Primitive Archer
Main Discussion Area => Bows => Topic started by: E. Jensen on January 15, 2016, 09:48:30 am
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Hey everyone. The other post about the science of steam/dry heat bending has inspired me. My current academic interest is understanding wood, and not just industrial uses. Being a bowyer, knife maker, and general wood worker has driven my focus. Actually, the process of heat treating the belly of a bow led me to my Master's paper, thermal modification of wood.
I want to understand why wood does what it does and what properties are desirable for what purpose, and usually outside industrial applications. I like the little guys, like myself and you all, that have different ideas about what is desirable in wood. The eggheads up in the ivory tower think that only Dr's have anything to say about it, but I know that a degree is a piece of paper, and often times the redneck with 20 years experience has much more to say about something.
We were talking about effects of heating on wood in the other thread. I don't know what we'll talk about in this thread but I have an example of what I mean above. First, a short background. Wood is made up of cells, as you all already know. The cell wall is where the action happens, and it is made up of several layers. Each layer is made up of smaller bundles called microfibrils. The main layer we care about is the S2 layer, which is by far the thickest, and has microfibrils mostly vertical. About 10 degree off vertical to be precise.
Let me introduce something else and then I'll bring it all together. There is something called juvenile wood. I prefer the term crown wood. Basically, the first 10-30 rings from the pith, wood formed within the crown. When the crown recedes in a section, the absence of hormones from the crown causes mature wood to form. In juvenile wood, the angle of the microfibrils is much greater. This makes sense, the stem is thinner at the crown, and in the sapling, and needs to be more flexible. The angled fibrils act like a spring. Boinginginging!
In the wood industry, this is very very bad. Other properties of juvenile wood result in problems with warping, shrinking, checking, and makes for a really weak board.
Months ago I made a post, asking about using this wood (saplings) for bows. The answer I got was that it wasnt a problem, which at the time was confusing. Inferior wood, making a great bow? But it makes sense now. I made an elm sapling bow and it shot great! In mature wood, with upright fibrils, I'm betting the wood is stiffer but can be crushed easier. So in bows, I bet they are stiffer and take set easier. And in sapling bows, the opposite, needing a tad more wood for the same stiffness, but not taking much set. With a whopping sample size of 2 (1 sapling, 1 mature), that has been my experience with elm. I'm tracking down some scientific articles about this, but its slim pickings because in the industry its bad so no one cares.
Thanks for reading all that and I hope it wasn't too boring or irrelevant!
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Looking forward to you sharing more of your finding here!
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Thank Badger! And please anyone and everyone chip in, because like I said, a degree is no match for years of hands on experience!!!
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So this juvenile wood grows towards the top of the tree as opposed to sapwood which grows on the outside of the tree? And even if the tree lives 500 years the center 10-30 rings at the stump would still be considered juvenile?
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Interesting stuff. I wonder how much growing conditions alter these properties. I have seen Elm trees lining a city street that were about 3 feet in diameter and yet the cut cross section revealed them to only be 40 years old. By contrast I have cut elm that was little past the sapling stage and yet it was almost 100 years old.
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Same story here in Arizona with ponderosa pine. Open grown pines can get fat quick, and then we have 1" poles 100 years old. In softwoods, tight rings increase density. They put on a set amount of latewood each year, more or less, so the slower they grow, the less earlywood they put on, increasing the latewood/earlywood proportion. In ring porous hardwoods like elm it is the opposite. They put on a set amount of earlywood (pores) in the spring and then the rest is latewood, so the less rings it has, the less earlywood it has, and so the more dense it is. That's why we want tons of rings in yew, and big honking rings in osage. There are exceptions to the rule. There are always exceptions. But more or less the rule of thumb.
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It is often the case with bowyers that we gradually find out what we like but we are not always sure why. It is not when you can quantify something. It makes it easier to apply to situations that don't exactly match your prior experience.
One example for myself is that in recent years I have taken a liking to thin ringed osage. I suspect it may be more elastic and lend itself well to my style of tillering where I like to crowd the bending portion of the wood into as short of areas as I can get away with but I am really not sure.
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I'm curious as to whether there is a difference, either in performance or longevity, between a freshly cut but dried bow and the same stave that aged years or decades and then was made into the same bow? My experience has been that the bow most likely to break is an older bow, of any species. Not sure if that is because it is wore out or that the wood gets brittle with age. That's just my limited experience.
My theory is that the younger bow is better in every way.
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Well we know old bows get stiff. How many facebooks posts have we read 'hey I found my grandads old bow and it broke when I tried to shoot it!!!' I imagine an old stave is the same story. Maybe it can be excercised back into shape like old bows. I'm not sure.
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Thanks, E Jensen. I've never really got into the science of wood. 35 years of teaching science and chem was enough for me.
Sapling bows have worked for millennia.
Interestingly, I've noted when hand planing shafts from white pine that the smaller tighter ringed boards make for stiffer spines.
I agree that being able to explain the "why" on a cellular level is useful.
Jawge
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Jensen, curious as to what your thoughts are on what we call the nuetral plane. I know most consider that to be in the center of the bow but if the back is only stretching a small fraction of what the belly is compressing wouldn't that throw the nuetral planne much closer to the back of the bow?
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Nice breath of fresh air! I've been interested in this part of wood science since I started making bows almost 20 years ago. Steve (Badger) and I have exchanged thoughts on the relationship between the resiliency of compression and tension in woods. Not sure we ever got any definitive information there.
I don't suppose you know anybody at the Forest Products Laboratories under whom you could light a fire that would get them to test the mechanical properties of dry Osage? They recorded the green values, but have nothing on dry values. I tried several times to hassle them about it but got nothing but whimpering about budget constraints. (I suspect there is no longer anyone there who knows how to do the tests or the math:(
Anyway, thanks for you insight on young and mature crown wood. It will be interesting to observe the principles when making bows.
Jim Davis
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In your graphic above, ...is the 'compression wood' bottom side of a limb? ...as in pointed toward the ground and loaded up in compression from the weight of branch?
...and perhaps for a different discussion, but I'll ask anyway because I'm sometimes rude like that, ...how does bamboo or cane differ at the cellular level, if at all, from more typical wood.
OneBow
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Yes it would be the underside of a softwood branch or the underside of a leaning softwood stem. People often confuse this but confiders only form compressionwood and hardwoods only form tensionwood. They are different and each has 'opposite wood' but compression and tensionwood and exclusive. Another phenomenon I suspect is different for bows, but 'inferior' for general wood products.
And I'm sorry I havent a clue about bamboo.
I'm thinking about building a bending test machine to test elasticity and breaking point. Its basically just load vs deflection of a 1x1x16" sample, but it has to be a set load applied over a set time. I'm thinking a really nice fish scale and a winch of some kind would do the trick. But otherwise, I know no one at the product lab.
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there have been many claims that older wood in instruments has a better sound,,Stratavarious for example,,the older wood producing the best sound,, that cannot be duplicated with any "newer" wood,, only the aged wood having the sound,, produced by curing and 100's of years of being exposed to musical vibration,, I have read of some curing wood with music playing to duplicate the effect so desired,,
I am not sure I am convinced older wood is more likely to break,,and the old bows mentioned may have been stored in such a way to compromise the wood,, Ishi shot quite a few older bows to test them,,, and some more than 100 years old,, overdrew and shot well,,
I think the wood has changed since it was made 100 years ago,,in a positive or negative way I am not sure,, and I am sure it depends on the type of wood as well as to how older wood holds up as a bow,,,, no need to open that can of worms :)
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I dont know if wood changes over time. I really don't. Not simply from time, all things being equal. But I do know the wood growing now is different than wood from the past. Yew is a great example. There just is not a lot of 100 rpi around anymore. Similar story across the board. Stuff available to cut now is faster grown, which for a lot of woods means less dense, less stiff, more knots. That could be part of the reason with the instruments. I bet that ring count has a big impact on sound quality.
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I did not think of that, and that makes sense ,, infinitely fascinating ,,
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I know that even the type of finish can have a huge impact on tone quality of wood. I don't suspect it comes into play much with bows, but I may be wrong. It may factor in if we're talking about sound dampening.
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there have been many claims that older wood in instruments has a better sound,,Stratavarious for example,,the older wood producing the best sound,, that cannot be duplicated with any "newer" wood,, only the aged wood having the sound,, produced by curing and 100's of years of being exposed to musical vibration,, I have read of some curing wood with music playing to duplicate the effect so desired,,
I am not sure I am convinced older wood is more likely to break,,and the old bows mentioned may have been stored in such a way to compromise the wood,, Ishi shot quite a few older bows to test them,,, and some more than 100 years old,, overdrew and shot well,,
I think the wood has changed since it was made 100 years ago,,in a positive or negative way I am not sure,, and I am sure it depends on the type of wood as well as to how older wood holds up as a bow,,,, no need to open that can of worms :)
The Stradivarius sound doesn't hold up under scientific testing. ;) The only time it sounds better is if the player knows it's a Stradivarius or is told that this is the instrument being played.
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wow that is very interesting,,, :)
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Compression wood? Isn't it different on softwoods vs hardwoods? What about branch bows vs. sapling bows? Making branch bows would kill less trees;_; and branches are probably easier to get?
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Hardwoods only have tension wood, never compression wood. Softwoods only have compression wood, never tensionwood. Both are different from juvenile wood. I'd imagine branch wood is also usually juvenile wood since it is in the crown.
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More of this. I love it.
The difference between scholarly research and red-neck know-how is the redneck learns from tradition and and personal experience, and the scholar tries to figure out the why behind it. The danger for the scholar comes when he ignores what experience has taught and thinks he knows better despite all evidence. The danger for the redneck is the same, but the other way around. Both can and should learn from the other, with a healthy dash of humility on both sides.
I once watched a group of engineering students at my school try to design a radio controlled airplane for a contest. I have nearly 30 years of experience building and designing r/c aircraft, using mostly rule-of-thumb experience and that-looks-about-right engineering, and I have been pretty successful. The students were building with techniques and materials that I knew would result in complete failure--but in their minds, they were engineers, and the math told them it was going to work, no matter what I told them. You can guess at the outcome.
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More of this. I love it.
The difference between scholarly research and red-neck know-how is the redneck learns from tradition and and personal experience, and the scholar tries to figure out the why behind it. The danger for the scholar comes when he ignores what experience has taught and thinks he knows better despite all evidence. The danger for the redneck is the same, but the other way around. Both can and should learn from the other, with a healthy dash of humility on both sides.
I once watched a group of engineering students at my school try to design a radio controlled airplane for a contest. I have nearly 30 years of experience building and designing r/c aircraft, using mostly rule-of-thumb experience and that-looks-about-right engineering, and I have been pretty successful. The students were building with techniques and materials that I knew would result in complete failure--but in their minds, they were engineers, and the math told them it was going to work, no matter what I told them. You can guess at the outcome.
This is why we still see engineering failures on a grand scale. All the math used is right except for the intuitive stuff that gets left out entirely. ;) Engineers are also notoriously lousy builders.
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Actually, when there is an engineering failure, it is because there was a factor that wasn't recognized or somebody made a mistake with the math. The principles are rarely at fault, just the execution.
With the trial and error method, a best practice may result, but sometimes accompanied by no understanding of why it is best or how it might apply in other circumstances.
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Actually, when there is an engineering failure, it is because there was a factor that wasn't recognized or somebody made a mistake with the math. The principles are rarely at fault, just the execution.
With the trial and error method, a best practice may result, but sometimes accompanied by no understanding of why it is best or how it might apply in other circumstances.
The factor not recognized is the intuitive part. ;)
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Something I would like to see done and would be fairly low tech and easy to establish would be a rating system designed specifically for bow woods.
Example take a piece of wood that you might expect to come out mid range. Cut it 1/4"thick, 1' wide and maybe 24" long. Cut into a pyramid shape. Clamp the wide end and then progressively bend it to correspond with a 1" incremental measure board that would accommodate the shape of the arc we are using. 1" back up, 2" back up. 3" return etc, measuring the weight each time, monitor at what point it first registered a weight loss due to set and continue until it has taken 1/4" set. The mark on the measure board would be the rating for elasticity.
Each sample would be thickness ground not to match the test piece in thickness but to match it in weight at say the 6" mark. The thickness measure would express the stiffness. Some designs look more for stiffness and some more for elasticity. This would help to identify better woods for various designs.
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Tim Baker did related tests many years ago and the results are listed in (I think) Traditional Bowyer's Bible Vol I.
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I am interested in what happens on a cellular level when a bow is heat treated, when it is drawn and when it is released, or what happens that causes set. Things like that. I think some things are mostly untestable, because each piece of wood is different, so you have difficulty repeating an experiment. Other things would be testable.
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Actually, when there is an engineering failure, it is because there was a factor that wasn't recognized or somebody made a mistake with the math. The principles are rarely at fault, just the execution.
With the trial and error method, a best practice may result, but sometimes accompanied by no understanding of why it is best or how it might apply in other circumstances.
Yep. Math doesn't lie, but bad or incomplete math will kill you.
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in yew, the juvenile wood has the best compression strength properties (have a look at this post I wrote earlier: http://paleoplanet69529.yuku.com/topic/61477/Mechanical-properties-of-European-Yew#.VU02YvntlBc), possibly because of its higher lignin content.
Heat treating the belly reduced the hygroscopic nature of the belly wood but not of the back. I posted a thread some time ago on the physical properties of heat-treated wood: http://www.primitivearcher.com/smf/index.php/topic,51962.msg703208.html#msg703208
Cheers
Joachim
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I guess I was a part of that second chat and didn't even recall! Could you PM me a link to the paper in the first chat? There is evidence that heat treatment increases compression strength independent of just lower MC, but its a small window.
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I found one by Bjurhager, is that the one?
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Right, the Bjurhager paper deals with the properties of juvenile versus mature wood for bow-making (in yew and common juniper).
I made a temporary link to it here on my google drive, for those interested in the paper: https://drive.google.com/open?id=0B3YYA3Sr_3gqeVQ3Sk1yd21wc3M
cheers
Joachim
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It is asking permission to ask your drive. I have requested the paper through my university anyhow. If you come across any other similar readings, let me know. Surprisingly, wood science people don't care as much about bows as you'd think, considering bow making is the working epitome of every aspect of wood science.
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OK, permissions for the google drive pdf changed, so it should be available to anyone now.
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E.Jensen or joachimM
hope the more knowledgeable might comment on a question that has been in the back of my mind for a while.
Most all the local hardwoods nearby are considered ring diffuse porus. I have found examples of different densities between staves I have collected, but as yet do not understand what causes these differences. What would you look for when searching for dense staves?
age?
protected vs exposed location?
elevation?
something else?
thanks
willie
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The rule of thumb for ring porous is the thicker the rings, the denser it is. Think about it, earlywood is another way of saying lots of vessels. Vessels are empty space, air. The more empty space, the less dense. The more rings, the more earlywood, the more empty space. As someone else commented, there could be exceptions like white oak. I wonder if that has something to do with tyloses, which is basically the clogging of those vessels in the heartwood. Which is very important, and why whiskey barrels are made from white oak, and not red oak which does not have the clogged vessels. Basically, all your beer would leak out!!!
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Paper is printed and I look forward to reading it. It has already led me to a few others and I'm sure I'll find more in the references.
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E jensen
my bad, I meant to say diffuse porus
looking at easy to see rings makes early/latewood judgements easier for sure
willie
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with the very limited experience I have, my feeling is that in diffuse-porous woods, slow growth gives denser wood, just like in softwoods. Hazel, for example, can grow very fast. When well-watered, well fed and with plenty of light it can give 3" staves in 3-4 years. Such a stave I got lately only had SG 0.36... I've had staves from below a canopy in the shade of an oak forest closer to 0.55. These 3" staves are sometimes 30-40 years old. These are the ones I'm after lately.
In ring-porous wood, there seem to be quite some exceptions to the rule of thumb that the thicker the rings, the denser the wood. I have had black locust with rings of 1.5 cm that was less dense than other trees with >5 rings per cm. In many oaks, you will find that the density of the older branch wood, which is typically very fine-ringed, is very high.
So really, what you should look for is density or specific gravity, how much the wood weighs (dry) for its volume.
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Beats me
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I can add this to what Joachin said, but can't vouch.
I cut hazel from a landscaped yard once, two staves that grew in a row of others in the sun along a fence, and twio that grew from a round cluster along a fence. The sunny ones were younger and less dense. The shaded ones were older, more dense, and had only a few sparse leaves. The staves were equally straight, but the shaded THICKET on average had straighter trunks and fewer knots, of course.
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Something I would like to see done and would be fairly low tech and easy to establish would be a rating system designed specifically for bow woods.
Example take a piece of wood that you might expect to come out mid range. Cut it 1/4"thick, 1' wide and maybe 24" long. Cut into a pyramid shape. Clamp the wide end and then progressively bend it to correspond with a 1" incremental measure board that would accommodate the shape of the arc we are using. 1" back up, 2" back up. 3" return etc, measuring the weight each time, monitor at what point it first registered a weight loss due to set and continue until it has taken 1/4" set. The mark on the measure board would be the rating for elasticity.
Each sample would be thickness ground not to match the test piece in thickness but to match it in weight at say the 6" mark. The thickness measure would express the stiffness. Some designs look more for stiffness and some more for elasticity. This would help to identify better woods for various designs.
Badger, this pair of Australian threads was pretty interesting reading re. testing bow woods and rating, though not quite the same as your suggestion
http://www.ozbow.net/phpBB3/viewtopic.php?f=34&t=13765
http://www.ozbow.net/phpBB3/viewtopic.php?f=34&t=5450
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Lot of my Botany classes are coming back to me.....
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Joachim-
thanks for your observations about slower growth with diffuse porous woods. Of course density is what counts, and I have also found an understory tree is desirable for bowmaking, but I have always wondered if slow growing was a just another symptom of a small crown caused by light deprivation? And small crowns need less water vessels, hence denser wood?
If light deprivation produces denser wood, what about feed and water?
Do drier conditions affect density?
Do poorer soils affect density?
I am guessing that drier environments might need more or larger transport vessels to maximize water transport when water is available? And trees in poorer soil need to move more nutrient poor water requiring more or larger transport vessels? I could well be all wrong in my thinking, and would welcome a more scientific viewpoint.
I have found certain trees that are stunted by wind to have very tough wood, but are so twisted and gnarly that one can not get a stave out of them. I have been looking to find similar growth in less windy locations. Are other factors involved in producing dense wood besides empty vessel ratios?
willie
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For an understory tree I would imagine juvenile vs mature wood would have more to do with things.
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Willie, yes what I wrote was close to the obvious and more likely common knowledge ... Mostly, I tried to convey that I'm not convinced there are easy rules of thumb, and that density is the only really reliable proxy left.
Sorry I can't help you with the remaining questions, no experience on those subjects. Only a biologist's logic. So here's a try.
Nutrient poverty will lead to slower growth, but it doesn't necessitate larger or more transport vessels: the roots don't suck up water randomly like a straw and filter out later the required nutrients. Most of the transport of minerals and nutrients happens selectively through specific ion channels along the cell membranes of the hair roots, so the plant concentrates the nutrients by itself at the desired concentration.
likewise, drier environments will require a lower crown to root ratio: more roots are needed to find and suck up water. But I don't think the plant needs more or larger vessels to transport the water in the stem and branches. The amount of crown that can be produced is then a product of the amount of water the tree can gather, as a larger crown (~larger leaf surface) loses more water through evaporation.
Now that I think of it, it could even be the opposite of what you suggest: a fast growing tree with plenty supplies of light, water and nutrients needs lots and large vessels to transport all that water to a crown that is large (because fast growing) compared to the diameter of the stem. so its density must be low. But since the crown is large, the girth can also increase rapidly. But at the expense of having large vessels, so low density.
I'm freewheeling here, so stop me if you think it doesn't make sense.
Ring-porous wood is still a bit different. there the amount of early ring growth seems (to me at least) determined by the crown size and the concomitant water uptake required for bud burst in spring. For a same crown size, A tree strongly limited by nutrients and water and not light will require the same amount of early growth for this bud burst as a tree having access to the same amount of light but a surplus of nutrients and water. The former tree needs all the water/ nutrients it can get just for the maintenance if its machinery (the leaves, roots etc) and will have little summer growth (late wood), hence narrow growth rings. The density of the summer growth may be good, but the porous spring growth (early wood) may be disadvantageous. The well-watered and well-fed tree with plenty of light will have much wider growth rings with plenty of late wood. The summer wood may be a bit less dense than that of its poor-soil neighbor, but, the overall density may be higher because it has less early wood.
Like I said, just freewheeling
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Generally density is the best indicator of wood quality, however I just read yesterday that density and MFA combined can predict 95% of wood quality, but MFA alone predicts ~85%. The thing is density is easy to determine, MFA not so much.
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Generally density is the best indicator of wood quality, however I just read yesterday that density and MFA combined can predict 95% of wood quality, but MFA alone predicts ~85%. The thing is density is easy to determine, MFA not so much.
What is MFA?
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Sorry, micro fibril angle
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Sorry, micro fibril angle
Quite a few of the very dense exotic woods we have tried for bows are horrible because of tendency to chrysal. Can the MFA identify woods that are prone to chrysal??
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E Jensen
Have you come across any information about juvenile wood, or steep MFA in hardwoods?
If determining MFA is beyond the ability of the average stave hunter, then perhaps there are growing conditions that could be identified that would increase the likelihood of finding juvenile wood?
I have always associated juvenile wood with second growth conifers from clear cuts, but that was with construction grade doug fir.
joachim- thanks for the freewheeling thoughts
Willie
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Knots. Juvenile wood is also called crown wood because more or less it's formed in the crown. Something with hormones. Should be about the same between softwoods and hardwoods. It is associated with second growth because old growth trees had their crowns recede long ago, forming lots of clear mature wood.
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E.Jensen
Thanks, that makes good sense about the knots.
Earlier you stated "juvenile wood..... makes for a really weak board." I presume that you are referring to the MOE or stiffness. Have you come across any info that might indicated whether a high MFA increases flexibility by improving the tension qualities, any more or less, than improving the compression qualities?
Is there a way you might be able to share some of your reading sources? I came across a paper years ago about MFA, but its lost now.
thanks
willie
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Not specifically. I don't think it would be good in tension but that is just my guess. I'm starting to think of the way the cellulose chains are arranged as a spring (a high angle wrapped around). Springs compress well and bounce back. Idk about tension though. I'll round up that paper about MFA
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E. Jensen, a paper or two I have seen assumed that the amount of compression equaled the amount of extension in bending. I wonder if that is ONLY an assumption or if there is experimental evidence to support the idea? I'm sure it could be measured and wonder if it would be the same with all woods. Most of us think hickory, for instance is stronger in tension than lots of other woods.
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Jensen, for the most part with bows and arrows bows tend to be a work in work out type spring minus hysteresis losses. I did a little study on hysteresis last year and came to the conclusion that it is much lower in wood bows than previously reported until the wood has been overdrawn into the plastic range at which point it starts going up dramatically depending on how badly the wood has been overstressed.
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What happens to the internal structure of (say) a northern hardwood if it freezes while green? Let's say it was a summer cut log.
If it does make a difference, at what lower moisture level would it be undamaged by freezing?
What's the moisture content if cut while standing in winter?
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heres an easy reading page about live trees and cold......
http://msue.anr.msu.edu/news/how_do_trees_survive_in_the_winter
when it come to already cut trees, about all's I know is that if you wait until it is sub-zero to split green birch firewood rounds, it will just about fly apart with minimal effort
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Yes the frozen moisture in green wood keeps the splitting maul from losing energy by crushing the wood. Far and away the best situation for splitting wood.
Jim Davis
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looks like a nice place you had there, Jim. Bet you don't miss having to get up that much firewood now a days
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Still interested if anybody up on the science of wood structure knows whether cut green hardwood, if frozen, suffers structural damage at the microscopic level compared to dried wood subjected to freezing.
Also since willie's link mentions cellular changes in lipids and proteins to prepare for winter, whether winter cut green wood would be less subject to cellular level freeze damage (if it exists) than summer cut green wood.
Splitting frozen firewood in winter I'm very familiar with. And I own an old green tractor, too! Cool photo. :)
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Plan B, I can't imagine why freezing would damage cut wood, when it doesn't damage standing timber. I had staves in my shed all the time in Maine and never had subzero temperatures hurt it.
Right you are Willie. I didn't mind feeding the stove, but I sure don't miss having to drive in half a dozen storms a ,or having to get the driveway cleared to go to work, or coming home from work at 1 a.m. to a driveway I could barely get into!
That's an early picture of the house. It got a deck and all decked out and pretty later, but that's WAY off topic. I'm very glad I Don't have to worry about it any more.
Jim
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Asharrow, thanks, I'm not saying it makes a difference .....I'm asking if it does.
The link willie gave indicates that a living northern hardwood tree takes active measures to change some of its internal chemistry. reduce free water content, etc. in order to prepare for winter. So I'm asking if that type of timber,cut in a warmer month, which hasn't taken these internal biological measures might see some internal structural damage and reduced physical properties. Maybe so, maybe not, I personally don't know .... that's why I'm asking.
Here's why I'm thinking about it. Some woods which grow well further south of here -- I think hickory is an example -- you guys know which ones better -- don't work well as bows subjected to subfreezing use. Some others, like Finnish birch bows , made from woods which grow far more northerly, apparently do well in subarctic areas.
So what's the difference in the woods internally, and were they cut for bow use in summer or winter. Lots of interesting questions to me since, those very woods grow here, and maybe I'd like to shoot 12 months of year without worrying so much about the selfbows I make for that.
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Plan B
I do not have an opinion based on science, but just a few observations. Having worked with quite a bit of wood in some really cold conditions, I would say that frozen wet wood seems to often have internal stresses not found when warmer. The example above of the firewood birch round is typical. I think Jim is right to a point about most all wood being easier to split when cold, but I have observed green birch with good bark do strange things when cold. Splits travel almost by themselves once started. I once was removing the bark from spring cut (warmer) birch, and as I released the hoop tension in the outer bark, I could actually watch splitting in the inner layers, follow my knife. My guess is that there are often pressures in trunk associated with different conditions. The article mentions 900 psi. And as far as being detrimental, I guess it's a matter of degree. I have heard of cottonwood trees (cottonwood is very wet when green) "exploding" during rapid temperature drops in the mid west. "Exploding" probably being a bit of overstatement, so under more normal conditions, perhaps trees can adjust to freeze/thaw conditions faster than you might expect.
I have shot self bows in very cold conditions, and have always been leery of "cold breakage", but have never had one break (hickory board bow included) in a way that suggested cold might be a factor. but you can find a lot of opinions on this. Also, my worst cold weather enemy seems to be low winter humidity
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Found it.
http://onlinelibrary.wiley.com/doi/10.1017/S1464793103006377/abstract
Also found this
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