Reaction wood

[Chrispy]

A few days ago I mentioned reaction wood in a answer to a query which got me thinking so thought I would start a thread on the topic, please feel free to correct me if go wrong :lol:

There is I think two main kinds of reaction wood, that that is in compression and that that is in tension.

It can be found in the buttressing around the base of large old trees, or where a tree has been pushed over by land slip or being leant on by another fallen tree, it is also found in branch wood.

The job of this type of wood is to support the weight of the tree and to return a leaning tree to a upright balanced standing.

You can sometimes see it as an extra layer between the spring and summer growth rings, especially on a horizontal branch.

If you have a plank of wood that is very bent it is almost sure that is due to reaction wood, and as soon as you cut into it it it will move even more binding on the saw and leaving a woolly cut due to the fibre of the wood being in great tension or compession.
This is sometimes in error seen as bad drying.

As far as I believe you find tension wood in softwoods and compression wood in hardwoods but l may be wrong.

hope you find this useful.

 

[marcros]

would you not get compression on ons side of a leaning tree, and tension the other. Every action has an equal and opposite reaction and all that?

 

[Chrispy]

would you not get compression on ons side of a leaning tree, and tension the other. Every action has an equal and opposite reaction and all that?

If you look at a cross section of an Oak branch the growth rings will be much wider underneath not on top this is because of the extra bands of compression wood.

 

[CHJ]

I had quite a surprise yesterday when cutting a small piece of Walnut.

Kiln dried, a short length of 150 X 32mm thick perfectly flat planed/thicknessed/sanded plank, off-cut from a furniture manufacturer.
Moisture content below the 6% reading on my cheap meter.
Was re-sawing to 2 X 15mm, within 70mm of starting, the wood moved, closed the kerf and jambed the blade and had to be wedged.
Subsequently the independent slices moved 4mm out of flat, needless to say not to be risked for a box lid.

Green wood I expect it and have wedges at hand, but never expected a flat bit of well dried wood to move so dramatically.

 

[Phil Pascoe]

I had a piece of really hard, well seasoned African Mahogany of some description that I sawed down the middle to get 2 x 4" x 2"s from an 8" x 2", it was about 9'. One half could have been used as a straight edge, the other twisted and bowed by 2".

 

[Sgian Dubh]

Tree species that normally produce good usable timber don’t always do so due to special circumstances. Trees that grow on a bank can adapt and grow successfully but they react to their location by growing in a distorted fashion. Similarly trees on coastlines where the wind predominantly blows strongly landwards compensate by bending downwind. In extreme cases tree trunks from these locations sometimes bend almost parallel to the ground.

Trees and branches distorted in this way are full of reaction wood. Reaction wood is abnormal wood produced in trunks and branches that are not erect- they grow at something other than parallel to the pull of gravity, i.e., not vertical.

Reaction wood is either compression wood or tension wood. Compression is a crushing force and coniferous trees with leaning trunks form compression wood mostly on the underside of the trunk; the underside of their branches similarly develop compression wood. Tension is a ripping apart force and leaning broad leaved trees and their branches react differently to gymnosperms and produce tension wood predominantly on the upper half of the trunk or branch.

Reaction wood, whether it be compression wood or tension wood is frequently unsuitable for the woodworker, the dryer or kiln operator, and the sawmill. It produces unstable wood that, even at the planking stage, releases stresses causing it to bow, cup and twist.

Compression wood tracheids (of gymnosperms) have thicker cell walls and less cellulose than normal wood and it is weaker and more brittle too. It’s typically denser and doesn’t polish up the same as normal wood. Also it shrinks and expands appreciably in the longitudinal direction, 10 to 20 times normal, whereas this movement in unstressed wood is usually insignificant, i.e. < 1/8” per 96” length or < 3 mm in 2.5metres.) Dense reaction wood in coniferous trees is often brash or brashy, i.e., weak, and snaps easily across the grain.

Tension wood of angiosperms, or deciduous trees, has its own problems. Unlike gymnosperms where reaction compression wood is concentrated on the underside of the stem, angiosperm reaction wood, although mostly concentrated on the upper side, spreads more evenly around the whole stem. Tension wood has more cellulose and is usually stronger than normal. Machining and cutting tension wood is difficult because it tends to be fuzzy and woolly and won’t work up to a fine surface. Stain, dye and polish uptake is uneven leading to blotchiness. As with compression wood, tension wood is unstable with longitudinal shrinkage factors much increased, along with a tendency to bow, cup, and generally distort.

With both forms of reaction wood there is a propensity for growth rings to be eccentric with the pith off-centre. This latter characteristic is typically more pronounced in softwoods than hardwoods. Slainte.

 

[devonwoody]

And how do you spot this stuff at the timber yard? I reckon I got a board of maple and another of American black walnut a couple of years ago that has caused me trouble.

 

[Chrispy]

And how do you spot this stuff at the timber yard? I reckon I got a board of maple and another of American black walnut a couple of years ago that has caused me trouble.

Well it will be bent along it's length and may have a woolly or fluffy feel and look to the sawn faces, that should ring the alarm bells.

 

[Chrispy]

Thankyou for putting the facts on the subject Richard.

 

[devonwoody]

Well we now know what Bee & Queenie supply. :wink:

 

[Sgian Dubh]

And how do you spot this stuff at the timber yard? I reckon I got a board of maple and another of American black walnut a couple of years ago that has caused me trouble.

Reaction wood is so obvious to the sawyer at the sawmill that he or she would probably have abandoned boarding up the sawlog after just a few cuts. A tree full of reaction wood would have likely been spotted by either the buyer, the forest manager, or the logger before ever it got near the mill and rejected for firewood, pulp, man-made board, et cetera.

As these are both North American woods processed there you are more likely to have found some boards that had stress put into them by the kilning process, e.g., through improper final stress relieving procedures used in kilning operations which, when done correctly (which is almost always), take out the case-hardening induced in the wood in earlier drying stages. Or you may have come across some reverse case-hardening which can sometimes happen if the stress relieving operation is overdone, although reverse case-hardening is rare. Slainte.

 

[devonwoody]

Thanks SD, I recall however on one visit to the timber yard in question there was a consignment on a large transporter of ABW logs uncut. So if the timber merchant got a bad log I don't think he would be selling it as firewood, but I bet he sold it retail :twisted:

 

[Sgian Dubh]

DW, that's a rare occurrence indeed to see American black walnut in log form over here in the UK. I've only seen that a couple of times in a timber yard where they milled it into boards, dried it and offered it for sale; and not steamed at that, so that the full range of colours were preserved in the heartwood as opposed to it being the ubiquitous all over purple'ish stuff that's been kilned and steamed for colour adjustment in North America. Of course the sapwood was highly visible too which is rather different to almost every other stick of black walnut sold here.

Anyway, even if your yard had a bad log that was full of reaction wood, they would have found out about it as soon as they tried to mill it into boards because it would have turned into bananas, propellers and pretzels as it came off the mill. So I still suspect the problem you described earlier was caused by the drying rather than reaction wood. Obviously I'm only speculating, and I could be completely wrong. Slainte.

 

[devonwoody]

I decided to not purchase anymore timber from the merchant direct but to collect only, a bit of a bind.