Shrinkage & Splitting in Drying Wood

wade clark · #1 06-20-2005, 09:00 PM

If the heart of a log doesn't shrink much compared to the outer wood when it's drying, thus the outer wood's tendency to split as it dries,...can anyone tell me if this would help: Sealing the end only on the outer wood and not sealing the heart.
Help me think this through: The heart is older and dense; the outer wood is newer and growing fast, so it's not so dense and is full of moisture/porous openings carrying water/sap. Therefore prone to more shrinkage because it has more water when alive and less fiber...correct me if I'm wrong please, I'm trying to figure this out in my mind. Doesn't the Cadmium Layer, the slippery layer on the underside of the bark, carry the sap, or just most of it, and is the rest of the tree just storing moisture or is actually flowing through the tree too?
I guess I should have done a search for this info rather than trying to straiten out my understanding of a tree here, but I think everyone here feels a kinship of one kind or another to trees and their wood.
Again my question is about how to most effectively seal the end grain to prevent splitting re: seal it all or leave the heart exposed.
Any thoughts?
Wade

Colin_Partridge · #2 06-20-2005, 09:37 PM

My understanding is the outer layer of sap wood checks because wood dries from the inside out so when the inner heart wood dries it pulls on the sap wood and thats why it cracks. Not trying to confuse any one that is how I have always been told it works. I have tried to stop logs from checking but without a whole lot of sucess. I do find if I carve them green and carve them quickly then seal them with two or three coats of spar varnish that they dont check as much and if thin enough dont check at all.
Colin

wade clark · #3 06-20-2005, 09:45 PM

Colin,
Two or 3 coats of spar varnish while it's still green or does it have to dry first? Does the spar varnish breathe enough that it continues to dry slowly?
Wade

Colin_Partridge · #4 06-21-2005, 11:23 AM

Wade I would guess yes or the moisture is kept inside. I cut down a piece and carve it right away probably within and hour of cutting it down. I think the spar varnish acts as an outside strong coating and holds everything together. Mind you if the wood is really large it will check some but not near what it does if you leave it dry naturally.
Colin

KettleKarver · #5 06-21-2005, 01:28 PM

I found this to be an interesting discussion and decided to look for some definative information. The USDA's Forest Products Laboratory has some great information, although some of its a bit technical. Below are links to the home page and to a PDF document on drying hardwoods. I hope these are helpful.
Jim

http://www.fpl.fs.fed.us

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr118.pdf

wade clark · #6 06-21-2005, 05:45 PM

Whew! I read alot of the second link and understand a tree a whole lot better!
Lumber w/less than 20% moisture has no risk of developing stain, decay or mold....
...Sapwood is very permeable to liquids and for this reason dries faster than heartwood....
...As the sapwood becomes older it becomes less permeable as various chemicals (resin, tannins, oil...) are deposited in the cells. These darker cells become heartwood....
...fluid conduction in the heartwood is greatly slowed compared to sapwood....
...Shrinking is moisture loss....
Swelling is moisture gain....
...Denser woods are harder to dry....
...Drying too slow causes mold, stains, mildew, decay....
DRYING TOO RAPIDLY CAUSES CHECKS, SPLITS, CRACKS!
I don't know if I answered my question or not, but I know more than I did!
The article is worth reading...someone spent their life figuring that stuff out! It's really written about drying boards/lumber, not logs or larger carving pieces. I started to doze before I got through it, though.
Wade

AlArchie · #7 06-22-2005, 10:06 AM

I don't often dissagree with Colin, but I think he's got tyhe drying process reversed....the outer layers dry first, thus shrinking more than the inner portions. Maybe not more, but more quicly....thgis results in the outer layers splitting and pulling back from the inner. if you watch the end of a log, you will find the wider splits toward the outer circumference. Thge outer lauyers dry quicker, simply because they are closer to the air

Al

wade clark · #8 06-22-2005, 11:00 AM

From the website article, Outer wood, sapwood, holds lots of moisture and is very open celled compared to denser heart wood, which dries slower because of all the minerals and oils that have built up in the cells. Beyond that, it makes sense that dry air will absorb moisture from what is next to it, the sapwood, and after reading the article, I understand the basic cell make-up is open and wet, but the only truly live part of the tree is the cambium layer outward including leaves. The rest moves some moisture and stores starches and sugars, and it's wet, but it's technically not "live". The article is worth reading just for understanding. There's tons of info I never mentioned, and lots of it affects how it carves! There are Folks that can look at a tree and say,"Whoa! That'd go everywhere and fight me all the way! It's going to be pretty too!"
MY ORIGINAL QUESTION was, would it help to seal the sapwood and leave the heartwood exposed? I still don't know. I guess I need to cut a log and try it and post the results here a year or so from now, unless someone here has tried it and knows the answer.

Hugh · #9 06-22-2005, 11:49 AM

No disagreement here, but something I read quite a few years back. In Australia they had a huge pine tree plantation catch fire. Mostly only scorching the tops of the trees. 600 Acres of pine. They logged the whole thing, as the trees were going to die anyway. Dumped them in a lake, where they waterlogged and went to the bottom. When they want pine logs they drag them up, and saw them. The part of the story that most interested me, is that the lumber wasn't prone to splitting after this treatment. Apparently they decided that the sap had been washed out, and replaced with water, and the water evaporated from the lumber more evenly, thus no splitting. don't know how much relevance this has to your question Wade, but thought it interesting..

KettleKarver · #10 06-22-2005, 01:15 PM

Sealing the ends will reduce the amount and severity of checking. I remember reading in the article about the need to wax the log ends after harvesting but before going to the kiln to improve yield. That's because moisture moves most easily in the longitudinal direction, allowing the end of the wood to dry fastest. Sometimes too fast. I have a small birch log that was given to me green. Within a week in my garage it was horribly checked. I hadn't sealed it.

Jim