Mechanics of steam vs dry heat bending

[Badger]

  I was discussing this today with one of our members here and something occured to me. Lately I have been relying more and more on steam for tight bends as I have a higher rate of success without lifting splinters.

   The natural properties of wood are such that when moisture is added to the wood it becames weaker in the compression aspects and stronger in the tension aspects. If the steam permeates the surface of the wood for an eight of an inch or so this would be a big advantage in bending. The hot steam would soften the lignins and the moisture in the wood would better position it to compress.

  When using dry heat we would further dry out the surface of the woods leaving the compression aspects stronger and the tension aspects weaker, just the opposite of what we are looing for in a bend. The added heat will still soften the wood but the result of moisture loss would reduce the qualities we look for when bending.

  Just a theory, does this seem logical?

[PEARL DRUMS]

Maybe this will help further prove your theory Steve. When I bent those statics in my mini bow the back wrinkled on both ends and the belly never cracked or crinkled.

[Badger]

Maybe this will help further prove your theory Steve. When I bent those statics in my mini bow the back wrinkled on both ends and the belly never cracked or crinkled.

  Did yo use dry or steam heat?

[DC]

You may be on to something. I've often thought that steam was used only because it was a convenient method of getting wood hot enough to bend without getting it hot enough to scorch. I was considering building an oven that would only go to 220 degrees or so just to test the difference of wet vs dry at the same temp. Never did it, it was more trouble than it was worth. I've often wondered how far the steam does penetrate dry wood. I know that dry firewood can take a lot of rain before it doesn't want to burn so I didn't think it soaked in that fast. On a vaguely related note when I was fuming some OS the speed that the color change went through the wood amazed me. 24 hours the color went though an inch of wood. That mechanism may be entirely different than steaming but it does show that some effects can penetrate very quickly.

[PEARL DRUMS]

Maybe this will help further prove your theory Steve. When I bent those statics in my mini bow the back wrinkled on both ends and the belly never cracked or crinkled.

  Did yo use dry or steam heat?

Duhhh, steam. I knew I forgot something!

[Badger]

  Usually osage will cooperate pretty well with dry heat but I have had some that just would not bend, once they were steamed they bent like butter.

[PEARL DRUMS]

I have never even attempted a static bend with dry heat. I wont for a few reasons. I don't like my tips looking burnt and the amount of heat it takes is more than I care to apply. It works, I've seen it plenty. But I have my preference.

[Badger]

I have never even attempted a static bend with dry heat. I wont for a few reasons. I don't like my tips looking burnt and the amount of heat it takes is more than I care to apply. It works, I've seen it plenty. But I have my preference.

  I agree with you, I have screwed up more with dry heat than I care to admit to.

[bradsmith2010]

when I first starting making bows 30 years ago,, most used steam ,,
I think the dry heat does have its applications,, but I have better luck with steam for some things,, even if i do have to coat with glue to prevent checking ,, etc,,
so much depends on the relative humidity of wood and the air,,,but I really don't know of an instance where steam would not work,, but the dry heat bending has more limitations in my opinion,,

[PatM]

No question steam or even boiling is the way to go for tight bends. Any walking stick maker will tell you that. A bend is actually accomplished by the inside wood compressing not stretching of the outside wood.

[JW_Halverson]

Maybe this will help further prove your theory Steve. When I bent those statics in my mini bow the back wrinkled on both ends and the belly never cracked or crinkled.

  Did yo use dry or steam heat?

Duhhh, steam. I knew I forgot something!

Pretty bad when the drummer is a beat off!

 

[huisme]

Some drummer, I'm never off beat, ever

I've been under this impression ever since I read about exactly why we don't heat the back aside from I've been told so, it all makes sense and I do all my tight bends with steam now.

[RBLusthaus]

A bend is actually accomplished by the inside wood compressing not stretching of the outside wood.

Not to quibble, but I dont think the bend can happen without both the inside compressing and the outside stretching, at least a little. 

Like DC said, I always operated on the theory (I may be wrong) that the steam was just a good method to get the heat deep into the wood without burning the outside, since it can only be 212 tops.  The water part of the steam really plays no part in the bend except as a conduit for the heat to travel.  That is why green wood steam bends so well, since the liquid water in the wood cells carries the heat in deep, faster than I am able to accomplish with dry heat as I do not have the patience to sit with a heat gun long enough to not scorch.   

Russ

[blackhawk]

I just think its cool to steam a piece of wood and then bend it over near 90° like butta!!! It amazes my simple pea brain every time    and yes on the tight bends you can get compressed wrinkles on the back,and a stretched concave side on the belly which proves the belly side is under some tension and stretching...it also will "thin" it some due to it compressing and stretching as well....fun stuff!!!

[PatM]

A bend is actually accomplished by the inside wood compressing not stretching of the outside wood.

Not to quibble, but I dont think the bend can happen without both the inside compressing and the outside stretching, at least a little. 

Like DC said, I always operated on the theory (I may be wrong) that the steam was just a good method to get the heat deep into the wood without burning the outside, since it can only be 212 tops.  The water part of the steam really plays no part in the bend except as a conduit for the heat to travel.  That is why green wood steam bends so well, since the liquid water in the wood cells carries the heat in deep, faster than I am able to accomplish with dry heat as I do not have the patience to sit with a heat gun long enough to not scorch.   

Russ

   The stretching is negligible. The reason that a metal strap is often used is because the wood will break as soon as it  is forced to stretch beyond that minimal amount.
 Hoadley addresses this in his book.