Mechanics of steam vs dry heat bending

[Danzn Bar]

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

[H Rhodes]

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.

[DC]

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.

[stuckinthemud]

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

[BowEd]

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.

[PlanB]

Some useful information here

www.fpl.fs.fed.us/documnts/usda/ah125.pdf

[stuckinthemud]

Thanks PlanB, really interesting read.

[BowEd]

That pretty much tells it all Plan B.Useful info to say the least.

[Badger]

  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.

[Badger]

  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.

[PatM]

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.

[Badger]

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.

[willie]

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

[Badger]

   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.

[PEARL DRUMS]

Its always about which is better Steve

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