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Fire-hardening wood?


Favinger

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Does heating wood harden it? I remember learning that primitive man hardened wooden spears over fire and that violin bridges could be harden by heating. An anthropologist friend of mine says that it is thought unlikely to actually harden the spear. What is the heating suppose to do to the wood? Does it just drive off water? Does it harden resins? Does it cross-link polymers of sugar or lignin? Does it do anything? I have never seen much difference after heating my bridges; am I doing it wrong? Any opinion or reference is welcome.

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I thought the objective of burning spear tips was to build a layer of hardened charcoal, not to harden the wood.

I don't think that heating bridges hardens them, though it's commonly alleged to--what I do notice, however, is that if I don't cook bridges first, sometimes their water content is excessive, and sometime after I'm finished the wood consequently shrinks, and I'm left with strings sitting on the fingerboard--especially cello bridges.

The other day I did a test to confirm this, and fit a cello bridge immediately after drying it, then did my usual water routine to carve the bridge (a moist bridge carves easier). When I'd finished, the bridge was almost 2mm too high, and then a few minutes on the hotplate pulled it back to where I'd originally set it. I've had cello actions drop 2mm in a week or so, and now suspect that the (humid) bridges were at fault, not the cellos.

Because of this, today I had to fit several violin bridges, and left them sitting on my hotplate on low until I needed each. I think that's going to become the regular strategy.

[This message has been edited by Michael Darnton (edited 03-01-2002).]

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One can also microwave bridges. This is a way to straighten a warped bridge; wet the concave side and heat it. Sometimes this works, sometimes the bridge warps back, so let things settle on the violin for a couple of weeks before trusting!

The technique of charring, then scraping, violin plates was patented at some point. I don't have the patent, but a search at www.uspto.gov [NOT a commercial site!] will reveal the patent. Many claims of improvements, but it didn't convince me to chuck any violins in the woodstove to see if they sound better charred. Does make some sense, but one would need to rough a plate, then char it, then scrape it down. I don't mind some slightly singed spots on a bow, but a violin seems different!

Steve

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Most woodworkers like kiln-dried wood because it is more stable and less likely to absorb moisture than air-dried wood. But they also don't need it for acoustic properties, which is why violins are made from air-dried wood. I don't know the exact mechanism, but the cellular structure is changed during the kiln-drying process, and this detrimentally changes the acoustic properties. The longer wood is air-dried the more stable it is and the less likely it will absorb moisture, one reason to use wood dried at least 7 or more years. But since violin wood will expand and contract, even after being varnished, is one reason to use quarter-sawed wood, it expands tangentially less than slab-cut wood. This makes the tops and backs less likely to expand and contract. Excessive movement of the plates could cause problems in glue joints at the sides. Of course, there are also slab-cut backs that work well and have no problems. Quartered wood is best for blocks and linings as well as tops, backs, necks, and bridges.

Concerning bridges, they will take on moisture and expand. If they are heat dried before cutting they will shrink to the smallest dimension. So when moisture causes them to expand it raises the action of the strings, which is a much better alternative than the problem Michael had where the bridge shrunk, leaving action of the strings too low.

I wouldn't fire/heat harden anything but a bridge. If a top or back were heated it would shrink to a minimal dimension, even causing an imbalance between the heated inside and unheated outside, and when as a violin it went out into the world there would be excessive expansion of the wood, stressing the glue joints, and causing much potential for disaster.

Does anyone know of one of these violins and how they perform?

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I do exactly the same as you do, and was therefore surprised at the effect it had. "Local water" I assume to be water obtained locally? :-)

quote:

Originally posted by Barry J. Griffiths:

Michael,

Can you pleae further exlain your "water routine" used on cello bridges. It sounds like your whole bridge is getting moist. I usually fit the feet by cutting them while moist, then a little local water to thin the ankles and whatever we're calling the large 'eyes'.

Barry

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Hank--When I originally got interested in this subject a couple of years ago, I did quite a bit of research on the effects of kiln vs air drying, and though I read much of the same stuff that you've said from various more informal and trade-related sources such as woodworking books, it was denied by the textbooks and wood technology stuff I found. I came to the conclusion that there's perhaps essentially no difference between modern properly-kilned and air dried wood--especially as regards future tendency towards expansion and contraction and moisture content, though I still do have my doubts about my doubts. . . .the written evidence is extremely inconsistent, however, depending on who's talking, and if nothing else, I've decided to suspend worrying about it. That is, I'm not agreeing, or disagreeing, but the more I looked the less clear the story was.

Now I notice that Andres has just added to the confusion, in just the way I meant. :-)

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Andres,

The information I gave is from research and experience. One source I have used extensively is The Encyclopedia of Wood, Wood As An Engineering Material, by Forest Products Laboratory, U. S. Dept. of Agriculture. I used this book as a guide when engineering a house I built. It contains physical and mechanical properties of domestic and imported woods and much related information including a section on seasoning of wood, the advantages and disadvantages of air and kiln drying. "...air-dry stock used without kiln drying may have some residual stress and set that can cause distortions after nonuniform surfacing or machining." Concerning kiln drying "thus minimizing shrinkage-related defects." "Further advantages of kiln drying are the setting of pitch in resinous woods..." It has to do with moisture content-the percentage of water weight to the total weight of a wood sample. Air drying typically can only dry lumber to about 14-18% and kiln drying can get wood to 4-6%. Wood shrinks as it loses moisture. I tested several samples of wood in my shop with a moisture meter- 30 year old spruce 6%, recently purchased European maple from International Violin 6-8%. This is very low for air dried wood and is the reason it takes many years to properly dry violin wood. This wood is stored in an air conditioned shop, mostly to keep out moisture, and if this wood was stored in a shop with no moisture control the moisture content would be higher.

I have a small sawmill and have researched drying methods for lumber. From experience I found many problems associated with using air dried lumber, mostly concerning warping and shrinking. Typically, these problems are not associated with kiln dried wood.

This manual is for lumber and does not consider long term drying as with violin woods. The longer wood air dries the drier and more stable it becomes, but it will always be acceptible to moisture and dimentional changes. How much is the question? How long were the cello bridges Michael used air dried and how much was the dimensional change with additional heating/drying? There was a definite problem with shrinkage with the one bridge. And when a cello is played outdoors on a damp night how much will the bridge move? I don't really know how much, but luthiers have to keep in mind that wood is an organic, alive substance that is subject to hot, cold, dry, damp conditions. I am always awed by how the old luthiers were able to design a violin keeping all these factors in mind.

I hope I haven't rambled too much, but this is a subject that is very interesting, and like Michael said it is one that has no definitive answers. I would be interested in knowing more about how long-term air drying affects the physical properties of wood.

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In his book "Understanding Wood", Bruce Hoadley tries to disarm misconceptions about wood drying, and one thing he points out is that the equilibrium moisture content of wood is connected ONLY to storage conditions, not method of drying. That is, kiln dried wood reaches lower levels because it is dried in lower levels, but quickly adjusts itself in real life in exactly the same way as air dried wood, arriving at exactly the same point, dependent only on the humidity of storage conditions, which have exactly the same effect on kiln and air dried woods.

Somewhere along the line, people have gotten the idea that kiln drying harms cell structure. The reading I've done indicate that the primitive methods used 100 years ago did, but that modern methods are much more carefully regulated and gentle, and don't.

Setting of resins is a different issue, and I believe that's the main reason that instrument makers prefer their wood aged more than furniture makers. In that regard, it seems like kiln drying would be more desirable, but only in the short run, for the impatient.

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I'll just add a remarkably unscientific note here. I am currently using 3 year old maple. 2 years in the shop, and 6 months in the back of my subaru outback in summer time. The day time temperature was often 110, but never much higher. I'm currently building my second violin from this wood, and find it to be very stable. that is no discernable ill effect from poor stablity.

so I now run around with 6 wedges of maple in the back at all times :-)

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By the way, just to clarify my last post--I'm not saying that I believe Bruce Hoadley, either. I was just throwing a third, different, opinion in on top of the two that were already there. This is truely one of those topics where if you quote one authority, I'll find another who disagrees, no matter what's been said. :-)

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Michael,

Last night after reading the last posts I was all set to do some research to find more about kiln vs. air drying, and now in the light of day realized that since I'm not drying wood anymore it's not such a pressing matter. What seems important is how the already seasoned wood we get will work and react under a variety of conditions. One thing I love about wood is the sometimes mysterious and always wonderful ways it behaves from the sawmilling, or splitting, to the drying, to working it into an instrument, to how it behaves when being played. I do my best to understand the properties of wood and materials but realize it's often an open-ended discussion.

Storage after drying is an important issue, making sure that this precious wood stays as dry, and unaffected by bugs and fungus, as possible. It took a long time to get it that dry and it is worth the effort to keep it that way. If the maple bought at 6% moisture content(MC) is stored in a damp place the MC can soar to 18%, causing dimensional changes that will affect its stability as being worked and after its made into an instrument.

I always wonder what it was like being a luthier in the Middle Ages and how they dealt with these problems.

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A short article in the current Strad magazine talks about a fellow in Asia (Hong Kong??) who uses some sort of "carbon" treatment of the wood. He brushes several coats on and the wood loses 30% of its weight. It becomes resistant to the high humidity in Hong Kong.

I wonder what he is using?

[This message has been edited by violins88 (edited 03-04-2002).]

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I read that, too. Brushing "something" on and losing 30% weight doesn't sound credible to me--at least it's contrary to science as I know it. It implies that "something" converts something in the wood which isn't volatile into something which is totally volatile on its own. I've stretched my mind, but the chemistry doesn't work for me.

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In most places in the world producing tonewood, kiln drying is considered a must to preserve the sapwood in the non-Sitka spruces (sapwood is discarded in Sitka), and to prevent fungusing in the maples...

If you not artifically dry maple immediately after cutting, you'll turn a very valuable stack of violin backs into a moldy worthless pile of nothingness...I speak from experience....(g)...

The problem is to do it correctly and very gently to prevent brittle texture in maple, or a collapsing of the cells in spruce...

Bruce

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  • 3 years later...

Here is an unscientific suspicion. !.Fire or heat would drive out moisture. Dry wood tends to shrink . Therefore volume of solid material per given volume increases.The more solid it is the harder it becomes.

Now for the other side. Take a peice of soft balsa wood and cut a slice off off it. Now compact it as much as possible by crushing it in a mechanics vise. Cut another slice.The second slice will still be a slice of soft wood but it will be harder than the first. While carbon (in diamond form) is the very hardest of materials,the carbon on the end of a burnt stick is so structurlly soft that it will sluff off in your hand easily. Brush up against it and brush the black off your hand.

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