drying conifer woods for arrow shafts

[willie]

I recently tested some various conifer woods that I had set aside for arrow shafts.

after ripping air dried boards to square arrow sized blanks, I placed the stock into a warm box for a week. (about 120 degrees F.) and tested for deflections. I then baked the blanks for an hour at 300/325 F and tested again

 
white spruce         lost 5 and 10 % strength after toasting  (two samples)

lodgepole pine      one gained 3% strength and the other remained unchanged

western larch       three samples remained unchanged after toasting


testing will continue, but I am not sure how at this time. any suggestions or recommendations? questions welcomed too.

[Badger]

  Did the baking have a toasting affect on them. Interesting regardless, I would have expected close to 10% gain from the larch, not so much from the pine

[willie]

Steve, sounds like you have been down this road before. I was disappointment in my spruce results, but the board I chose may have had too high a ring count.
I guess I shoud have said I baked them, cause toasting implies that they got browned maybe? I was not hot enough for that, although it did bring some brownish spots to the surface.
why would you expect better results from larch?

[Badger]

  I thought the larch might have more lignins to harden up if that actually happens when we heat.

[JNystrom]

I understood that pine would benefit from heating, spruce not so much. At least a friend told me so, when he tried it.
Spruce didn't work at all in my brief test's.
ps. I wish i had that long oven to bake the shafts. Maybe i should just cut them shorter, experiment with oven and replicate the good ones with heat gun.

[avcase]

Willie,
Just to clarify, did you measure the impact of heat treating on the wood strength, or was it change of stiffness?

I measured a pretty dramatic change in stiffness for heat treated spruce arrow shafts. I heated them around 375-385f for a shorter period of time. Maybe 20 minutes. It left the arrow shafts a shade darker. They were also significantly stiffer, smaller in diameter, and lighter than they were before going in the oven. Unfortunately, much of the benefit faded away as the arrow shafts re-hydrated over the next week.

The amount of change due to heat treating will probably depend on the humidity level where we live.

Alan

[willie]

lignins to harden up if that actually happens when we heat.

I seem to recall some papers that indicated that might be happening, but if I remember correctly, the effect was not very consistent, and not all the experiments started with wood at the same moisture content, so it might be hard to isolate the "lignin hardening" effect from stiffness gains from simple drying. I deliberately chose a lower temperature than one normally uses to heat treat a bow limb, so that I could observe any discoloration that might be hidden by the  browning that we expect at the higher temps. the spotting on the heated samples seem to suggest something melting or vaporizing (besides water, as the the wood was fairly dry to begin with), but if it is lignin, I have no way of knowing.  Just guessing, but if lignin is formed  by the tree to reinforce the cell wall, and it is subsequently displaced, than it's possible the cells could become relatively weaker.

I have since retested the samples, and after a few days, the arrow blanks that lost strength a few hours after heating, seemed to have regained their strength back to pre heatreat levels. The idea of a curing or hardening that happens over time rather than changing with temperature or moisture seems reminiscent of the "seasoning vs drying" debate of bow staves.  At any rate,  the samples will go back in the warm box and be tested again later.

Alan, I have been measuring deflections as one would with a typical spine tester. The strength differences reported are % change in deflection. Are there strengths in different directions that might be useful to look at?, or a more useful criteria to quantify stiffness?

[avcase]

Alan, I have been measuring deflections as one would with a typical spine tester. The strength differences reported are % change in deflection. Are there strengths in different directions that might be useful to look at?, or a more useful criteria to quantify stiffness?

Thanks for clarifying. I was just thrown off track when you mentioned the heat treatment effect on strength, but meant stiffness. I think strength often goes down as a result of heat treating, but stiffness almost always goes up.

Perhaps the spots you saw on the surface of the wood was due to the outgassing of moisture, and this carried various resins with it to the surface. Just a guess.

I would like to try heat treated spruce arrow shafts again and see if I can maintain the stiffness benefit by keeping the arrows stored in a sealed box with a desiccant in order to keep the arrows from rehydrating.

Alan

[willie]

Alan, Did you see significant reductions in MOR when you heated your sitka spruce?
I guess I think of stiffness (bending strength) as governing design, but I best not ignore ultimate strength if my bow hand is going to be in the line of fire.

Btw, In the flight community, is there a commonly used spine test standard for short arrows? Otherwise I can make my spreadsheet  accept any length/thickness etc.. And calculate MOE to predict spine.

[Badger]

      As for spine I seldom measure beyond the finger pressure test but when I was measuring I just measured at actual length of whatever arrow I was testing. Figuring out optimum spine requirements is another ballgame. I think around 25# spine for 26" arrow of about 200 grains works pretty well, even a little less has shown good takeoffs.

[joachimM]

baking wood, or toasting, changes the hygroscopicity of the wood itself by the early combustion process of hemicellulose molecules, which have a high affinity for water molecules.

toasting beech for example, reduces its equilibrium MC from 10% to 5% at an ambient MC of 66%.
Since drier wood is stiffer too (in compression), toasting arrows increases spine.
That's exactly the reason bamboo for splitcane fly-rods is baked during the manufacturing process.
For the technical info, see http://www.primitivearcher.com/smf/index.php/topic,51962.0.html (and second page).

Since all woods contain hemicellulose, this affects all wood species. But there are strong differences in types of hemicellulose among wood species.
I noticed that some woods require a deeper / longer baking than others, for the same result. I long thought that my fir boards didn't respond well to heat treatment, until I forgot a slat in my pizza-oven at 300°C (but cooling down), and only recovered it after half an hour when I also removed the burnt focaccia I had tried to make. The slat had changed (on the belly side, which was lying on the oven stones) from cream to dark brown, had become concave at the toasted side. The change was dramatic and didn't disappear after some time.

So maybe if someone's toasting results only seem temporary, I guess this is due to a mere change in MC (and a rebound afterwards), and the toasting wasn't done long enough or hot enough to markedly affect the hemicellulose content and structure.

[willie]

Joachim-
thanks for posting that link to the earlier discussion and an explanation of your discoveries. The extreme temps do seem to offer hygroscopic reduction benefits, but not so much for stiffness, at least not with the conifers I have been looking at.  I do wonder if the improvements with bamboo are more than just drying ? 

I have been drying in a hot box that is nothing more than a Styrofoam lined cardboard box with a old florescent lamp inside. Seems to maintain about 40 F. above ambient temps, it's enough to lower relative humidities considerably. The larch has stiffened up nicely. Still looking for some better spruce, and some  doug fir is still drying.

Steve, is optimizing spine requirements mostly about a smooth release?

[Badger]

   Willie, stiffer spines allow me smaller diameter arrows, I prefer dense woods with stiff spines when I can find them. I have had pretty good luck with purple heart when I can find straight grain. I had some 11/32 larch with 100# spine. That was also good stuff to work with.

[avcase]

Alan, Did you see significant reductions in MOR when you heated your sitka spruce?
I guess I think of stiffness (bending strength) as governing design, but I best not ignore ultimate strength if my bow hand is going to be in the line of fire.

Btw, In the flight community, is there a commonly used spine test standard for short arrows? Otherwise I can make my spreadsheet  accept any length/thickness etc.. And calculate MOE to predict spine.

I didn’t test the strength of my heat treated arrows, I only tested the stiffness.
   The info I have seen on the effect of heat on strength is from research by the forest product industry or supporting government agencies, or from those who make fly rods.

[willie]

Larch that tests 100# (.26" deflection with 2# weight at 26") is quite the find in 11/32 dia.
75#  seems like the best I will be able to get out of this batch. My preliminary sampling shows a 25% difference in the relative stiffness of spruce, pine, doug fir and larch. The doug fir and larch is heavier on an absolute and relative (stiffness per density) basis, and in spite of the adage that spruce is supposed to be the "strongest for it weight" , the lodge pole pine stiffness seems to be coming out on top of the spruce sample currently have in the drier. I guess I misunderstood what was meant by "optimization" if one is simply seeking to find best spine per diameter. Aero dynamically, spine per diameter seems to be desirable, but at the expense of how much extra mass seems to be the $64 question.

Perhaps I should  ask if "normal" wood arrows are still competitive in classes that allow carbon, or for that matter, how about the composite bamboo arrows that you make, Alan?
 
Btw, I have recently located a commercial supply of sitka/lutz spruce that might have some potential. It seems like the slower growing variety is in demand for its strength. Available up 50 rings per inch and "dehumidification kiln" dried. The drying process limits ambient temps to 120 F. max for quality purposes.