Processing fallen wood for turning

[OldWood]

There's a lot of it around !

To a certain extent this came out of the Laburnum thread, but it will apply to all recovered timbers.

I understand that the pith of the timber is the core of the problem and needs to be cut out ! There is I see a thread running on creating a rip saw chain for saws which certainly will be useful.

But it is worth asking the question as to what is happening that the pith needs to be removed ? Why is that the cracking starts at the pith point - I noticed it only yesterday in some sycamore logs that had been cut some time back for the fire and I'm now looking at again for a hollow form. But then I also had access recently to some almond tree wood that sadly had been cut to fire logs - I kept a couple hopefully but they split horrendously from the outside in.

So given that some timbers we retrieve can be quite big, is splitting (saw or wedges, etc) essential, or should the pieces be cut to a twice (?) diameter length and sealed. And what about sealing split lengths ?

Thanks for any guidance

Rob

 

[CHJ]

The object is to slow the moisture loss to the local atmosphere down to the same rate that it migrates out of the core of the wood.
If logs are whole then seal the ends to mimic the bark covering at least.
Place in paper sacks or similar to reduce the risk of splitting from the sapwood inwards.
If split seal the ends, with luck they will just distort (fold back on themselves) as the sapwood shrinks and can close up rather than it splitting.

Closely stacked logs, split or otherwise seem to fare better, a loose covering of the stack to allow some air movement but increase the localised humidity in summer can help.

I manage to salvage considerable quantities by placing them in my greenhouse for twelve months or more, it's warm and higher humidity than outside and it appears to control the moisture loss rate better down to the point I can further prepare it into blanks or rough turn.

 

[bobham]

The reason the cracking starts at the pith is that a log will lose moisture much more readily through the end grain than through the sides. That basically means that the first couple of inches of the end of the log is trying to shrink because it is drying, while the rest of the log is staying the same size. In effect the radius of the end of the log is trying to shrink while the circumference is staying the same. The stresses focus at the pith and it has to let go to relieve them.

I just wish it would let go in the same direction I wanted to cut it......

Take care
Bob

 

[Roger C]

Hello Rob there already exists rip chains for chainsaws. You can also rip withe the cross cut chain but not long pieces.
As with all tools a sharp chain is needed. 1 Ask your local garden centre what is used to seal trees when branches have been cut off, and check if the sealer is toxic. 2 The Yanks use Anchor seal ll is apparantly beter than the lll. 3 Another method of sealing is with wax. 4 Ask the nearest sawmill if they use a paint on product to seal the log ends. 5 Seal all exposed areas of the log, no need on the bark. Hpoe this is of some help. Rgards Roger in RSA

 

[OldWood]

Hello Rob there already exists rip chains for chainsaws. You can also rip withe the cross cut chain but not long pieces.
As with all tools a sharp chain is needed. 1 Ask your local garden centre what is used to seal trees when branches have been cut off, and check if the sealer is toxic. 2 The Yanks use Anchor seal ll is apparantly beter than the lll. 3 Another method of sealing is with wax. 4 Ask the nearest sawmill if they use a paint on product to seal the log ends. 5 Seal all exposed areas of the log, no need on the bark. Hpoe this is of some help. Rgards Roger in RSA

In the UK my understanding is that the arborists and foresters do not now consider that sealing a cut branch end with sealing chemicals is good practice. The tree is perfectly capable of doing it itself. The only advantage of sealing is that it masks the colour of the freshly cut wood.

The 'paint on product' is just that - paint!

I think perhaps where I was looking for guidance is in the initial sawing preparation, and also why splitting occurs. Bob Hamilton has explained the splitting and why correspondingly sealing the end is critical.

The question then is still on the table about whether one is best to split the timber lengthwise or cut into shorter lengths to dry ?

Rob

Rob

 

[CHJ]

Keep lengths as long as possible, as long as possible. That way any end splitting is a smaller proportion of the whole.

Never cut a piece for storage less than a third longer than the diameter even when you think it is well on the way to drying, and reseal the ends immediately, even 50% longer than the diameter is not too cautious.

If you think it is down in the 12-14% range and worth a spin then if not putting on the lathe immediately coat all the endgrain of the blank in sealant.
Normal practice is to roll a standard circular blank in hot wax, that way you have all the endgrain covered, and a bit of side grain, but the two faces are open for continued moisture loss as would be a larger billet in sticks.

Don't forget to seal any branch spurs or cuts, they are in effect end grain exposure.

 

[OldWood]

Thanks Chas - but I'm still wondering whether I am better to leave timber in its round form off the tree, and seal its ends, or take a chain saw with a rip chain and cut it lengthwise - and then seal it?

Rob

 

[CHJ]

How long is a piece of string.
Everybodys storage conditions and geographic location is different, so what works for one may be disastrous for another.
And that's not considering the moisture content of the timber due to harvesting time etc. which will vary with every batch.

I could only recommend trying some in various states and see what you end up with, I'm afraid it's one of those dark arts for the home drying woody that only time will teach with any chance of guessing right next time.

If you see a cut log starting to core split within minutes of cutting through then it would seem prudent to split the logs or slab it as soon as possible.

 

[Aled Dafis]

Personally, I'd split the tree down the pith if you intend to turn bowls, this gives you the best chance of avoiding serious cracking, whilst maintaining nice large sections to cut as blanks in the future. If you intend to turn some hollow forms then you must decide what size forms you'd like, and how best to achieve them, either by splitting the tree or not.

Cheers
Aled

 

[chipmunk]

Hi Rob,
I still don't think anyone's really answered your question about why the split comes from the pith so I'll have a go.

The main reason is that all timber tends to expand and contract with moisture content more in circumferance than in radius. So, as it dries the circumference of the growth rings effectively becomes shorter than the 2 Pi R that it needs to be to stay in one piece. This is why the long splits from pith to bark form down the log as it dries. It's the only way to relieve the tension.

By splitting the log down through the pith, the two semicircular cross-section lumps can shink without so much stress building up. Often you will see that when it's dry the pith will be raised up along its length indicating the result of the different shrinkage rates and the fact that the total circumference is no longer 2 Pi R but some value smaller than this.

HTH
Jon

 

[Aled Dafis]

Hi Rob,
I still don't think anyone's really answered your question about why the split comes from the pith so I'll have a go.

The main reason is that all timber tends to expand and contract with moisture content more in circumferance than in radius. So, as it dries the circumference of the growth rings effectively becomes shorter than the 2 Pi R that it needs to be to stay in one piece. This is why the long splits from pith to bark form down the log as it dries. It's the only way to relieve the tension.

By splitting the log down through the pith, the two semicircular cross-section lumps can shink without so much stress building up. Often you will see that when it's dry the pith will be raised up along its length indicating the result of the different shrinkage rates and the fact that the total circumference is no longer 2 Pi R but some value smaller than this.

HTH
Jon

Good explanation (for those of a mathematical bent). I've been trying to think how to word it and have had a look for some diagrams that explain it but failed miserably! "Turning green wood" by Michael O'Donnel is the best book I've read on the matter, but i leant it to someone years ago and never got it back. :evil: :evil:

Cheers
Aled

 

[Neil Farrer]

Thanks Chas - but I'm still wondering whether I am better to leave timber in its round form off the tree, and seal its ends, or take a chain saw with a rip chain and cut it lengthwise - and then seal it?

Rob

Rob,

I'm fairly new to turning but I have been working with wood for about thirty years building boats (dried so that experience is worthless!) but more recently for the last fifteen years I have worked designing interior and exterior facades/ceilings/ details from wood for architects and designers. Once designed and specified and subsequently ordered we have to go through the procurement process, dry it, impregnate it if fire retardency is required, cut it plane it etc and finish it. With that in mind I would use that experience to suggest that:

If you dry wood in the round and then cut it to turn bowl blanks it is still likely to split as the wood remains, although relatively dry, under stress. If you then cut the wood the stress has an outlet but it will be uneven and the wood may still split. (I purchase wood that is kd and then needs to be further cut to rough sawn dims before being planed and this still splits - or rather some woods do more than others and also depending on when and where they were cut, but thats another story). If you dry wood in the round and then make a vase or the like from the wood then the dimensional change to the wood is the same all round and the pressure is stable and you will generally not get splitting - I emphasise generally.

If you cut the wood for a bowl blank all thats been said, cutting long etc is excellent advice. If you cut a series of say, 2 inch slabs from a 16 inch log, don't stack the planks as if they were still one log, turn every other plank over as the drain rings will try and shrink on drying, keep them stretched. If you do cut for bowl blanks, cut either side of the pith and use either side of the pith on the middle slab for something else or small blanks. Cutting down the pith is in my experience a false economy, so much greater presure is applied on drying to the area around the pith that the wood discarded as a reult of subsequent drying is greater than that originally discarded.

So I would suggest that you store some in the round and use it subsequently to make vases or similar, and slab some to use for bowls or subsequently to cut blanks for spindle turning, but don't change your mind later!

 

[OldWood]

Hi Rob,
I still don't think anyone's really answered your question about why the split comes from the pith so I'll have a go.

The main reason is that all timber tends to expand and contract with moisture content more in circumferance than in radius. So, as it dries the circumference of the growth rings effectively becomes shorter than the 2 Pi R that it needs to be to stay in one piece. This is why the long splits from pith to bark form down the log as it dries. It's the only way to relieve the tension.

HTH
Jon

Jon - I had to go off and doodle to think about what you were saying, and pick you up on the statement above as I consider that you have it the wrong way round.

If you take a large piece of round wood and look at it as it dries you do see two failure modes in operation - some woods show it much more distinctly than others. What needs to be explained are the two modes and why splitting, and effectively cutting out the core of the timber, is beneficial. The latter you have done.

The two mode are the splits from the core on the end face and the circumferential splits along the length of the timber.

Your description address the longitudinal splitting so common in fruit wood although your wording is back to front in that you say the splitting occurs "pith to bark" down the length, whereas it should of course be "bark to pith" as the bark area dries first, shrinks as you say and splits longitudinally - you cannot get the moisture out of the core before that at the bark !

Bob Hamilton has addressed the splitting we all too commonly see at the pith point on the timber face; the moisture rapidly leaves the first cm or so of end face - it shrinks radially, but cannot do so at the circumference as that is held in place by the whole log, so the stress is expressed radially at the pith point (nominally the centre), which is weak point anyway due to the lack of bonding across the 'zero' point, and hence it splits there. This also explains why so often the shakes in this area can disappear with quite a short removal of timber along the length.

While on the subject, it is worth googling and reading up on Equivalent Moisture Content (ECM) (The Wikipedia page on Wood Drying is as good as anything). It is interesting that on felling a tree is typically 60% moisture with half of that actually in the lumina (moisture tubes). This free fluid drains out first and in it's pre-drying stage the timber is around 30% ecm - I recently got some holly that had been felled and then dumped within the last 6 months or so, and it is reading 30% which seemed to back up what I was reading.

Rob

 

[CHJ]

Your description address the longitudinal splitting so common in fruit wood although your wording is back to front in that you say the splitting occurs "pith to bark" down the length, whereas it should of course be "bark to pith" as the bark area dries first, shrinks as you say and splits longitudinally - you cannot get the moisture out of the core before that at the bark !
Rob

Ahhhhmmm, how come every log that I have sawn through star splits within hours if not minutes from the core ends and migrates down the log if not sealed. Long before any moisture loss has had a chance to take place in the bark let alone the sap wood.

I believe this to be the splitting referred to above and makes good sense as grain shrinkage across the grain is a factor of 5-10 that longitudinally in my experience.

Yes, Bark Inwards if moisture loss is quicker than moisture migration but core splitting takes place from end grain exposure within seconds in some instances.

 

[nev]

at a slight tangent can i say that also dry blanks are not necessarily stable. i have just turned a blank that has been a 7x2 'blank' since 1993 into a bowl with a flat base. and guess what? 19 years as a blank and a week as a bowl and now it wobbles :evil: . not much but its there.
i guess what i am saying is even if it is prepared cut waxed stored (sold!) and kept properly theres still a chance of movement.

 

[chipmunk]

I'd be the first to admit that my explanation doesn't have all of the answers.

As Chas says it doesn't really address the differential drying between the unsealed ends of logs and the centre which will obviously set up other stresses. But I'd argue that the difference in circumferential shrinkage and radial shrinkage is at the heart of most defects in timber. In my mind pith to bark and bark to pith splits are fundamentally the same failure. It all depends which parts are drying out first.

Also removing the pith doesn't solve it either because from my experience splits if they occur even without the pith will be radial and perpendicular to the growth rings.

Jon

 

[OldWood]

Your description address the longitudinal splitting so common in fruit wood although your wording is back to front in that you say the splitting occurs "pith to bark" down the length, whereas it should of course be "bark to pith" as the bark area dries first, shrinks as you say and splits longitudinally - you cannot get the moisture out of the core before that at the bark !
Rob

Ahhhhmmm, how come every log that I have sawn through star splits within hours if not minutes from the core ends and migrates down the log if not sealed. Long before any moisture loss has had a chance to take place in the bark let alone the sap wood.

.

Interesting - that I've not experienced except perhaps in fruit wood. I had some almond that cracked horrendously but from the outside in.

I went back to the Wikipedia article on Wood Drying as I wondered if what you report, Chas, was due to the free moisture leaving the lumina, but no, that has no impact on the timber's dimensional stability. I can see why you should think that such internal cracking down the length must be due to the moisture egress from the ends - The fact that, as I see it, that is not consistent across all timbers, suggests that some hold their moisture more than others. It would also seem that it was a significant moisture equilibrium misbalance in your case that was drawing moisture out too fast.

Rob

 

[gregmcateer]

at a slight tangent can i say that also dry blanks are not necessarily stable. i have just turned a blank that has been a 7x2 'blank' since 1993 into a bowl with a flat base. and guess what? 19 years as a blank and a week as a bowl and now it wobbles :evil: . not much but its there.
i guess what i am saying is even if it is prepared cut waxed stored (sold!) and kept properly theres still a chance of movement.

Nev,
Didn't turn it first thing New Year's Day, did you?! :lol:
Greg