Mechanics of steam vs dry heat bending

[Badger]

Its always about which is better Steve

I got your point, hopefully my tid bit helped in some way.

  Your post was in line with the thread. The whole point I think is sometimes just knowing how to do something is good enough. Getting a little better understanding of why it works is icing on the cake. I just think it is kind of cool sometimes to take a little closer look at how things are working for no other reason than enjoying understanding the process. Most of us already know we don't want to stretch the wood when we bend it but we do want to compress it. It just occurred to me for some reason that moisture helps the back resist stretching by a good margin and offers a little more thorough explanation as to why moist heat works better than dry heat. 

[Aaron H]

When I steam bend (mostly osage), I normally seal the staves with two coats of shellac to help seal out any excess moisture, it seems to help with checking, but do you think this is hindering the steam from doing it's job?

[bushboy]

Thinking in. Reverse,when bending I a. Recurve the belly becomes the tension side and back compression using steam.then lock in the bend with dry heat increasing the. Compression value. On the belly side of the recurve.i hate auto correct,just saying

[BowEd]

I see your point Steve.It is nice to know why something works better the way it does.

[Lumberman]

What is actually helping with the checking Falcon is that the shellac  is keeping the moisture in!  Checking is caused by the surface moisture leaving too quickly for the interior moisture to replace it. The thicker the stave the more it will be encountered. Seems to me like having the coating would lessen the difference in using dry or steam heat because you would essentially be using heated moisture for both..  You would have to have some very very  dry brittle wood for  steam to  put moisture back in.. I use steam for conditioning kiln loads and to put 1.5% MC back in the wood dried down to 5-7% takes 12 hours of steam!

[Badger]

  Checking is an issue with steaming, not sure what effect shellac has, it may very well help to keep some moisture in. I know that a too dry back can weaken it a lot, not sure how moist a belly really needs to be once it is plasticized with heat. I would be nice to get more detail on this.

[wizardgoat]

Anither reason why steam is better, throw your stick in a pot, do something else for 15 minutes and your stick is now a noodle. 

[JW_Halverson]

Anither reason why steam is better, throw your stick in a pot, do something else for 15 minutes and your stick is now a noodle. 

You got a hot date with a beaver?  Wood noodles in sap sauce?  You serve red or white wine with this?

[Pappy]

Good topic, I use dry heat most of the time if the wood is seasoned except for the more radical
bends especially on white woods. Don't know which is better, both have there place for me at least.
 Pappy

[stuckinthemud]

  Checking is an issue with steaming, not sure what effect shellac has, it may very well help to keep some moisture in. I know that a too dry back can weaken it a lot, not sure how moist a belly really needs to be once it is plasticized with heat. I would be nice to get more detail on this.

The document posted by PlanB earlier in this thread goes into lots of detail on this, also the use of an end block to prevent checking, which is a new technique on me, as well as a reverse lever instead of a compression strap looks like a really useful technique - its a really useful document, wish I'd been the one to find it ! Actually wish I'd found it years ago.

[E. Jensen]

I imagine it has a lot to do with limiting the temperature.  Under normal circumstances, I think it would be difficult to exceed 212F, and you can thus soak the wood and get even temperature throughout.  Lignin softens starting at about 175F.  But the wood will start changing chemically and physically at the cellular level at about 300F, so with dry heat you have to worry about even temperature, assuming it's coming from one side like a heat gun.  Sometimes we want this differential change, like when heat treating the belly, but maybe not with bending.  If I recall correctly, 300-400F can increase compression strength but 400F starts to weaken it.  And I was also under the impression that steam dehydrates wood. 

Just a bonus, if you heat wood with steam above 350-400, it will have different chemical changes than dry heat.

If you guys saw my other post, my masters paper is about thermally modified wood, which is basically wood cooked with steam or dry heat at 300-500F.  Its not exactly the same as when we dry heat bows, but I suspect similar.

Key changes:

Degredation of hemicellulose (the part fungi like to eat)
Reduced hygroscopicity (ability to absorb/desorb water) due to the microfibrils in the cell walls contracting
Slightly increased lignin concentration, and condensed lignin
Slightly lower elasticity, greatly reduced breaking strength

In TBB, the author talks about wanting brown on the belly, not black.  I think scientifically this is for two reasons.  1st, its getting the compression side much hotter than 400F so the compression strength is lowered, not raised.  Second, its getting hot enough through the second half of the bow, the tension half, to change some of that wood, lowering tension and breaking strength.  Double bad.  As opposed to 300-400F chocolate brown, where the back remains normal and the belly increases in compression.  Double good.

I hope all that is in alignment with the OP?  I've read about 400+ pages on thermal modification of wood in the last week for my paper so I sort of eat/sleep/breathe/dream this stuff lately.

[Badger]

  That was a very good post Jensen and it does explain some things that I wasn't always real clear on but suspected. I changed my heat treating technique several years ago. I like to heat slowly going from one end of the limb to the other with steady passes. The color change is preceded by an unmistakable smell which lasts for a few minutes and then passes. At this point the wood has only darkened a few shades and is easily soft enough to manipulate the gentle bends we use when reflexing on a cawl.

  In the article that Pat provided I found it particularly interesting when they talked about the ideal mc of the wood prior to bending, between 12% and 15%. I find a green stave when roughed out into a bow will hit that 12% to 15% range pretty quickly and does seem to respond best when I am doing dry heat bending or steam bending. That was a very useful tidbit.

[E. Jensen]

That smell is a great indicator.  I'm just guessing here but extractives burn off at raised temps and I'm betting that's what that is.  When heat treating the belly, I look for that, color change, but also sparks.  I can't explain how solid wood without dust or raised splinters sparks but when I see the color change and then it sparks or has that little puff of smoke, I know it's done.