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Heat treating demystified (Somewhat)
H Rhodes:
On the same subject of moisture loss and heat treating.... I heat treated a couple of bows that I built last spring. They turned out to be good shooting bows, one that weighed in the low forty lb. draw weight, the other right on fifty. I toasted the belly on both of them. I revisited these bows after they had been hanging on a hook in the air conditioned interior of my house for about six months.... Both of them had gained about ten lbs of draw weight! I reasoned that the wood was dried outside in my shop. Not heated or air conditioned - higher humidity shop never allowed the wood to reach optimum moisture content. These bows did reach their proper moisture content inside the house and gained draw weight - well AFTER the heating they received during construction..... That made me realize a couple of things.... 1. I need to dry my wood inside before I attempt to make a bow out of it - that and check to make sure it is completely dry; and 2. normal heat treating doesn't remove that much moisture from a bow. There is a blundering budding bowyer's take on it....
lostarrow:
Thanks for the comments Marc. I am not surprised with what your friend found for moisture readings.As you wrote in your findings you were using well cured hardwoods for specimens which probably had a reading around 9% or so If it were in the late fall or winter where you are . What I am saying is that when you shrink the Physical size of the vesel , you can't get the same amount of water in as you could before. The vesel in question isn't full of water when you heat treat it .It merely contains some water. From 9% down to 7% would make a good bit of difference in a piece of elm or hickory, probably just right. If you continued down to say 6% or 5% that would likely cause trouble. If your heat treating continues too long you may be drying too far into the back working portion of the bow, causing a very dense belly and a less elastic back. Which from your own research once again it looks like you found from the samples that didn't make it. Tension failures on the back and cracks instead of crysals on the belly, on the samples that were Carbonized(charred ) instead of Caramelized( lightly toasted ). I like the idea you had with the turpentine /varnish being applied while hot . I would expect this finish to penetrate deeper into the wood to add more long term protection. As a side note ,I was in your neck of the woods last weekend.(Pembrooke)Beautiful trip , lots of HHB.Some day when I get up that way I may even get a chance to look you up.
lostarrow:
So I just read through the post again because I was confused about the comments about heat treating not drying out wood very much.I 'm not sure we are on the same page as far as how little a difference a couple of % points of moisture will make in a piece of wood.Here is an example. A Hardwood floor for example,if laid in the summer with a moisture content of 12% has a nice tight fit .That winter the temp drops down to -20deg Celsius with a relative humidity of around 15%.With no artificial means of moisture(humidifier) ,the content in the wood now drops to a drastic 8%.The floor boards now have a gap of 1/8" -3/16" between each one.The next summer sees relative humidity as high as 90%, but the boards will be only scarcely larger than they were in the winter.They will never again return to their size they were when laid. The transverse would be fitting a flush drawer in a cabinet. The cabinet made in the winter must take into account the up coming summer swell of wood.if you leave 1/16" all of the way around , it will likely be jammed tight in the summer .If you had years to wait ,it would settle in and stabilize to an acceptable rate of swelling and contracting ,diminishing over the years to where it just can't absorb hardly any moisture at all. This all varies greatly with species and even within the species based on growing conditions and orientation of grain etc. In short , 6% MC in a bow will likely explode,while 8-9% would be ideal,12-13% would likely be sluggish. Again ,just examples. That's why people would prefer a board that is air dried for 7 years over one that is kiln dried 6 months ago. It has gone through several cycles of high and low moisture and is more stable, more predictable.
TBod:
Well spoken again!
PatM:
lostarrow, The shrinkage and decreased moisture uptake of the heat treated wood still doesn't explain things. A heat treated bow can at least be brought back to an intolerably high mc for a regular bow and still perform better. You may not be able to turn a raisin back into a grape but a moist heat treated bow performs better than a bone dry regular bow.
I'm not sure if anyone has dried a stave for 7 years in forced reflex and compared it to a bow heat treated into reflex though.
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