Another Drying Question

[Noah Schlag]

Guys,
i have read multiple articles regarding dna drying methods, but i still have a question. do i need to air dry blanks to a certain moisture percentage before rough turning and dna drying or can i chop down trees saw into blanks rough turn and dna dry? i do understand i need to leave a proper wall thickness and wrap with paper bag etc. i have probably overlooked the answer to my question in my reading of dna articles so forgive my ignorance.
thanks for your help,
nschlag

[Terry Quiram]

Noah

Rough turn straight from the tree then DNA soak. You can turn wall thickness about 80% of the major diameter when you DNA soak. I stick my soaked bowls in a brown paper bag and roll the bag opening over the rim of the bowl leaving the inside center of the bowl exposed to air. Stack and wait for your bowl to dry.

Terry

[George Tokarev]

First go to http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm and read up on how wood dries and why things happen the way they do, as illustrated in chapter 3, fig. 3-3 (or any 2x6 at Home Depot).
Then forget your "rules" of thumb about 10% of diameter and an inch per year. Go to table 3-5 and take a look at the average shrinkage figures. Take for example the yellow birch I posted yesterday, which had been turned about three months ago from a freshly felled tree. Yellow birch has a volumetric shrinkage of 16.8% from FSP to 0% moisture. At 30% relative humidity that means it's at 6% moisture by weight, so it's lost 80% of all it can lose in dimension. That translated into ~.75 in a 15 inch bowl, so the ten per cent is pretty far off. A cherry bowl from the same shelf lost about .375 in dimension, which equates pretty well to the prediction one might make from the 16.8% volumetric of birch versus the 11.5 for cherry.
That difference has to do with the shape. Loss in diameter is a function of contiguous wood, and we've hollowed the bowl, so the contiguous distance is much less than the diameter. Broader the bottom, greater the shrink, which is why we like to contour down to narrower bases for best success. It also depends on how closely the shape of the rough follows the annual rings, which is why part of the diameter loss is an illusion. The sides drop in relation to the heart, so the loss in depth is part of the loss in diameter. Contouring down to narrower bases helps in speed of drying as well as success in drying, since wood loses moisture along the grain (and vessels) ten times faster than through face grain and twelve times faster than quarter grain. We want to keep the distance to open air through end grain as low as possible for fast drying.
I think you can tell from this that the straw men conquered by soaking are indeed straw men. What determines the wall thickness we leave in a rough is the compromise between restyling possibilities and desire for speed in drying. The boys over in Wisconsin have given us some good information to work with, so all we have to do is apply it. If I had wanted to settle for less than a half inch of final wall thickness in that birch, I could have taken the thickness below the 1" or so that I did, and it would have dried faster than the eight weeks it took in reality. Of course, I would have had to use nearly the contour I originally cut, sacrificing the undercut rim I like in popcorn or passing bowls or possibility of leaving a greater thickness outside on the top for style. The flare in the spalted bowl was left in the blank, but had I wanted a big undercut, like the semi-jug types I make occasionally, I would have had to take into account the drop on the shoulders, making the rough a bit thicker up top. As you gain experience with green wood you'll be able to game the piece a bit more accurately, so look and take at least mental, if not actual measurements of pieces you dry and compare them to the information compiled by the FPL. I think you'll find that their work is both accurate and thorough.
Once again, burls are a different animal. Where they are random-grained, they tend to shrink only volumetrically, while areas of non-burl exert drying stress pretty much as figure 3-3 shows. Of course, cracks are non-contiguous wood, and may open if the drying stress produces expansion, or close if compression. That's why you don't need to coat or wrap the inside of a bowl if you don't want to. The circular shape puts that under compression, while keeping the exterior plastic and expanded longer allows existing stresses to be take up, and prevents new ones caused by too-rapid drying of the surface relative to the interior to form and open.

[Mark Pruitt]

Noah,
If you don't rough-turn a blank right away, it's best to seal the end grain to prevent it from drying out (and consequently splitting). Attempting to dry the blank to a certain percentage beforehand would actually get in the way of your desired outcome, so don't put yourself to the trouble of doing that.

The only thing I could add to what Terry said is to stack 'em upside down after DNA'ing. That helps them to dry more uniformly.

And BTW, DNA drying is effective!

[Joe Melton]

Mark Pruitt said
...
And BTW, DNA drying is effective!
...
Well, that may work in practice, Mark, but not sure it works in theory.
Joe

[Mark Cothren]

Well, that may work in practice, Mark, but not sure it works in theory.

That's some funny stuff right there.....:D :D :D

[Mark Pruitt]

...
Well, that may work in practice, Mark, but not sure it works in theory.
Joe
You know, you're right. I need to forget about reality, experience, and all that evidence-based, emperically proven stuff and go bury my head in a book. I'll never get anything done, but hey I'll be smaaaaart!;) :rolleyes: :D

[Noah Schlag]

thanks for the help, i am anxious to try dna drying. i will post my results when i try it.
noah