichspillmann.com
Ken
Your funny, not sure how you put a 2000 lbs piece of wood into a plastic bag with some extra water. I have admired Virgil's microwave for a while and have been wondering if and how I could adapt his technology, but even so the logistics are a bit daunting. I also have been following with interest in the alcohol thing ( that sounds odlyy wired) but there too the logistics are a bit daunting. There is also a wood turner, Ron Kent who turnes beautifu l bowls and then soaks his pieces in a dish wash liquid solution I think. But again the logistics of applying any of these methods ar a bit daunting. For now I believe Gene's old postulate of letting time do its thing in shady and well ventilated place is still the best advise. Hopefully that shady and well ventilated place is on the property of a well endowed client.
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