Heat treating demystified (Somewhat)

[lostarrow]

I know some guys will probably jump all over this one but here it goes............ When you "heat treat a piece of wood ,what you are doing is removing the moisture from the cells. The more you do it , the less moisture. Wood is made mostly of water while growing  ,and when we cut it ,it starts losing that water immediately. When they talk about a piece of wood reaching equilibrium they are talking about it losing or gaining moisture ,relative to the ambient humidity present in the air.(RH-  relative humidity) When a piece of hardwood is brought down to say 10-12% MC(moisture content) the cells have shrunk significantly already. They will never again be able to contain as much water as when fresh cut.Even if submerged in water.If that piece is brought out of the kiln in mid summer when the RH is 80-90% it will absorb some moisture and maybe creep up to 18% or so depending on species.After the winter when the air is drier and the board has come down to 6-8% in some areas ,in the spring and summer of the following year,it will not get back up to the 18% it did the following year.And so it goes through time until the wood has lost more and more ability to hold the moisture that once made it flexible.Old and brittle. Some woods fight this better than others . When we heat treat we are artificially collapsing the cells(shrinking them ) making the wood more compact but in turn ,less able to absorb moisture. The wood turning brown from heat is the sugars in it caramelizing,just like onions in a fryingpan or sugar heated with a torch on Creme Brulé. MMMMMMMMMMM yummy! the more sugar ,the browner it gets. When the wood reaches a point where it no longer contains enoughwater to keep the temperature below the woods flash point, it cumbusts.You don't need a spark or flame to make fire, you just need enough heat. If you leave the heat gun on the same area too long you shrink the cells to the point where they can no longer bind to each other .Too much of a good thing.If you can heat a piece quick enough,you can harden the outside without collapsing the cells all the way through to the back.If its really thin and the wood doesn't contain much sugar, you may wind up drying the whole piece through before you see any toasting happen. the cells on the back have then collapsed and are no longer able to regain enough moisture to become elastic again.You now have a dry brittle piece of wood. Try this for an experiment,it might give you a better idea ,visually of what is going on. Take a piece of wood 3/4"x3/4"x24" long. Straight grain(ish) Square and straight.Heat one side only , in the very center, for about 6" or so .Heat it until it becomes light brown on that side only.Maple would work good for this.The two ends on the side that is heated will draw into reflex because the cells have shrunk.The piece of wood will now be narrower in the middle as well . The side closest to the heat will be  narrower than the side farthestThe piece may also twist ,depending on the grain.The cells closest to the outside of the tree will be larger and therefor shrink more. On a larger sample you can shrink one side until it cracks the opposite . This is what happens to your staves when they dry and split. The larger cells shrink more than smaller ones near the center of the tree or around knots.Back to the experiment......... Measure the size at the middle and both ends. Now throw the piece into some water for a few days. Extreme ,I know , but leave it for the amount of time that you would leave a bow after heat treating. Take it out when you are satisfied that it can gain no more moisture . Dry it off with a towel and stand or hang  it somewhere to dry so that it dries even all around. When it no longer appears wet on the outside, measure your spots again. The ends should be larger than 3/4" the toasted side is probably the same and the side opposite might be marginally larger but still smaller than the3/4" you started out with. The cells will never be the same ,even on the side you didn't think you heated. You've changed the specific gravity of the wood in essence, as the same amount of cells are still there, they just occupy a smaller space and are more compact.They are now more resistant to compression ,but also less resistant to tension.Some guys use oil when they are doing heat correction ,as the oil has a higher evap. temp and keeps the cells from collapsing while bringing the wood to a temp.high enough to bend.Just make sure it's compatible with your finish.Contemporary accounts of native bowyers of the 19th century said they applied grease or oil from animals to the back of the bow to keep it flexible while they were making it and throughout it's working life .Once again ,if you wanted to sinew back it ,this would not be advisable. Hope this helps. Sorry if it was long winded,but there is allot of info for one small aspect of wood.
Cheers, Dave.
 This was taken from another post ,I just didn't want to type it all out again.Thought this info might help demistify heat treating

[TBod]

It makes sense, thanks for posting!

[PatM]

Not really. No amount of drying duplicates heat treating. It changes cell structure in ways other than just shrinking. I'm not really sure why people are quite so obsessed with explaining how it works rather than just accepting that it does work.

[jimmy]

So what is the verdict here?  It almost sounds like heat treating would compromise a bow's longevity.  It's always worked for me, but my bows have not been around all that long.  Does a well treated bow last just as long since it supposedly can't gain good moisture?

[Pat B]

Marc St Louis wrote a chapter in TBBIV on just this subject. Marc did years of studying and lots of experimentation before he wrote the chapter in TBIV. Read that and you will understand the process and the results much better.
  One thing that Marc does is substitute the moisture in the wood cells with a mixture of pitch and turpentine, while it is still hot to seal and protect the cells that have been scorched in the heat treatment.
  Heat treating does probably shorten the life of the bow in the long run but it also increases the compression strength of less strong woods(mostly whitewoods) which can increase the life of a bow. There are trade offs in every aspect of wood bow building. It is up to you to decide which trade off is best suited for you as a wood bow builder.

[BowEd]

Well put fellas.To conclude my end of thinking here.Wooden bow building to me is a process of problem solving a lot of times.You can't fix nothing without tools.Heat treating is a very valuable tool in the bag a bow maker has to get the best out of a piece of wood.Without it you would'nt see a lot of the beautiful bows shown on this forum.I think Marc St. Louis has investigated the process and the durability of it to the extent noone else has done.He stated many times how tension strong wood is best suited for heat treating.So it can keep in step with the bellys increased compression strength it gets from heat treating.If a bow fails for me because I heat treated it too many times I blame myself.

[druid]

Great article lostarrow, thank you very much for posting.
Did you collected these datas or you copied it from somewhere? If it is yours, are those the things that you tried personally or from some other documents collected?

[lostarrow]

Hey guys, I,m not so sure why some are getting defensive on the subject.This is presented as information to aid you should you decide to heat treat.No where in this post do I contradict the findings of Marc St.Louis ,but on the contrary support his findings with a little background info on wood cells . Knowing how something works will save you much time and aggravation. I Just reread the chapter that Marc wrote in TBB ,based on Pat's reply to see If I had somehow missed something, and as I recalled he simply stated the results of his experiments. No B.S. , no wild theories , just the facts. That's what earns Marc the respect he has achieved over the years. Thanks Marc ,for taking legend and making it fact! If it wasn't for Marc we wouldn't have this tool in our bag of tricks. It may have been lost for eons until someone stumbled onto it again.

Bead man ,the point you made just once again supports the info here and as presented by Marc.

Jimmy, heat treating shouldn't shorten the life of the bow if done correctly. It will end it immediately if done wrong. That's why I posted this, to hopefully save someone the heartache of breaking what could have been a good bow.

Pat M , I try to present things in a way that is easy to understand, so I will not be going into the complex world of lignins,cellulose,antibiotics, minerals and the effects of heat thereon. Yes ,heat affects all of these ,but as they are in unknown quantities for  any given species in various regions and growing conditions ,we shall overlook them . The more important point I was trying to make is that once those cells have shrunk ,they hold less water. Water is the main ingredient in flexibility(for our purposes).Desirable to some extent on the back, but can be detrimental on the belly.(increased set and sluggish cast).

Druid, I present only the info that I have gained from over twenty five years of cabinet and furnituremaking. Some from college ,much more from hands on experience.Over the years ,I have learned from many skilled artisans and learned more from people that decided to throw centuries of knowledge to the wind to increase production, only to have it all bite them in the butt when things started to fall appart. I have studied traditional ways of craftsmanship in many fields and found that much info can be transferred from one trade to another. Timber Frame construction to Furniture,Boat building to timberframing, Stained glass to Cabinetmaking, and so on.Some info comes from High school science class,(teacher  didn't like that I asked so many questions,but I needed to know why,or the info didn't stick)and some I can't recall where I learned it , because I use it every day.Like trying to remember how you learned to walk or talk.Bow building is just my latest obsession.The above is what I believe happened to that piece of "tree of heaven" . too much heat for a tension weak wood.Low sugar content meant much more heat to "toast " it . by the time that happened,it was over cooked, and cause a tension failure.

Please , if anyone finds anything I post to be in error, feel free to quote the statement and present your argument for the contrary. I'm only human and am not offended by being proven wrong. Humbled perhaps, but not offended. When we stop learning , we die.

[SLIMBOB]

  I for one enjoyed reading your post.  I, like you and many others enjoy the pursuit of knowledge for it's own sake.  It's how I roll.  I see the same quality in my 7 year old boy.  I need to know the whys and hows of a particular problem in order to fully understand it.  I at least need to be able to question the whys and hows.  Not taking sides here so much as acknowledging that, at least for me, your post was informative and interesting.  Not sure about your facts (not that I question them, I just don't know their validity yet) but a worthwhile read.  Thanks.

[Marc St Louis]

There was quite a bit of controversy on this subject when I first started to post my findings and moisture loss was at the top of the list as an explanation.  One person in particular, I believe his name is Lennie, did some testing at the time using fairly precise instruments to measure the moisture loss from heat-treating, he measured the wood before heat-treating and after.  He found that the actual moisture loss was less than 2%, not a very large amount.  It was concluded at the time that moisture loss was NOT an explanation for the increased compression resistance from heat-treating.

All of these old posts were on the old PA message board and I wish we could have access to it.  There is a huge volume of interesting posts and information sitting out there somewhere

[H Rhodes]

On the same subject of moisture loss and heat treating....  I heat treated a couple of bows that I built last spring.  They turned out to be good shooting bows, one that weighed in the low forty lb. draw weight, the other right on fifty.  I toasted the belly on both of them.  I revisited these bows after they had been hanging on a hook in the air conditioned interior of my house for about six months....  Both of them had gained about ten lbs of draw weight!  I reasoned that the wood was dried outside in my shop.  Not heated or air conditioned - higher humidity shop never allowed the wood to reach optimum moisture content.  These bows did reach their proper moisture content inside the house and gained draw weight - well AFTER the heating they received during construction.....  That made me realize a couple of things....  1.  I need to dry my wood inside before I attempt to make a bow out of it - that and check to make sure it is completely dry; and 2. normal heat treating doesn't remove that much moisture from a bow.  There is a blundering budding bowyer's take on it....   

[lostarrow]

Thanks for the comments Marc. I am not surprised with  what your friend found for moisture readings.As you wrote in your findings you were using well cured hardwoods for specimens which probably had a reading around 9% or so If it were in the late fall or  winter where you are . What I am saying is that when you shrink the Physical size of the vesel , you can't get the same amount of water in as you could before. The vesel in question isn't full of water when you heat treat it .It merely contains some water. From 9% down to 7% would make a good bit of difference in a piece of elm or hickory, probably just right. If you continued down to say 6% or 5% that would likely cause trouble.  If your heat treating continues too long you may be drying too far into the back working portion of the bow, causing a very dense belly and a less elastic back. Which from your own research once again it looks like you found from the samples that didn't make it. Tension failures on the back  and cracks instead of crysals on the belly, on the samples that were Carbonized(charred ) instead of Caramelized( lightly toasted ). I like the idea you had with the turpentine /varnish being applied while hot . I would expect this finish to penetrate deeper into the wood to add more long term protection. As a side note ,I was in your neck of the woods last weekend.(Pembrooke)Beautiful trip , lots of HHB.Some day when I get up that way I may even get a chance to look you up.

[lostarrow]

So I just read through the post again because I was confused about the comments about heat treating not drying out wood very much.I 'm not sure we are on the same page as far as how little a difference a couple of % points of moisture will make in a piece of wood.Here is an example. A Hardwood floor for example,if laid in the summer with a moisture content of 12% has a  nice tight fit .That winter  the temp drops down to -20deg Celsius with a relative humidity of around 15%.With no artificial means of moisture(humidifier) ,the content in the wood now drops to a drastic 8%.The floor  boards now have a gap of 1/8" -3/16" between each one.The next summer sees relative humidity as high as 90%, but the boards will be only scarcely larger than they were in the winter.They will never again return to their size they were when laid. The transverse would be  fitting a flush drawer in a cabinet. The cabinet made in the winter must take into account the up coming summer swell of wood.if you leave 1/16" all of the way around , it will likely be jammed tight in the summer .If you had years to wait ,it would settle in and stabilize to an acceptable rate of swelling and contracting ,diminishing over the years to where it just can't absorb hardly any moisture at all. This all varies greatly with species and even within the species based on growing conditions and orientation of grain etc. In short , 6% MC in a bow will likely explode,while 8-9% would be ideal,12-13% would likely be sluggish. Again ,just examples. That's why people would prefer a board that is air dried for 7 years over one that is kiln dried 6 months ago. It has gone through several cycles of high and low moisture and is more stable, more predictable.

[TBod]

Well spoken again!

[PatM]

lostarrow, The shrinkage and decreased moisture uptake of the heat treated wood still doesn't explain things. A heat treated bow can at least be brought back to an intolerably high mc for a regular bow and still perform better. You may not be able to turn a raisin back into a grape but a moist heat treated bow performs better than a bone dry regular bow.
 I'm not sure if anyone has dried a stave for 7 years in forced reflex and compared it to a bow heat treated into reflex though.