Just wondering ... which surface loses moisture quicker on a per unit area basis? My guess would be end grain. That's where checking occurs first during drying. How much faster is one than the other?
End grain dries much faster. For some reason, 8-10 times faster is stuck in my head...not sure why. As with most things, there are a lot of variables. To what depth of end grain are we dealing with, and so on. I'm sure a more scientific answer is coming...
Thanks, I knew end grain must dry faster due to the exposed pores but I had no guess for the magnitude.
Quote from this article: http://owic.oregonstate.edu/sites/default/files/pubs/for55.pdf
"Wood dries along the grain up to 15 times faster than
across the grain. Therefore, a board will dry at a faster rate
from its ends. However, because a board is usually many
times longer than it is thick, most of the moisture loss
occurs across the grain and out the surfaces of the piece. In
other words, the moisture travels across the grain at a slower
rate, but it has to cross a much shorter distance and, except
near the ends of the board, it dries more through the
surfaces."
Generally about 8 to 10 times faster than flatsawn face grain and double that for quartersawn.
Then cookies should dry very fast. Is that a contributing factor in their cracking rate?
That and the fact that the tangential part of the disk dries at about twice the rate of the radial part, setting up stress which is relieved by cracking. You have to slow things way down to keep the stress from building up to the cracking stage.
Speed doesn't change the difference in shrinkage amount on those different directions though, that stress is built in no matter what the drying rate is. You can lessen the drying gradient stress but not the underlying radial/tangential shrinkage stress.
This was a thin slice from a 6x6 post that I dried on the dash of the truck. The orange tangential grain lines shrinks about twice as much as the red radial lines. In this case the slice was thin enough the stress was relieved by going into the third direction, up, shown in the top view. If you imagine pushing that down flat, it would have to open up a check from one edge to heart to be able to flatten.
(https://forestryforum.com/gallery/albums/userpics/10017/dr6x6drysection.jpg?easyrotate_cache=1489552345)
Nice illustration, Don.
Quote from: WDH on January 20, 2019, 08:21:36 AM
That and the fact that the tangential part of the disk dries at about twice the rate of the radial part, setting up stress which is relieved by cracking. You have to slow things way down to keep the stress from building up to the cracking stage.
What I meant to saw was that the tangential part of the disk
SHRINKS at about twice the rate as the radial part.
Consider a wood disk made of a species that has 8% tangential shrinkage and 5% radial, green to dry, typical for some hardwoods.
When the disk dries, the diameter, which is the radial direction, will shrink by 5%. Now, the circumference will therefore also become 5% less with the 5% smaller diameter, as circumference = pi x diameter.
However, the circumference which is tangential direction really wants to shrink 8%...that is, more shrinkage than the radial shrinkage causes. The stress builds up and soon there is a crack.
If radial and tangential shrink were the same, no crack, but usually T is much greater than R.
This T greater than R ais why drilling a large hole in the center stops most cracking...thereis room for the extra tangential shrinkage...the hole will get smaller as the wood disk dries.
Tangential shrinkage in eastern red cedar is 4.7%. The radial shrinkage is 3.1%. The difference is smaller than in most species. This is why eastern red cedar makes the best cookies for me. Smell good, and are pretty too.
Smells like gerbils to me. ;)
WDH's comment had me wondering about cupressaceae and FSP, I was remembering that a number of them have a low fiber saturation point, seem to grow "dry" and have low shrinkage, and then there is cypress so not sure there is anything to that line of thought. On the way to the forum though I happened across more FSP numbers @GeneWengert-WoodDoc (http://forestryforum.com/board/index.php?action=profile;u=20498), I remembered you were looking for more data;
https://tropix.cirad.fr/en (https://tropix.cirad.fr/en)
Click the "access to species data sheets in pdf" tab. On a number of species pages under the physical properties heading is FSP.