Science

[E. Jensen]

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!

[Badger]

  Looking forward to you sharing more of your finding here!

[E. Jensen]

Thank Badger!  And please anyone and everyone chip in, because like I said, a degree is no match for years of hands on experience!!!

[DC]

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?

[PatM]

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.

[E. Jensen]

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. 

[Badger]

  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.

[steve b.]

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.

[E. Jensen]

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.

[George Tsoukalas]

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

[Badger]

  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?

[Jim Davis]

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

[Onebowonder]

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

[E. Jensen]

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.

[bradsmith2010]

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