Replies

1. 
   JohnWW | Jan 10, 2005 08:38pm | #1

   Ron,

   A section of tree trunk will have more severe checking than a glued up block, but a considerable amount of checking will still occur in a glued up block that large exposed to the weather. 

   Gluing up the block with a large hollow in the middle would help considerably in reducing checking as would choosing an especially stable wood species.

   Except for using a marine epoxy as the finish, no other finishing material will appreciably improve the stability of the wood and even the epoxy may not help especially.

   I'd recommend reading Bruce Hoadleys book, "Understanding Wood" to help you with the problems involved.

    

   John W.

    

1. 
   RonBev | Jan 10, 2005 09:45pm | #4

   Thanks John. I failed to mention that a large portion of the center of thre sculpture will be removed so this should help in either case.

1. 
   JohnWW | Jan 10, 2005 10:09pm | #6

   Keeping the shell of the sculpture thin will help considerably, especially if the air can circulate freely inside. 

   I forgot to mention polyethylene glycol, PEG, which was suggested in another post.  It works very well for stabilizing wood, but the block needs to be submerged in the chemical for quite a while to saturate the wood, though hollowing out the block would help to speed up the process.  You can pull the wood block out of the solution to carve it and then resubmerge it between sessions, PEG is fairly non-toxic I believe. 

   If you want to learn more about using PEG, post a question here with PEG in the topic line. I don't have any hands on experience with it, but I'm sure at least a few Knots people do. 

   PEG is available from several mail order sources, in woodworking it's primary use is to stabilize large bowls turned out of green wood and to stabilize wooden artifacts salvaged from historic ship wrecks.

   Hope this helps, John W.

2. 
   Rich14 | Jan 10, 2005 09:01pm | #2

   Ron,

   I second John on the recommendation for Hoadley's book. It's a must for anyone who needs to understand how wood responds to the atmosphere once it stops functioning as a living tree.

   You are discussing two separate but related problems. A single tree, 36" in diameter will check badly because as it loses moisture to the air, the huge dimensional changes will result in physical stresses that can only be relieved by cracking. That is why a tree is immediately cut into boards for drying to the state that can be used for lumber. The boards will still check if the ends are not sealed, but the problem is much less and can generally be controlled.

   Theoretically, a larger log could be stabilized for drying, but the process would take a very long time and the practical limitations would be prohibitive. It might take a human lifetime to accomplish if at all. Of course, if you have a large log that has been drying for many years, use it without too much worry. Large sculptures have been made of logs that have been driftwood or otherwise dried to a useable state.

   Once boards have equilibrated with the ambient conditions, no further checking will occur. Such boards can then be glued up into a larger mass which could remain stable. The issue of the glue to hold it together is another thing. Epoxies have been developed which have been highly successful. You may or may not need a final "varnish" protective coat. A lot depends on the wood you are going to use. Several woods have excellent resistance to the elements as a consequence of their own internal oils. Teak is one such wood, other tropical hardwoods are also weather resistant. What wood are you going to use?

   I think the best advice on this topic can be had from The manufacturers of the WEST Epoxy System who have developed many products that are pretty exotic, but straightforward to use. In any case, you will need to give consideration to the UV protection included in the epoxy (a specialty of WEST), the actual conditions that the sculpture will need to "weather," and some routine "maintenence" program of applying whatever outer protective coat you decide to use.

   Rich

3. 
   BenM | Jan 10, 2005 09:13pm | #3

   You could look into using a single log that was treated with PEG.  However, I don't believe you can apply a finish over the wood then.  If you want to laminate thinner boards together you will need to joint them, otherwise you can expect some unsightly gaps, especially as you begin to carve away the waste.  You also need to consider the color and grain of the laminates or your sculpture will have a bunch of unsightly glue lines which could detract from the overall effect.

4. 
   GHR | Jan 10, 2005 10:07pm | #5

   People tend to carve rather large animals from standing tree trunks.

   I don't know how much checking your work can tolerate.

   Epoxy will work as a glue and as a sealer for the finish work.

5. 
   SawdustSteve | Jan 11, 2005 06:42am | #7

   Hi Ron and Welcome to the board.... I don't know where you're located, but if you are near a Maritime Museum such as Mystic Seaport in CT, see their figure-head carving demonstration. They make large sculptures, the size you are talking about and they MUST last in harsh environments. SawdustSteve Long Island, NY

1. 
   RonBev | Jan 11, 2005 04:40pm | #8

   Thanks Steve. I'm in Quakertown, PA about 50 mi north of Phila. When is this figure-head carving demo? Is there a web site?

1. 
   SawdustSteve | Jan 11, 2005 09:57pm | #9

   Mystic Seaport is in Connecticut, a bit east of New London. Do a google search. They are a major maritime museum and run various ship related demos all the time. Someone there will know the names of people who can answer your questions. You could also try South Street Seaport in NYC or Baltimore Harbor Seaport which is MUCH closer to you. SawdustSteve

6. 
   AlbionWood | Jan 12, 2005 12:03am | #10

   You can reduce checking substantially by hollowing out the center of the log before it dries. The worst checking is caused by the difference between radial and tangential shrinkage rates - most wood shrinks much more in the tangential direction. Hollowing out the center and leaving a thin shell of carved wood will reduce this effect considerably. I've seen a lot of large medieval sculptures, with very deep relief on the front, that were hollowed out from the back. (Some of them still had cracks, though.)

   Welcome to the forum!

   "Everything should be made as simple as possible, but no simpler."  A. Einstein

   http://www.albionworks.net