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Heat Treating - Moisture Re-Entry

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The West Virginia Division of Forestry has a wood drying kiln at one of their offices.  Apparently they have quit using it due to the fact that the wood returns to equilibrium within a matter of hours.  It comes out at 6% moisture content then quickly returns to 16%.  The kiln is basically useless.  (Lee Orr)

The research I think that  M-D did a few years ago or maybe it was someone else says the same thing. Soak Bamboo and in 26 hours later it returns to the original air moisture.  But there is a difference in the texture of the bamboo after heat setting dry or soaked.  I heat set and then soak. After heat setting the strips will be straighter and stiffer, it stays on my MHM much better.  Heat setting, in my opinion is not as much a moisture debate, I don't see much to debate there the cane will return to the air moisture, but a tempering debate.  I believe heating will temper the cane, make it stiffer and keep it from taking a set as easily.  But others have other opinions, to each his own. I have learned the hard way and from Tom Morgan do not try and plane the cane after heat setting especially on a MHM.  It is too dry and brittle and does not plane well.  Tom sets his aside for a few weeks, I throw mine in water.  (David Ray)

You just said exactly what I was thinking!  The feel of cane (while cutting) is so much different before and after the heat treating.  I was following the "tempering" train of thought.  (Scott Wolfe)

As I understand it, there are 2 kinds of moisture in wood, some is inside the cells, and some is between the cells.  Kiln drying is supposed to remove both kinds.  The moisture that is regained is only the part between the cells.  Kiln drying also kills the insects that may infest the lumber.  (Neil Savage)

A few years back, Rick Crenshaw was kind enough to ask his wife to detail the effects of heat treating for us. the result was a veritable treatise. I know it is in the archives. It did prompt a rather nasty bout of notes for a week or so as list members debated what she had said. I don't recall her exact credentials, so I won't mention them, but if anyone turns up that post, Rick did specify a rather extensive and impressive list of qualifications.  (Larry Blan)

Subject: Tempering: was HT Ovens

Guys (and gals),

I have often wanted to jump in to the discussions on tempering cane and the physical and chemical changes which may or may not occur with heat treating.  I'm still not sure that I have much to offer but let me tell you what I do know.

My better half is a chemical engineer and has been employed for the last 23 years with a company that processes raw plant fibers into specialized cellulose materials for industrial, food and paper manufacturer's.  Her company's cellulose interests are with cotton fibers, softwood pulps, and hardwood pulps.  Bamboo is a plant which has a very high percentage of cellulose compared to many plants.  As you probably already know, the woody parts of plants is composed almost entirely of two materials: 1) cellulose, the relatively long chained fibers and 2) lignin, which acts much like a glue holding the cellulose fibers together.  Buckeye's business is primarily geared to chemically and physically cooking the wood pulps in order to break the lignin-cellulose bonds in the wood chips to refine cellulose for other products.

Heating wood chips alters the properties of the lignin and causes some cross-linking' of the cellulose fibers.  I am convinced that heating bamboo does more than drive out moisture.  We are most likely altering the bonds of the cellulose chains themselves.  Just the right amount of heat and we increase the cross-linking between cellulose chains.  Too much and we begin to break the bonds and possible destroy the integrity of the chains themselves.

Don't ask me the magic temperature and time.  I've asked my wife and she starts throwing in so many factors and lignin types and cellulose lengths that I gave up long ago of getting any kind of real answer from her.  After all, her job has been 23 years of full time work to find the magic answers for her own cellulose problems.

What we are left with is to find the answers on our own.  I don't have the time, but the tempering study done by Lloyd Cross.  I think Mr. Cross leaves out many of the chemical changes caused by heat treating and focuses on the carbonizing effect which gives us the change in coloration.  I am convinced there is much more going on such as changes in chemical bonds between actual cellulose fiber chains.  We may never know. Who would fund such a study?

My position has been to believe that some amount of temperature increases the bonds between cellulose fibers and therefore the rigidity of the bamboo and the resistance to sets, but that too much breaks the bonds down.  Trial and error.  I heat treat and hope it is not too much.

A final word about Ammonia Browntoning.  You know that many 'plastics' such as screwdriver handles are actually cellulose.  Yes, if you take pure cellulose, dissolve it in Sulfuric solutions, then regenerate the fibers through another

chemical bath, you can get cellulose fibers which are cross linked in all sorts of directions and form a solid 'plastic'.  Think of rayon, cellophane, sausage casings, etc.  All cellulose regenerated solids.  Ammonia Browntoning may cause some of this and crosslink fibers as well.

Something to think about.  (Rick Crenshaw)

Yes, Neil is correct.   As it is explained, freshly cut wood (and bamboo) contain what is called "bound water" as well as "free water."  The former is contained within the lignin cell structures, while the latter is distributed all around those cells.  Over a very lengthy period of time, all water will be transferred to the atmosphere, leaving the wood "cured."  This is a relative term, however, as the moisture content of wood is constantly attempting to equalized with the ambient humidity.

The term, "cured," refers to the transformation that takes place relative to the cells that once contained "bound water.  With our heat treating,  we greatly accelerate (exaggerate) the curing process, and the "bound water" that's driven from the so-called power fibers of bamboo leaves the resins within those cells hardened and largely impervious to subsequent moisture absorption.

The "free water" between the cells and in the pithy parts, however, while also initially driven out, is certain to return (though in greatly reduced measure), as these parts of the structure will quickly seek a state of equilibrium with the ambient humidity.  Nothing can prevent this latter process from happening (which is why protecting your cane with desiccants, while possible, is an irrelevant effort).

So, heat treating remains a very important step in our building process, PROVIDED that one knows how NOT to overdo it, cooking the fibers past their optimal strength.  (Bill Harms)

On page 16 of "Fact Fiction etc" Milward says, referring to heat treated dry strips:

"Of more interest is what happened afterwards. Six of the strips were immersed in water for 2 days, retested, then left in the water for two weeks. They were retested and no change in deflection was noted from just after heat-treatment.

As already mentioned, heat treated bamboo will still soak up water, expand and gain weight, but this process will not rehydrate the molecules. The benefits of heat-treating remain. The bamboo can subsequently be dried out and used as normal."  (Henry Mitchell)

I also noted this point in particular when reading his book. If it is true I wonder whether it is possible to heat treat first, then soak the strips and plane wet strips to final taper (or at least just over final taper to allow for swelling). Has anyone successfully done this?  (Stephen Dugmore)

A friend of mine is a polymer chemist. A very good polymer chemist. I was talking to him about bamboo heat treating, and he says that it is really very simple: when you heat treat, you are creating double bonds from single bonds in a variety of compounds within the cane. This is what stiffens it. Water would not undo the double bonding process. It of course affects the cane, but once redried you still have a strip dominated by double bonds from the heat treating process. I believe him.  (Jeff Schaeffer)

My wife is a chemical engineer in the cellulose industry (wood pulp, cotton, etc.)  The lignin in woody plants and grasses is the 'glue' that holds the cellulose molecules together.  The lignin is a polymer.  Heat treating creates dehydrates the lignin (drives off hydrogen and oxygen molecules) causing the weaker bonds to reform to these double bonds.  Heat treating essentially 'plasticizes' the lignin and it is for all practical purposes irreversible.  So... you are driving out moisture, but you are also creating new stronger bonds with the heat energy as well.  Simply drying cane will achieve this to a lesser degree, but heat treating is not the same as just desiccating or drying out the cane.

So... I agree with Jeff's friend.  My wife will be attending the CRR this year with me, if anyone wants to speak with her about this.  She is no expert on cane, but she knows wood pulp chemistry fairly well and bamboo is cellulose and lignin material so there's no reason it should be an exception.  Only the temperature and time duration and the desired effect of the treatment should vary.  Basic chemistry and physics of the material are similar.  (Rick Crenshaw)

I have no problems. I am starting to do all my rods that way.  (Tony Spezio)