Tempering- Does anyone REALLY Know?

[Chuck Hoopes]

Ok,We know tempering works very well on some woods, but not so well on others.  We know various ways that are effective means of tempering.  We know it  adds poundage to a bow, and we know it increases compression strength.  What I have never seen is an explaination of why it works-- Is it because resins are being crystalized, and hardened in the wood?  Is there changes taking place at a molecular level that result in chemical changes?  Really does anyone know how or why tempering is often so effective?-- Magic? Soccery? or what?  Whats your theory?

[talkingcabbage]

I'd say, mostly sorcery, with a tablespoon of magic.  Throw in a dash of guesswork, and wal-lah!

Can you tell I have no idea?

[ChristopherO]

It hardens the surface being toasted upping the compression.
On the resin issue:  I belly toasted an osage bow just yesterday and added reflex at the same time.  I, too, was thinking that the resins must have released while heated and hardened again in a manipulated shape after cooling.  Just my thoughts.

[Pat B]

I believe that it hardens the wood cell walls and solidifies the resins, etc in the cells making it harder but more brittle. This adds compression strength to a compression weak wood. That's why it works better on whitewoods and to a lesser extent on osage. It still solidifies the resins in osage too but osage is already strong in compression so it is not as noticeable.
   As kids we used to fire harden pointed sticks to make spears. By fire hardening it made the points twice as hard as the rest of the stick.

[Chuck Hoopes]

Originally posted by Pat B:
I believe that it hardens the wood cell walls and solidifies the resins, etc in the cells making it harder but more brittle. This adds compression strength to a compression weak wood. That's why it works better on whitewoods and to a lesser extent on osage. It still solidifies the resins in osage too but osage is already strong in compression so it is not as noticeable.
   As kids we used to fire harden pointed sticks to make spears. By fire hardening it made the points twice as hard as the rest of the stick.

So if your hunch is correct: " that it Hardens the Cell walls" then tempering is essentially a way of quickly aging the wood.  I say this, because my recollection of Botany 101, was that the cell walls become thicker and harder w/ age. Perhaps microscopic exam. of cells from the same piece of wood, part of which is heat treated, v.s. a non-treated section, would reveal weather this is the case or not.  I asked the question orginally hoping that better understanding of what is actually occuring, might indicate some methods for even more effective heat treatment- i.e. what is the optimal time and temp.  Like others I just make a w.a.g.-- Something like 30 min. per limb, and reaching a pt. where the back of the bow is so hot, I don't care to hold my finger against more than a second or two.

[Pat B]

Marc St Louis wrote a chapter in the TBBIV. He is the guy that popularizes the use of tempering bow bellies. His chapter has lots of good info and explanations.
   I think the tempering does more than just quick aging but I have no proof.

[Chuck Hoopes]

Pat- thanks - I'll check it out.

[shamus]

Heat treatment of bows has been around since the 1930s. Nothing new there. No one really knows how/if it works. It hasn't been scientifically tested. All we have in anecdotal evidence.