Thermally treated GC Build

[Joe Sustaire]

This is going to be a comparative build using the thermally treated wood I won in the monthly lottery a while back. White spruce top with african mahogany back and sides. Grand Concert "Stella" size with ladder bracing like the last couple of builds.

Here's the top and back with rosette and backstrip.









Simple rosette ring and backstrip I made up way back out of some kind of flecked light wood with black fiber on each side. Real accurate tech data I know.... And this time I used Ivoroid to bind the inset soundhole.


I found the thermo top wood to be pretty brittle stuff to work with. Had quite a few tight cracks to glue up with water thin CA. I don't know if they were in there originally or picked them up in shipping, but looks like they're going to be okay. Had a couple cracks in the back I had to glue also. Just handle with care....

Then we come to bending the sides. Grant originally sent me un-treated sides as he'd had trouble bending the therm treated wood. Recently he's had some success so he sawed me a set of treated sides at the gathering. First off they were sawed a bit thin it seems, had to take them down to .065" to get rid of all saw marks so at least they should bend like butter.....



My bending rig. Pretty much copied from Colin, and I use his bending formula laid out in a couple of his early build threads. Now these sides were brushed with Downey and wrapped in plastic for a day, then re-coated and wrapped for another day. Then on bending day I sprayed them down with water, wrapped in brown paper, then foil, and put between the bending slats with the heating blanket.

Started bringing down the waist at about 270, bent the lower bout about 300, then the upper bout at about 325, then tightened down the waist. Let cool down, them brought back up to 325 for about 8 minutes and let cool down again. And here's what I got....







Hard to photo the cracking but I think this shows it fairly well. I didn't feel any problem when I bent them, felt smooth and easy. After seeing the cracks in the first one I thought maybe I was too slow in getting to the upper bout bend and it dried out too much. So I was quicker on the second one and it felt good, but again cracks.....






And I'm also getting some of this stain/spotting like Dennis has a thread about. Tried sanding it a bit and it's not just on the surface, seems to go down a ways.

So, any ideas?

[Eben]

I don't know if those are saveable Joe - As discussed,when I get to it, I'm gonna do the Downy thing, but bend by hand on a pipe - I think it can't but help to be able to see, feel, and smell this stuff as it bends.

E

[Dennis Leahy]

Yeah, those damn black spot/stains...

I was sure that they were from steel filings that some slob left in my shop , but now, you're making me wonder. Maybe the fabric softener/Supersoft2 is reacting with minerals in the wood?

I still wonder if Supersoft2 is actually a superior product actually designed to soften wood (or modify or coat lignon, or whatever it actually does), or if clothing fabric softener is truly the same. Since I had purchased a quart of SS2, I used it, and once I had success on the batch of sides that I just bent, kept using it. Has anyone done any A/B comparisons of products?

Cracking/faceting:
You've bent enough sides and have a high-tech bending rig now, so the temperature was probably not the issue (unless TM wood needs a higher temperature?)

Sounds like you had plenty of water (and I assume that you wrap in foil, so it takes a while to boil off all the water.) I don't think time is the issue either, because of all the cutaways I just bent (and they take me at least twice as long as a non-cutaway side to bend.)

So, that leaves me with a few possibilities to think about: maybe the structure of the fibers in some species just doesn't lend itself to TM bending. Maybe the soaking period needs to be longer (although I did all of mine after 24 hours, but again, maybe species characteristics plays a role. Maybe some of the wood in the TM kiln gets "crispier" than other wood. I know the whole kiln is brought up to the TM temperature, but it seems like wood on the outside of stacks would come up to TM temperature earlier in the cycle, and thus "cook" longer.

Just a bunch of maybes, but that's all I've got.

p.s. p.s. : Maybe the thinness of the material had some bearing on the cracking. You know, loss of structural integrity? My worst cracking failure of the last batch was a too-thin Katalox side.

p.s., and not related: what is the width of the lower bout?

[Joe Sustaire]

This one is going to be a little bit deeper, 4 1/2".

Well I figured out one thing I did wrong. For some reason, I had the heating blanket next to the foil package under the upper bending slat instead of on top. So they didn't get proper support from the slat during bending.



Proper orientation.

Makes sense to me.... so I sanded, down to .085", and shaped the un-cooked sides that I had and bent them with the bending slats in proper position. Bending slat, foil side package, bending slat, then the heating blanket.

First side, bent perfect. Success. On to the next side.... then....



this. I don't know what went wrong here. Obviously I'm carrying some bad ju-ju today.

[whitespruce]

Hmm, well, something is screwy, specially since one non-TM side bent fine and the other destroyed itself. Makes me wonder if the TM is the problem or not. BTW, I always put the heating blanket inside the upper slat.

I will try some experiments on more TM wood next week. I resawed several species of TM sides last week.

At any rate, looks like you need some more sides.

I don't think variability in the TM kiln is an issue. The process is gradual and very carefully computer controlled. The whole process takes 72 hours and the high temp portion is only about 2 hours long during the last half of the time period.

My TM Beng sides bent very easily and flawlessly ( I did them at a shade over 325 F). John cooked several sets of already sawn mahogany sides for Dana Bourgeois, and they must have worked, because there were no bad comments back.

G

Oh, I just looked back and noticed that your bending forms are not solid across. That always used to give me trouble until I changed to solid forms. FWIW

[Joe Sustaire]

Yeah, I wondered if my forms were complicit, seems likely as that doesn't give you solid support all the way.
And interesting that you have the blanket under the top slat. So many variables.....

After sleeping on it, I remembered that I didn't do a smooth, easy bend on the upper bout that second time. The way I've got it rigged there isn't an even radius for my bolts to follow. I have to lengthen and shorten, and I fumbled the procedure pretty good that time. It went down, then sprang back up while I was juggling tightening/loosening my block. I think I'll go to some heavy springs at that end so it'll follow the form smoothly.

And yeah, I'm not sure this has anything to do with the thermal treating or not......

I'm going to go bend some more wood and try to iron out the bugs on the regular wood, then I can try tm wood again.

[johnparchem]

With the blanket on top of the slats, I can easily see that a slight gap between the slat and the wood could near totally insulate the heat from the blanket from the wood before you bent it. If the blanket is inside the slat then the weight and floppiness of the blanket tends to keep it on the wood. I say that because one half of the cracked board looks OK the other looks like it was bent cold. I often clamp the stack together on the ends while the blanket is warming up.

[whitespruce]

With the blanket on top of the slats, I can easily see that a slight gap between the slat and the wood could near totally insulate the heat from the blanket from the wood before you bent it. If the blanket is inside the slat then the weight and floppiness of the blanket tends to keep it on the wood. I say that because one half of the cracked board looks OK the other looks like it was bent cold. I often clamp the stack together on the ends while the blanket is warming up.

John, your comment makes a lot of sense. Bet that's the problem on the one cracked side.

G