Thread: Green wet wood vs dry wet wood

A week or two will be good in a lot of general work like table bases , not tops that are wide and glued up from a few pieces . It does also depend on what you were going to be making out of it . If it were a piece where it was crucial that there was no movement at all, Id leave it a month or two.

I once had some dry Oak that I wanted to put a lot of moisture back into so that I could turn it and watch it change shape after it dried again . like when stuff is turned from wet fresh oak . I didn't have any though. So I threw a dry square leg 50 x 50mm in a drum of water and left it a week , it only affected the first few millimeters and didn't go right in to far . I don't think it can re gain the large % moisture that was there when it was live .

Rob

Originally Posted by auscab

....... I don't think it can re gain the large % moisture that was there when it was live...

I reckon it will, Rob, but as you discovered, it may take a lot longer than you'd expect. Put crudely, if it takes a year for all the water to come out from an inch inside the wood, it's going to take a similar time for it to go back in. It's not that simple, of course, there are a number of factors that could influence the rate of 'rehydration', but you get the general idea.

But I think you nicely illustrated the point. My own experience with wood getting wet after it was 'dry' has been that it doesn't take anywhere near as long to settle back to a usable condition, compared with air-drying it from green, but exactly how long it will take is hard to predict, since it would depend on just how 'wet' the wood got (& how dry it really was to start with!) as well as the wood type. And I would certainly heed your advice to not use it in any situations where excessive movement might create problems!

Cheers,

I did think this might be the case, so i'm kinda relieved. I picked up some really nice bits of hardwood from someone that was chucking it out! including a piece of an old pier. The block is 50cm x 15cm x 30cm. He had it outside for a while along with some old hardwood fence posts, so I'll leave them in the garage for a few weeks and have a go at resawing them ( after a thorough check for nails ). Finally get to do some resawing on my bandsaw!

I'm just having a guess at this Ian, but I think it was once alive and was pumping moisture through its cells / fibers and once it died and they shrunk it wouldn't go back to what it was or exactly the same as what it was in size as when it was alive .

I just did a little searching, before posting this, and I found a site that does say I'm wrong though . So Id better go read some more


Rob

Originally Posted by auscab

I'm just having a guess at this Ian, but I think it was once alive and was pumping moisture through its cells / fibers and once it died and they shrunk it wouldn't go back to what it was or exactly the same as what it was in size as when it was alive....

No, not quite the case, Rob, but along the right track. The vast majority of the cells that we call 'wood' were dead long before the tree was harvested. My expertise is with animal structure, but we had to take botany in my first year at uni, so I have a vague idea of the structure of wood. Any botanists reading this please add-to or make corrections, but here's a (very) basic "Wood structure 101":

There is only a very thin layer of living cells in the trunk of a tree, called the 'cambium' which sits between bark & wood. These living cells produce both the bark on one side and the wood on t'other. The new cells 'live' for a very short time before becoming passive bits of the tree structure. At first they still take part in transport of 'sap', but over time, the walls of the dead cells become glugged-up (a highly technical term!) due to deposition of compounds and chemical changes in the cell walls, and less able to let water in & out. Other chemicals build up within the dead cells, too, which among other things, give the wood the colours we value. They are also often anti-microbial, and/or discouraging to wood-munching critters and help the tree to survive to grow old & produce lots of offspring. But the cells constituting the 'heartwood' are very definitely dead, by any definition.

Lots of water remains within the dead cells, and in their walls. Much of this is 'free' water (~75% give or take) which moves out fairly readily by diffusion once the tree is cut down and turned into lumber, and the cut edges exposed to air. How readily the free water diffuses is related to the density of the wood, which is dtermined largely by the amount of garbage deposited in the cells, but in general, this part of the drying process goes pretty quickly. When the wood hits around 25% total MC, the game changes, because much of the remaining water is attached to the compounds in the walls of the dead cells and needs a bit more encouragement to move. This is the part of the drying process that takes time & needs care, because you want to set up a moisture gradient that will get the water out as quickly as possible (i.e., surround the wood with air that is as warm & dry & constantly changing), but if you overdo it, i.e., the gradient is too steep, you're likely to get cracking & internal collapse, which can be quite severe in some woods. Other woods are much more tolerant, and can take very rapid drying, but with most woods, getting the 'bound' water out needs to be a pretty carefully controlled process, not always the case, and why kilning has a bad reputation with many folks.

Getting wood back to containing as much water as when it was cut involves the reverse of the two processes, but 're-wetting' the cell walls is as slow or slower a process as "un-wetting" them was. Until it becomes 'wet' the wood actually repels liquid water to some extent. You can speed it up by immersing the wood in hot water, but it's still going to take a lot longer than you might think for it to get all the way back into the middle of a board.

You can readily see why wood that has previously been dried is generally considered no good for bending. Wet wood is more plastic than dry wood, and hot lignin (much of what makes the cell walls) is more plastic than cold lignin, so it's better the wood be both wet and warm. "Steaming" wood isn't to get it wet, it's to get it hot, all the way through, which is much easier to do if the cell walls are 'wet' because water conducts heat better than dry wood. So putting it all together, you can appreciate why 'green' wood is so much better for steam-bending than previously dried wood.

That's very long-winded, but I do like getting into the nitty-gritty of things. Some (like my spouse), call it 'obsessiveness'....

Chers,