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Sound Board Warping.


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I have been thinking about this topic and I wondered if you considered that any flat piece of wood left on a flat surface for any period of time warps.I just wondered if you take your concertina apart for any repair work that the sound board left unclamped for any period of time ,you increase the possibility of it warping.Surely that if it is clamped back on the bellows ,or even clamped flat during the repair work would reduce the risk.Seems a bit of common sence to me but I am happy to be shot down in flames if you disagree.

Al

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This may well be true , and the idea of clamping disassembled parts has been discussed before, however, I fail to see that the clamping effect provided by the Bellows corner blocks and the End Bolts is sufficient to stop a Reedpan or Pallet board from warping if it is so inclined.

 

I recall Harry Crabb telling of the fuss they went to in the old days , of taking their stock of Sycamore boards down from the stack each year and putting them through the Planer, taking a small skim off each board so that they were very gradually reduced from about an inch thick down to the required 1/4" ( or there abouts) ... this process would be repeated every year for at least 25 years. This was probably during an explaination of why they had started to use Plywood .

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Pent up stress is released when the end bolts are loosened. The stress returns when the end bolts are re-tightened. In order to help prevent additional deformation due to humidity levels, I place the ends in a sealed plastic bag until reassembly.

Edited by michaelpier
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From my experience, clamping is not sufficient - my DIY concertina endplates had warped signifficantly even when screwed to the actual box. Stress caused by unequal drying of wood (or uneven distribution of this stress due to assymetric geometry etc) occurs whether or not the wood is clamped. It will just manifest when unscrewed, but the stress will be present in the wood nevertheless.

As a sidenote: plywood warps too, but the amunt of warp for given thickness is smaller in plywood than in solid wood. And from my experience plywood tends to twist rather than bend in single direction (due to crossed grain direction in each layer).

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As a sidenote: plywood warps too, but the amunt of warp for given thickness is smaller in plywood than in solid wood. And from my experience plywood tends to twist rather than bend in single direction (due to crossed grain direction in each layer).

Plywood can also delaminate.

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Timber cut "On the quarter" as against "Through and Through" has far less liability to warp. Most of the Chidleys that I own have the reed pans cut on the quarter, but then Rock Chidley was a Timber Merchant at some stage

Quarter sawn wood is more stable but might tend to twist rather than cup or bow in my experience.

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Twisting true quarter sawn wood results when there is some degree of spiral grain, which is not always obvious in thin stock. It is quite a common defect in lumber and from what I have read on the subject, may be genetic since individual trees in a uniform stand will have this growth pattern while others next to it won't. Quite a bit of effort is expended to figure out which is which before cutting since the milled product is sometimes unusable.

Wood also creeps slowly under stress, so if left in a stable environment, starting stresses tend to slowly decrease. Stable is the key word though. Any change in moisture content sets up a whole new round of stresses. Quartered wood preferably as far from the center of the tree as possible is least affected. ( as has been mentioned. ). Concertina reed pans and ends with fixed action boards are especially vulnerable because chamber walls or other parts are often glued across the grain leaving the main piece which can expand and contract quite a bit with the seasons to battle with a brace glued only on one side. Radial reed pans seem to suffer the most from this. Jeffries style reed pans generally have the partition walls in line with the grain and it is only the cross pieces that build up stress.

Edited by Dana Johnson
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Dana, what's your take on air dried vs kiln dried solid wood? I'm sure wood that has been carefully air dried over 25 years would be great to work with, but if you don't want to wait that long before you build your first instrument from solid wood...

 

I have been reading about a process that involves 'steaming' the timber at 70C for a week or so before drying it at 40C. The idea is that this relieves the internal stresses in the wood and permanently opens the pores(?) so that it subsequently dries quickly and evenly with minimal warping and splitting.

http://www.logosol.co.uk/all-products/sauno-kiln/

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Anyone experimented with thermally-treated wood AKA ThermoWood? The manufacturers claim the heat-treatment process makes it more dimensionally stable.

 

http://www.thermotreatedwood.com/what.html

http://www.thermoryuk.co.uk/thermo-treatment/

http://www.atmosafe.net/en/moisture-content/thermowood.html

Edited by alex_holden
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  • 2 weeks later...

A lot depends on how either was dried. Both can be done well or badly. Wood that is dried too quickly, or any combination of factors that allows the outside to dry hard and fast while the inside is still damp, freezes the outer dimensions larger than they would normally be. The inner wood dries attached to ynis outer "case" and often will do what is called honey coming. Many small cracks open up as the wood shrinks away from itself. These can close up again as the wood stabilizes, but remain as defects that allow the wood to open up the cracks after the wood is used. Steaming helps keep the case from forming and allows the dimensions to keep pace with the drying. Good kiln operators are very aware of all this and put effort into getting it right. Kiln drying got a bad name from early methods that did not take these things into account.

Careful slow air drying can produce fine wood, but can't be left to the vagaries of the weather. Since it is not subject to the steaming operation, overly fast drying can cause a lot of built up stress. Gentle drying over a few seasons will allow the stresses to work themselves out to a great degree.

My favorite wood is Vaccuum dried which is not as fast as kiln drying, but takes place in a cool humid environment and never risks developing a case. The natural pores in the wood allow the inside to dry while the outside is drying, which produces less stress. Unfortunately the only place I knew that did this couldn't eventually make it pay and started farming out their drying.

I did try steaming one of a couple pieces of wood from the same plank. I drew a line on them on the end grain and noted after milling them both, the steamed one's line had curved, but the finished piece stayed much flatter than the unsteamed one which reacted to the removal of the wood by moving to equalize the new unbalanced stresses. It took a long time to lose the moisture it picked up in the steaming process. ( months )

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Thanks Dana, that's useful.

 

I found a sawmill in Scotland run by a chap who sounds like he knows what he's talking about (he also teaches furniture-making). I initially contacted him because he was advertising "tonewood", by which it seems he means rippled sycamore that has been specially selected, quarter-sawn, cut into violin or viola size pieces, and carefully air dried for four or five years. However this is what he replied when I asked his advice about using (far cheaper) kiln dried sycamore instead: "We air dry all our furniture grade timber for a year per inch of thickness and only finish it off very slowly in a low temp kiln so it is unlikely it will behave adversely." The catch is that he doesn't specifically quarter-saw furniture grade sycamore, so I would have to pick and choose from his stock to find a board with suitable grain orientation.

 

It's tempting to try DIY kiln-drying! Perhaps an old chest freezer with a small fan and some tubs of desiccant crystals in it? No heater so it would dry very gradually.

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I would be a bit loath to use the thermowood in a concertina. The degree of heat the wood is subject to is high enough to cause chemical changes in the wood, especially the high temp version. The steam such as it is, is dry steam and while o have no reason to question the manufacturer' assertions, it seems it is used to displace the oxygen that could cause combustion, rather than to saturate the atmosphere to control the rate of moisture release unless it is in a fairly substantial pressure vessel. The pressure has to be high enough to inhibit boiling I order to control its rate. I don't know how may psi that is for 215C.( 419F). They do speak of having taken a long time to figure out the proper procedure to avoid all the usual kiln drying hazards. I think the big reason for this process is really speed.

While the wood becomes moderately plastic at those temps, which certainly de stresses it, long term stability still means using well quartersawn wood with as little ring curvature as possible. I'd put my effort into finding that and sticking to species that are close in structure and density to the traditional choices. You can certainly experiment, but the wood really does make a lot of difference in the tone. I think the traditional woods have lots of characteristics besides tone, like good machinability, availability in larger widths of quartered stock, low tendency to chip or splinter in thin sections etc. I did see a concertina made with black walnut for a reed pan that sounded alright. That also has good machinability, tenacity and stability, though it is considerably more expensive to no real advantage I could see. It certainly wasn't a magic bullet as far as tone went. I also don't know how easy it is to find well quartered, since it is expensive and is usually cut for a more interesting grain pattern that shows up better when plane sawn. I don't know any diffuse porous woods ( mostly tropical ) that fit the bill. The ones I've seen and used for other things are either too dense and high in lignin and hemicellulose to be stable or good for tone, or too open grained and split prone. The ones I can think of, ( some of the mahoganys ) are all protected species these days. And unless you can prove provenance, I'd leave them out of anything I ever intended to sell.

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You can find info on diy drying processes, but you want to do your homework to make sure you don't have to learn your schedules the hard way. Picking through stock is actually a good thing. You learn a lot in the process and get better results. Just a note, I've found that as long as the ring curvature is small, consistent across the width ring angle of up to 45 degrees works very well. You will get a little more expansion or shrinkage with the weather, but it still has little tendency to warp. Avoid curly grained wood if you can. It may look pretty, but behaves differently acoustically as far as concertinas are concernedm as well as being much harder to plane or machine since the grain alternately changes it's able at the surface. Thickness sanding works, but you don't really want the wood inside the concertina anyway. ( it is fine for end frames if you like the look ) the straight grained stuff is a lot less expensive anyway.

Have fun. Nice pieces of wood are a treasure and worth the hunt.

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