Primitive Archer
Main Discussion Area => Bows => Topic started by: Badger on January 13, 2016, 03:15:22 pm
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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?
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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.
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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?
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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.
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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!
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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.
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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.
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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.
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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,,
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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.
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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!
>:D
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Some drummer, I'm never off beat, ever O:)
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.
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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
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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 ;D 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!!!
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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.
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I've always thought that the steam or water/moisture helped as a lubricant to allow the wood fibers to slide after being released due to the temperature... ;)
DBar
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I prefer steam for any major bending operations, but I am not a science guy - I don't know why it works. For some reason I am under the impression that dry heat or steam heat deplete moisture content. It is counterintuitive that steaming a piece of wood removes moisture, but I have always been cautioned to wait a few days before stressing the wood to give the limb "time to rehydrate". Whatever is going on a cellular level is beyond my pay grade, I am sure. What Bill just posted about "lubrication" sort of makes sense to me. I know that I have had lots more success with steam.
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I wonder if the steam drying comes from steaming green wood. If you steam a piece of green wood the heat will push any water in the wood out the end. Quite a bit comes out, well, you've all seen wet wood in a fire. So steaming wet wood will "dry" wet wood somewhat.
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In the dim and distant past, I think I read that the process works when the water already present in the cells (intra-cellular?) heats enough to soften the lignin in the cell wall enclosing it, when the lignin cools down it keeps its new shape and the cellular water , ideally, remains in its cell. Therefore, the heat to allow this process (heating the cellular water) can come in any form, hot rocks (still used in ship-yards), toasting over a fire, or even microwave. The water between the cells (inter-celleular?) has relatively little to do with the process. However, any heating will dry out the timber, causing stress and damage if the timber dries out excessively. To avoid this, some sources recommend pre-soaking but others point out that swelling of the timber, if this is excessive, can also be damaging. I think that the reason steam is better than dry is because the drying happens after the bending process finishes, whereas dry heat stresses the timber while the bending happens. If you can spread out the stresses instead of hitting the timber with them all at the same time then it is less likely to fail??
Just to throw something else into the mix - I boil timber for moderate bends, and then add steam for more severe ones and I always keep the surface of the timber wet as long as I can when dry heating.....
ps, it really is a very long time since I read up on this and I don't mnd being corrected on any of the above comments :)
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I think the reason why steamed green wood drys quicker afterwards is because the steaming removes the sap and replaces it to a sort with just water moisture which in turn evaporates quicker than sap.Dry bending tightly is futile for me but steam will do it.I do dry heat temper my tight bends after steam bending though.I think it holds the bend better then.
Like said it is eye popping though to see the back wrinkle on a tight bend.This is sort of off track but I've heard but never tried to dry wood in deep water.I guess the pressure of the water forces out the sap and replaces it with water.
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Some useful information here
www.fpl.fs.fed.us/documnts/usda/ah125.pdf
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Thanks PlanB, really interesting read.
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That pretty much tells it all Plan B.Useful info to say the least.
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The conversations is not so much "which is better?" we have all known for a long time that steeper bends can be made with steam. It is more about why steam is better.
Heat dry or wet will plactisize the wood but dry heat enhances the properties of the wood in the opposite fashion we need, while wet heat enhances the properties in our favor. Wood getting stronger in tension when wet is not theory, wood getting weaker in compression when wet is not theory. Conversely we also know that dry wood is stronger in compression and weaker in tension. Thats not theory. The difference in the two methods is so substanial I think this has to be the case, I can't think of any other reasons.
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I just did a test on some linen string which is also cellulose. I tested wet and dry. Average breaking strength dry was 20# average breaking strength wet was 33#. There is a huge advanatge to keeping the tension side wet.
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Steve, don't however discount that hot wood is weaker in compression. You could probably have good results bending a sharp curve by using dry heat on the inside of the curve and keeping the outside of the curve wet but that's an awkward scenario to achieve.
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Steve, don't however discount that hot wood is weaker in compression. You could probably have good results bending a sharp curve by using dry heat on the inside of the curve and keeping the outside of the curve wet but that's an awkward scenario to achieve.
I think steam is ideal, not looking for a better way, I was just looking at why it is so much better. Kind of a useless study on why something happens when knowing that it works is really good enough. It also helps to explain why dry bends will often lift splinters.
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thanks for the link , PlanB
some of the shipwrights have found ways to adapt modern materiels to old skills, and
https://www.youtube.com/watch?v=--iPQIwSEJM
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I think as is usually the case most everyone missed the entire point of the thread. Not looking for solutions, not looking for better ways, not looking for anything just s simple discussion on the ways that steam favorably affects the bending characteristics of wood by using moisture to strengthen it in tension and weaken it in compression while using heat to plasticize it. Everything about steam compliments the bend while everything about dry heat does the opposite even though it still works a lot of the time.
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Its always about which is better Steve :)
I got your point, hopefully my tid bit helped in some way.
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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.
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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?
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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
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I see your point Steve.It is nice to know why something works better the way it does.
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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!
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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.
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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. :D
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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. :D
You got a hot date with a beaver? Wood noodles in sap sauce? You serve red or white wine with this?
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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
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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.
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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.
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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.
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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.