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Heat treated bridges

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Hi Michael....Great link you provided...It encourages one to carry on with what I'm trying to do..Hopefully I can try one on the old fiddle I had with me at the Amati Comference. Maybe it will sound even better...

How's the book coming along???I'm looking forward to it... Regards, Lonnie..

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Quote: “heat treated bridges are more dimensionally stable……”

The Stora Enso information clearly gives the reason for the dimensional stability obtained with their Thermo process. The equilibrium moisture can be reduced by as much as 50% but this is due to the fact that the wood cells are destroyed. Wood is a cellular material built up by tube-shaped cells oriented fairly parallel to the stem or branch axis. The inner side of the cell called the lumen will be (greatly) collapsed after the thermal treatment. This reduces the uptake of moisture and change of properties. The same report also mentioned the reduction of bending strength which is of interest when using their thermal treatment process on bridges. A bridge will have to withstand some bending. A brittle bridge on my violin is not my first wish. Also is said that the chemical reactions involved in the ThermalWood treatment are still in research showing loss in strength properties It may be clear: I have my doubts in using this thermal wood treatment for bridges on string instruments.

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I can't find the part about cell collapse in their literature--can you provide a link?

The rest doesn't particularly bother me (actually, the cell collapse part doesn't, either). Since "baked" bridges have been used successfully for decades in good shops on the best instruments the process isn't in the least questionable. "More is better" (bending strength in this case) is only an interesting philosophy if the effects match the philosophy, which they obviously don't, as thousands of successful bridges adequately demonstrate.

Another link showing even more definitely that this wood can work under stress (much more than in a violin, in fact): http://www.landola.fi/

So where, exactly, is the real-life problem here?

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The tension that six strings exert on a guitar top would be several times more than what 4 violin strings exert on a violin bridge..Also the stress would be a "pulling" ( on a pin type bridge) where as the tension would be "pushing" on a violin bridge..The only problen that I could foresee that the less of a breaking point for baked verses non-baked would be that of the strings pulling the top of the bridge toward the peg box.. Your not supposed to let that happen whether the bridge is baked, fried, boiled, or eaten raw.. Er...I mean just raw...Not a lawyer, could be wrong...Regards, Lonnie.....

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Quote: I can't find the part about cell collapse in their literature--can you provide a link?.........

You’r right it is not from their literature. It has to do with paper chemistry and paper physics. [paper is made of wood fibers]. But as you said it does not bother you, so I don’t want to annoy you with (a) scientific article(s). Instead the link of Landala you submitted says that the cellular structure changes!

But what counts is that even guitar makers use heat treated wood with excellent results as they say. The treatment seems to be at higher temperature and longer time than the normal time required for bending the sides. Also violin makers use some temperature treatment on their sides. Maybe a little shorter in time and lower temperatures? Just enough to allow proper bending. So, what is the big difference?

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Hi Lonnie

I would have thought that the *downward* pressure on a guitar top would only be a fraction of what it is on a violin top, due to the string angle??

I see you use the terms "pushing and pulling" tension - I imagine that "pulling" is the tension of the strings between the nut and the saddle, and the "pushing" tension is downward pressure? If that is the case, it should be borne in mind that the "downward pressure" is determined by the "tension" of the strings as well as the angle over the bridge. As such, one can have very high "tension" without any "downward pressure" at all (if, for instance, there were not bridge at all), or conversely, quite substantial downward pressure (a very sharp angle over the bridge) together with relatively low tension (tuning down the violin: with the same set of strings, the pitch is directly related to the tension, but only related in a secondary sense to the downward pressure).

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Just a couple of comments here: First, it makes perfectly good sense that heating a bridge would dry it, thus increasing its density, and the density of bridge material certainly impacts tone color and volume. What I don't understand is the mechanism whereby the bridge doesn't simply return to equilibrium with environmental conditions -- in other words, right back to the same moisture content/density/hardness that an equal but unheated sample would have -- unless there is some sort of physical change that happens (like cellular collapse) -- and even cellular collapse/prolapse would likely simply slow the process. The very act of heating wood and then packaging it with dessicant implies that simply exposing it to the atmosphere would undo whatever physical change occured in heating -- otherwise why the dessicant (which simply absorbs moisture in the packaging more efficiently than the wood does). I won't argue that heating bridges is useless -- there is too much about the acoustical properties of wood that is not understood, and it may well be that some sort of permanent change does indeed occur. Treatment of a bridge in some sort of solution is actually more intriguing to me -- perhaps followed by heating. It makes sense that such a process might actually fill-in the cellular pockets in the wood with whatever was in suspension in the solution -- whereupon drying would remove the water content but leave behind the filler (like a salt-stiffened, dry bathing suit). By the way, when it comes to protective finishes, there are very few (and certainly none that are commonly used on violins) that actually prevent water vapor penetration. Any fiddle will, given enough time, come to equilibrium with its immediate environment. What finishes do accomplish is to slow the process -- an important factor because rapid changes tend to create more uneven stress (and therefore damage) than slow, gradual ones.

Robert

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Hi Jacob...You are absolutely right....I didn't make myself quite clear as to what I meant..The pulling I was refering to is the tension that the strings put on the sound board causing it to belly up behind the bridge and bridge saddle when it ages...That is why pinned bridge guitars need to have a new (and usually a bit bigger) bridge plate installed when this happens..I'm not really comparing the string tension of guitars and violins as opposed to one another.. I was comparing the wood that the guitars(on the site that Michael provided the link) was made with as opposed to baked violin bridges.. Did I make it more understandable?? I can't always say (or type) what I mean .... Oh-oh, what's that I smell?? I think my bridges are burning...Maybe it's the biscuits instead..Gotta go...Regards, Lonnie..

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The topic of heat treating wood was recently discussed on the bulletin board, Left Brain Luthiers. Besides Landala, Taylar Guitars also heat treats its guitar tops. No one knows what the long term effects are of this treatment.

For spruce, there is an immediate increase in transverse stiffness (perhaps 10%) and a decrease in equilibrium moisture content to about 1/2 of what it was before the heat treatment. But it is not certain whether the wood recovers to its original properties if you wait long enough. The one big negative is that the wood is more brittle, but there was no physical measurement of brittleness. The wood gets darker, and luthiers have used this trick to help disquise a repair involving a visible insert.

There is this anecdote that Vuillaume used baked wood in his violins, and that it had bad results. I would like to hear from more knowledgable people on this subject.

I would also like to get a scientific paper citation for what happens to the wood properties and how it changes for various species. I promise it will not annoy me.

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Hi Robert,

you stated that "heating a bridge would dry it thus increasing its density...."

But: drying will reduce the weight of the bridge (moisture is evaporated) without changing the volume of the bridge. As the density is defined as g/cm3 (or kg/m3) the density will be decreased!

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Dutch,

You are so right -- poor choice of words on my part. What I ought to have said was rigidity, or stiffness. There would be a change in volume, however, as wood decidedly shrinks as moisture is driven off. In the case of a bridge this would be small but very measurable. In the case of a guitar top I'd be a tad worried about heat treatment -- it's a fairly large expanse of wood, and as it picks up moisture it doesn't have a lot of room to expand...

Robert

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Mike,

I did some search and found some articles related with wood cells and the effects on it during heat treatment. Some of hem are less scientific others are.

In:

http://www.bangor.ac.uk/safs-new/research/wofibsci.php

under the Chapter "Thermal Modification" (you have to scroll down a little bit) one can read that under heat the lignin which is the cement between the cell walls wil cross-link. In an other article -which I can't retrieve for this moment unfortunately- I found that heat treatment propagates delignification. And in:

http://www.eia.doe.gov/cneaf/solar.renewab...ual/chap02.html

one can read under Chapter "Changes in Process" that ..."the cell structure partly collapses in delignification..."

A less scientific article is dealing about the use of microwaves for timber:

http://www.crcwood.unimelb.edu.au/docs/fac.../microwaves.pdf

Hope this helps.

Regards,

Frits

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So are you implying that since the delignification part of taking wood completely apart to make pulp results in partial cell collapse, that any amount of heat, used in any way, on wood, results in cell collapse? Because if so, that's a definite non sequitur from the references you're citing.

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I'm no expert on wood, but I've worked with it a little. Wouldn't heating for a bit past a certain temperature harden any residual gums, pitch, or whatever you want to call it? That might well be beneficial, I suspect.

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I have never worked with heat treated bridges. But I have made bridges from a very large list of woods and other materials. Of all the various materials used the one that had the most notable effect on tone was a material called Micarta a linen/resin very hard material. The only one that came close was aluminum. This info is for what its worth. Also I had better results with vertical grained orietation then with horizontal grain. There was no difference in tone, but less warping or bending of the bridge.

Al;

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No, I'm not. The references indicate and describe that on heat treatment of fairly increased temperature and duration (so that's not the same as any temperature raise) propagates delignification. With respect to wood chips or pulp, high temperature (aided by steam) helps te delignify the wood cells. Pulp still contains wood with -on macro scale- billions of wood cells cemented together with lignin. If not so, than paper cannot be made because there is no longer a structure to keep the cells together and it will disintegrate. Furthermore delignification results in partly cell collapse, not a total collapse of all present cells.

Frits

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Thanks DutchViolins.

You gave me the keywords that I needed to research this subject (thermal treatment, wood). It is clear from these papers that the wood is subjected to very high temperatures (140-250 C), and the purpose is very different from what luthiers would want. The thermal treaters are trying to take inferior woods and improve their performance, particularly in resistanct to rot. There is a marked degradation in the properties that luthiers need--namely, stiffness. These woods also become brittle.

The left brain luthiers were subjecting the wood to much lower temperatures (90 C). Very few tests have been carried out by the left brain luthiers; so the issue has not been well investigated from the standpoint of the luthier.

The anecdotal story from the guitar builders standpoint is that Taylar Guitars does a mild heat treatment to reduce the number of guitars that are returned due to top shrinkage that results in cracks. If I was them, I would store the tops at 25% relative humidity before gluing the braces into place.

There are no free lunches (dang it).

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There is some focus on acoustic merit which is defined as modulus divided by density. Some guitar makers are using this--I am one of them; that is, we want it to be as large as possible. I suspect this would apply to violins as well.

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Can you be a bit more specific?

The reason why I prefer stiffer bass bar material and stiffer soundpost material is that I can always remove material for less stiffness as I progress with the carving (as with the bass bar) or with the set up (as with the soundpost), but, there's not much one can do to increase stiffness if it isn't there in the wood to begin with.

So, with wood that is really stiff to start with - there is an entire spectrum available if needed.

Granted that the need for stiffness isn't universal in all of these areas in all violins, since the requirement varies from violin to violin - but the ability to have it if it is needed is crucial.

With wood that starts out flabby, (as in cheap, flabby bridges) threre is no where to go but flabbier as wood is removed. That's why I like the stiffer Aubert bridges to begin with also, and why I start with them fairly thick. I attribute flabby bridges with all sorts of tonal weaknesses. How much stiffnesses is added by heating an already decent quality bridge?

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My only comment on this issue is that if you try the things that everyone says don't work, you're going to learn that a lot of common assumptions about what will and won't work are untested B.S. A lot of it is simple and untested "more is better" philosophy.

So don't ask me--try some stuff.

By the way, I didn't say that heating a bridge added stiffness. There's more to it than that. For instance, a Dobro doesn't sound like it's made of metal because of its stiffness--it sounds like it's made of metal because it's made of metal. You can make your bars and bridges weaker, but they're still made of stiff, hard wood--thinning won't get around that.

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Not to fan the fire... but I've noticed a difference depending on the way a bridge is heat treated (if the heat is applied to the outer surface as Michael described or if the wood is "baked"). I didn't scan all the articles, but did any mention a PH change from the heating process?

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