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Heat Treating - Technical Info

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My supervisor's husband is a PHD polymer chemist who is beyond brilliant. I talked about the heat treating issue with him, and he said that it all boiled down to conversion of single bonds to double bonds at the molecular level. When we heat treat, we are driving off moisture and oils, caramelizing numerous organic compounds, and altering the molecules that make up the "wood". All of these chemical processes represent conversion of single molecular bonds into double bonds. Without knowing anything about bamboo, he told me that he would fully expect heat treatment to make the cane stiffer and harder. This does nothing to answer the flame vs. oven debate, or the "how much is too much" question. But it leads to some possibilities:

I think that flamed culms develop a higher internal temperature, and that heat regimen either increases the overall number of double bonds, or it creates a different "mix" of double bonds within the cane. That is, the double bonds that form at high temps are in different places and forming between different molecules compared to oven baked cane, which although it gets hot, does not get as hot as a flamed cane. However, if you bake it long enough  you  may  get  the  same   effect.   It   could   be   a degree-minutes sort of phenomenon in which high energy for a short time (flamed) could be the equivalent of low energy for a longer time (baked) because the heat energy applied to the cane becomes equivalent. This is could be quantified, but not with any of the tools in my little shop. I could also see how variation in individual culms could have a major effect. A culm with low moisture content to begin with might turn out very differently from one with high moisture content, especially when a rodmaker is doing a flamed vs. oven experiment. Water is involved in many organic reactions, and if water is present it could affect what sorts of double bonds appear, where, and when.

Looking over this, I realize I all I have done is raise more issues than answers. But it does explain how Bill Harms (look at cane quality!) Bob Nunley (different and even extreme heat treatments produce identical effects) and Tony (Aboriginal empiricists) can all be "right".  (Jeff Schaeffer)

    It seems to me that if heating bamboo causes a permanent change in the moisture content of the bamboo (and I think Milward has so demonstrated that the ambient moisture content is changed by heating) then there has been a chemical change at the molecular level.  The perception of that change as Bob N so clearly points out is just that.  a subjective perception.  Therefore  I guess all we can really be sure of is that there is a change and I like/don't like it, so I heat/don't heat to achieve what feels good to me.  Oh hell,   I thought it sounded profound.  By the way did you notice that I didn't use any commas?  (Ralph Moon)

    As regards the debate of whether or not a change occurs in the heat-treating of bamboo, I would suggest that perhaps these particular biochemists are not specific to wood  sciences, cellulose, nor to lignin in particular, a field all unto itself. To be sure, there is considerable debate amongst chemists in these fields, and there is much speculation as to lignin formation in the cellulose/hemicelluse matrix, but it is certainly generally recognized that indeed bamboo undergoes a irreversible chemical change when heat treated, and there is no reason not to suspect this based upon conventional understanding of lignin structures, much less the empirical observations. That bamboo and wood are not the same thing does not go unnoticed. Lignins, regardless of their source, are built from phenyl propane units linked together by bonding patterns whose frequency and occurrence are still imprecisely known, thus the debate amongst chemists and biophysicists. Most of the conclusions regarding the bonding patterns of native lignins have been derived from chemical degradation techniques, such as hydrogenolysis and thioacidolysis, and to my knowledge no one has ever done the required analysis on bamboo to absolutely state that X occurs for Y set of conditions. Even so, it is without doubt that a reaction occurs within bamboo that accounts for the increased resiliency and indeed the tensile strength modification as noted by Bob Nunley. Using classical molecular dynamics simulations, solvation structures, stabilities, and diffusion movement of lignin monomers can be seen. Radial distribution functions between lignin monomers and water show the existence of strong hydrogen-bonding between the hydroxyl bases of lignin monomers and water, and the methoxyl substituents of the different  monolignols  influence  the  strengths  of   the hydrogen-bonding and diffusion properties significantly. By removing the bound water during heat treating we are, in theory, leaving the hydroxyl radical groups to further bond, creating varying chemical reactivities of the different intermonomeric cross linkages in the lignin. Therefore, while we have not added chemicals nor compounds, we have removed them, and forever altered the chemical and physical state  of  the  bamboo.  (Martin-Darrell)


As I have listened to the conversations among those who have experience with heating treating issues, I began to think about what I teach with respect to polymers. Now this thinking for me is probably dangerous. First we know that bamboo is a composite that is composed of two primarily components - a long fiber and a holding matrix - these are generally (I think) the lignin (matrix) and the cellulose (long fiber). We know the chemical composition of these structures are composed of carbon, hydrogen, and oxygen atoms in various long molecular chains with different bonding arrangements. These are also the basic elements of all plastics (sorry for the swear word). Now in plastics we know there are two different polymer types - thermoplastics and thermosets. The thermoplastics are ones that can be heated, molded, shaped, and remolded. They do not change their molecular bonding structure with processing (say heat treating). Now thermosets are plastics that when heated, molded, and shaped become molecularly rigid or fixed - that is they do not change after processing (say heat treating). As an aside, graphite rods are composed of graphite fibers (cooked carbon) and an epoxy matrix that is a thermoset that holds the carbon fibers together in a rigid bond. Once a carbon rod is "cooked", it has a rigid structure.

So what do we have. We might have a bamboo composite that may (and remember folks I am speculating based on what I know about plastics not bamboo) have dual properties. If the cellulose is the fiber that might be the thermoset that heat treating sets, and the lignin is the matrix that is a thermoplastic that changes with various heating regimes, then maybe all of you are correct including Milward. I do have the Milward book and questioned the same issue that Bob wondered about. Please remember that I am just speculating about how the bamboo material behaves when heat treated. As I said, thinking sometimes gets me into trouble, but it sometimes is right.

Now, consider the fact that we can heat straighten a rod that has taken a set. What are we really doing? I think we are just readjusting the fixed fibers in the matrix - that is sliding the fibers within the thermoplastic matrix of the bamboo. That makes me think that bamboo after heat treating maybe has two different molecular structures - one in the fiber and one in the matrix. Broken bamboo also gives one the sense that this may be true when looking at a fracture with the stringy fibers in a matrix structure.

I have no chemistry knowledge of the specific character of cellulose and lignin - thermoplastic or thermoset - , so I may be all "WET".  But on the other hand, this physical material thinking may explain what we see in heat treated bamboo.  (Frank Paul)

    The bamboo fibers move alongside each other during casting and I think the glue slips also, I think this because I found

    intermediates will stiffen a rod and my feeling of the reason for this is they more or less clamp the section reducing the ability for the glue and fibers to slip. When heat straightening I think the glue becomes malleable and that allows for the straightening as much as the fibers also  becoming more flexible, just like steam bending wood.  (Tony Young)

    I agree with your thinking about there being two kinds of material present. My own mental picture (with absolutely no scientific evidence) is that the power fibers behave like rebar (reinforcing steel for concrete) bundled together.  The bundle is held together by either chemical or structural elements that are imbedded in a matrix that keeps the the fibers from slipping.  The elasticity of the fibers allows the bundle to bend within limits.  When heat or moisture softens the matrix or if the internal shear allows the fibers to slip, then the bundle deforms permanently.

    Heat-treating modifies the matrix without affecting the fibers substantially.  I cannot guess what the nature of this modification might be -- perhaps the elimination of water, perhaps something else.  If you over treat, as Milward warns, you modify the fibers to the detriment of their tensile strength.  (Jim Utzerath)


I have been pondering your method of heat treating (tempering) for a while now and would like to pose a couple of questions.

I wish we knew at what temperature the cane really becomes tempered  (plasticizing the lignins and realignment of the fibers). Maybe  you know?

A few of the rodmakers, current and past, have browntoned their rods  in an oven while the culm is whole. Assuming some tempering. Similar  to your internal process, but different.

If the cane is really being tempered by such processes, doesn't that  set the fibers to the way they are in the culm? Not straight.

After such a process it seems any further attempt to straighten and  press the nodes in strip form would be detrimental to the cane. That  is unless the cane can be re-tempered any number of times without  harm. Who Knows?

Regarding soaking and straightening:

With as much heat and force as it takes to straighten a node, I would  doubt if air drying would have any effect on the nodes, but it would  be an interesting experiment.   (Jerry Foster)

    I'll promise everyone that I don't really know so actually I am posing a question for my own edification and Jerry,  I certainly would not want to challenge you but you raise a question that probably has been answered many times but as usual... I'm way behind on most things... so....

    When we talk about heat treating, it would seem to me and my feeble mind that heat may indeed plasticize or physically alter the lignins, assuming that these lignins are intra or extracellular fluids or fluids that contain structures that are linearly joined at a cellular level.  What I really wonder about is; can heat alter the position of structures that are macrocellular meaning the power fibers?  We can see what we know as power fibers with our naked eye but can we really change the position of them, say in the nodes, with heat or even heat and pressure?  I would suppose so but I wonder if anyone has done any microscopic studied of positions of fibers before and after heat and further, after heat and pressure?

    I hope no one accepts this as a challenge.  (Jerry Partrick)

      Yes, the structural change has been documented under a powerful microscope by a German rodmaker in a scientific paper by Wolfram Schott, Bamboo Under the Microscope. It's a large Adobe file, due to photos, that is available as one of the extras on the Power Fibers CD. I don't recall where there is an online link. No financial interest.   (Paul Franklyn)

        The document, "Bamboo Under the Microscope" is available on the Power Fibers site.  Just go to the Downloads section and you should find it.  It's even FREE!  (Todd Talsma)

      All I can say is - It makes one heck of a lumpy rod if you don't do something to flatten nodes before final planing.  (Darryl Hayashida)

    Every time this subject comes up I  a bunch of theories have been debated endlessly. Here is an idea from my friend, who is a senior organic chemist at BASF.

    When you heat treat bamboo, you are making double bonds from single bonds at the molecular level. Once the double bonds are created, they will not go away, which is why you can soak strips and they still feel snappy after they dry. If you apply enough heat, you create so many double bonds that the cane becomes brittle.

    He finishes his windowsills with tung oil.  (Jeff Schaeffer)

      A very good explanation, one I like myself, and yes we do go over these things again and again, but we also have new people joining the list, old members dropping off, old members rejoining, etc., but it sure is neat to see someone finally complete their first rod and be all excited about it.  (Darryl Hayashida)

    I straighten strips while they are wet by heating with a heat gun and pressing in a vise. Steaming the nodes essentially. I was just wondering if they were bound in a M-D fixture while they took up the water, then dried if they would straighten out. Bamboo is very pliant when it is wet.  (Darryl Hayashida)

      The strips would have to be rough planed to bind into the fixtures. That seems to be a bit of extra work and you would rough plane before pressing the nodes. I use the fixtures and they do not flatten nodes or remove kinks, but as we know, they will straighten the sweeps. Also, the wet strips will shrink before they are completely dry and the binding will probably get too loose to remove kinks.  (Steve Weiss)

        I thought there was someone, maybe Robert Kope, that had used a type of thread that actually shrunk with the strips during the heat treating/drying process.  I can't remember what type of thread it is, but I seem to remember it.  Anybody else remember it?  (Todd Talsma)

          Polyester.  (Wayne Kifer)

          There is heat shrink tape used by graphite rod manufacturers.  (Darryl Hayashida)

            No, this was thread.  Some type of nylon maybe?  (Todd Talsma)

              I've got a cone spool of nylon thread that I always use for gluing, as it is stretchy, and maintains a high clamp  pressure. I have been too scared to use it during heat treating, as I am sure it would melt. I suggest you would get away with using nylon at moderate 'drying' temperatures, but I would change to pure cotton for heat treating.  (David Kennedy)

                Depending on which web site you look at, nylon melts at 450 degrees F. to 495 degrees F. but it begins to lose strength rather rapidly at about 300 degrees F., so you're most likely right that it wouldn't be good for heat treating unless your regimen is more time at a lower temperature.  (Neil Savage)

        This is how I would envision my process would be if I had the MD fixtures.

        Inside flaming, split into strips,  soak in water for a couple days.

        Rough plane wet strips, bind into MD fixtures enamel side in, back into the water.

        Hopefully after a day or so in the MD fixture and water (hot water?) the roughed strips are straight, nodes flattened also. I might have to use an automotive hose clamp around the nodes to get them pressed in and flat. Maybe a couple inch flat metal bar on top of the node between the hose clamp and strip to press it flat as it is soaking.

        This would eliminate heating and pressing, not that big of a deal, but really straight strips does make final planing a lot easier, so if I can flatten nodes and remove kinks at the same time, so much the better.  (Darryl Hayashida)

          Here's a thought I've had for several months.  Just haven't gotten around to doing anything with it.  All still sitting in my head.

          Split the strips (flame the inside first if you like, something I'm going to try on a long one piece  - longer than my oven).  Cut to length and remove the nodal ridge however you like.

          Then, put the strips in a 1.5-2" PVC pipe hooked up to a steam generator.  Similar to something you'd use to steam a long piece of wood.  Lee Valley sells something or I think you could make one from a wall paper steamer.  Leave them in the steamer for a length of time (30 minutes?).  Or you could put in just a couple at a time.

          Then I'd build a 5' version of Robert Kope's node press (the one on the tips site).  Maybe make the clamps adjustable for the node locations.  Pull a strip out of the steamer.  Quickly put it in the press, and press the whole strip (pressing both ways at the same time).  Leave it in the press for 5 minutes or so to cool.  Take it out and put a new strip in the press fresh from the steamer.  While that one is cooling rough the first strip.  Then repeat the process.  (Aaron Gaffney)

            I recall someone making a steam contraption that clipped over a node so that he could steam and press nodes without having to steam or soak the entire strip. I also recall someone just holding a node in front of the spout of a boiling tea kettle to steam nodes.   (Darryl Hayashida)

              Before starting to soak I used combination (simultaneous) of boiling tea kettle AND heat gun to nodes. It was quite fast and eliminated totally charring of the node. I suggested it on List some years ago. Now I am soaking - and happy!  (Tapani Salmi)

                I tried using a steam iron a little while ago to treat the nodes, I've since gone back to using the gas torch. The iron was OK and I wouldn't dismiss it as an option but it wasn't for me. It definitely took longer to get the node up to temp. However I'm fairly sure that water transmits heat better than air, so I guess that those who soak the strips then heat the nodes are kind Of using steaming on them.  (Luke Bannister)

          Okay, time to fess up to a failure.

          Regarding steaming and pressing.  I took a different approach after reading some posts a few months back.  I had an old wallpaper steamer that I attached to a 1/2" copper pipe 6' long.  Put a Tee on the other end.  Fitted a cork into the straight through end of the Tee, left the 90 degree open to let steam/water drain.  Turned the steamer on and in about 20 minutes had steam going.

          Uncorked it and placed a strip in and recorked.  Left it in for 15 minutes and then took it out.  Very hot and limp.

          Previously I had fabricated a 6' long press.  Made from a couple of aluminum rectangles bolted together to form a 90 degree angle.  2 pieces of aluminum 6' long by 1 1/2", one 1/8" and one 3/16" thick.  These had slots milled to allow each of them to be bolted to one of the 90 degree faces and then be slide/adjusted in/down.  Installed radiused levers to apply pressure on each sliding plates.

          Placed the steamed strip immediately into the press as soon as I took it out of the steamer and clamped with as much pressure as I could.  Actually deformed part of the rectangles where there was too much spacing between bolts.  Should have welded those but I don't know how to weld aluminum. Left them in there for an hour.

          Net result was that neither were the nodes flattened (I had sanded slight moons behind them to allow them to be displaced) nor were any of the bends or sweeps removed.  I have a lot of aluminum scrap for other jigs now.  (Ralph Tuttle)

            Tim Abbott now sells a steam manifold that can hold a number of  strips. There was a write-up in Power Fibers 24.  (Jerry Foster)

            I can stop thinking about my idea.  You've possibly saved me time and materials at a point in the future.  Now I can free up room to think about something else that probably won't work LOL.  I'm working on a fly tying chest that's not turning out like anything I had envisioned.  Seems to go that way sometimes.  (Aaron Gaffney)


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