I am making a small pedestal table, and got a hunk of maple, split, planed, squared, and turned the pedestal, ans put three coats of shellac on it. Now this wood was pretty much as green as you can get, but I read that stuff should be turned green. I haven't been doing this for long, and this is the first large piece I have ever turned. I figured the coats of shellac would seal it, and keep it from splitting. Was i correct to assume this? anything else I need to know before I go ahead and put this thing together?
Yes that will reduce the risk of splitting, but keep in mind also that green wood is not as stable as dried wood. As the green wood does dry, it will change shape. Many turners like the effect of a perfectly round bowl changing shape after the green wood dries.
Drying generates a strain across the board, which leads to shrinkage.
If you dry a board evenly and at moderate speed, it shrinks evenly. If you dry it rapidly, or unevenly, then it will develop cracks (typically the honeycomb checking of poor seasoning). This creates a false impression that "all cracking is caused by poor drying technique".
If your board is flat and radial-sawn, then this stress causes linear shrinkage. You get a smaller, flat board. You may notice that radial shrinkage is less (typically half) the tangential shrinkage.
If your board isn't radial, and it's large relative to the curvature of the rings within it, then this difference in shrinkage causes the board to cup.
If the "board" is a ring, containing the centre, then the "ring" constrains it from cupping. The rings try to shrink more along their circumference than they do on their radius, so the board is stressed by circumferential tensions that can't go anywhere. It can't shrink, as the centre is in the way, and it doesn't shrink so much radially. If you cut a hoop with a central hole, or if the centre of the log is soft, then the ring _can_ shrink, by compressing the area of the centre. In a solid log though, it can only build up a tension.
In any solid disk of more than about 4" diameter, then drying it to typical "dry" levels generates enough strain to crack it. The strength of timber varies widely with species, but the strength/stiffness ratios less so, and so all species show this same problem.
If the disk is off-centre, then some of this compressive stress may be dissipated instead by warping instead (like the non-radial board). But any solid section that includes the heart is almost guaranteed to develop a radial crack.
You _may_ find a dry disk that's uncracked. But there's a huge stress in there, and any disturbance is likely to cause it to fail dramatically.
By using PEG-1000 to displace water, you may reduce shrinkage, and avoid cracking that way. If the disk is thin, then it may warp out of flat instead. But this radial cracking problem won't just go away, no matter how carefully you dry it.
The trick is not letting it split as it dries. I put my green turnings in a plastic bag filled with the wood chips to slow the drying process. Wood can be turned green or dry, and each has its pros and cons.
Well that is not absolutely true, it is far easier to turn green wood as it cuts easily. If you need the wood to be stable after turning go with dry wood. If you want to let nature aid in your artistic impression go with green.
Isn't it also easier to make large turned objects from green wood, because after turning the thin walls are flexible enough to survive the shrinking during drying by warping instead of cracking, while drying a lage blank without any crack is extremely difficult?
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.