Heat treating increasing set?

[jonathan creason]

I had a RO pyramid tillered out the other day, and ended up with just over an inch of set in each limb.  Nothing major, and I should have just left it alone, but I decided to break out the heat gun and toast it a bit to see if it would help.  I went down one limb slowly and to my surprise by the time I got to the fade the set had increased to 3-4".  Not a huge deal, since it was just a red oak board, but I'm kinda curious as to why it happened.  Has anybody seen that before?

[RyanY]

When heat treating, the heat causes the belly of the bow to shrink and become more dense (I think). So if you didn't put the bow in reflex or at least keep it straight, the belly must have shrunk and pulled itself into set. Putting the bow into reflex while heat treating still compresses the belly but keeps it in the reflexed state.

[Mangeur de lard]

Hello Jonathan,

If the wood is not clamped or tied down somewhat before you start the heat treatment set will increase because the wood shrinks as it is heated (loses some humidity I think). Be sure to tie it down at least flat before you start, let it cool thouroughly after the heat treat and let it rehydrate a few days before you string it again.

Cheers!

[PatM]

The cells shrink when exposed to heat the same way plastic shrivels up. Have to at least hold the stave straight if you want it to not curl the wrong way.

[eflanders]

If I may suggest:

When heat treating or tempering the wood, I have found that a fast hot heat works better than a slower heat-up process.  While it does take a lot of control, I have been using a propane torch as of late for several good reasons: 1). The torch heats the wood much quicker than a heat gun which in turn minimizes the risk of the glue letting go on laminated bows.  2). Wood gets somewhat elastic once it reaches a certain temperature.  By heating it quicker, it is not elastic for as long and minimizes deformation issues and you rarely need to back the heated wood to maintain shape 3). The torch is cheaper to use and gets the job done much quicker.   

Using a heat gun almost always requires the use of a caul, form or something to maintain the original shape of the bow.  Also if you did not add tung oil to the heated wood as soon as it reaches the right color, the wood takes longer to naturally rehydrate and the bow will take additional set if it is flexed too soon as the others have eluded too.

[jonathan creason]

Thanks for the advice guys, what ya'll are saying makes a lot of sense (if only I'd had the sense to figure it out before hand).  Oh well, at least it wasn't a primo osage stave or anything.