Flattening warped back plate

[polkat]

I'm just getting back into violins after a long layoff, so I'll have numerous questions for a short while. Thanks!

I've got one in the garage that I took the back plate off of quite a while ago (for some repair I think), and forgot about it. I went today to glue it back up. However, over the time it's been off, it has warped slightly across the plate. The drawing below shows it flat on a table, looking towards the button end......

I can compress this flat by hand though it takes a little pressure, but I don't want to glue it up that way. How would you straighten this?

plate.bmp

[duane88]

Dry clamp it to the rib structure, as if you were going to glue it, and leave it that way for a week or so, then check to see if it has complied.

[Joe Christian]

What is the measurement of the warping? Are we talking a few millimeters, 10mm, 20mm, more? I'm guessing that you should be able to just glue it as it is.

--Joe

[polkat]

It's very evenly warped down the centerline so that the highest spots are the middle of the button and the equvalent back end. Roughly about 4.5mm.

Actually, much like duane88 said, I was thinking of slightly dampening the wood everywhere except the glue line, and clamping it to shape on a flat surface for a few days. Think that might work?

[lyndon]

its not ideal but you can try clamping it flat or slightly warped the other way then carefully hitting it with a hair dryer, and thoroughly letting it cool down before removing the clamps

[duane88]

I didn't say to dampen it. I'd dry clamp it to the rib structure first. You don't know what water will do, and it may go the wrong way! You could dampen it with a sponge and use a hairdryer to dry it and manipulate the plate, but if you've never done that before, you might just do more damage than good.

I was restoring a Scottish violin last summer, and the back was twisted around so badly that I chose to glue the plate to a flat plywood board for a couple of weeks before trying anything else.

Just consider dry clamping it first. Patience grasshopper...

It's very evenly warped down the centerline so that the highest spots are the middle of the button and the equvalent back end. Roughly about 4.5mm.

Actually, much like duane88 said, I was thinking of slightly dampening the wood everywhere except the glue line, and clamping it to shape on a flat surface for a few days. Think that might work?

[Wolfjk]

It's very evenly warped down the centerline so that the highest spots are the middle of the button and the equvalent back end. Roughly about 4.5mm.

Actually, much like duane88 said, I was thinking of slightly dampening the wood everywhere except the glue line, and clamping it to shape on a flat surface for a few days. Think that might work?

Hi,

Before you do anything, turn it the concave side up in a dry room and leave it for a few days. It might traighten out.

[Joe Christian]

I'd agree that water isn't the way to go. Time, heat and clamping. If your garage is like mine, it's probably just dampness that's causing the warping and once the back dries out it'll flatten on it's own. If not do what the others here said.

I'm assuming that this fiddle isn't the most valuable since it sat in a garage for a while. So, I'm also going to suggest that after letting it acclimate to an indoor environment you just glue it up even if it still has bowing. 4.5mm is pushing it, but not out of the question. If you can close it using two fingers with an amount of pressure that wouldn't hurt if applied the pinching your own skin, the glue will hold it fine. You have the large glue surface of the block right where it is needed most.

Just to be thorough, you should also let the rest of the instrument acclimate in the same room as the back. I'm sure you would have thought of that but just sayin'

--Joe

[Jeffrey Holmes]

If it were me, no matter what the value, I'd check the flatness of the rib-set before doing anything. Are the ribs twisted or bowed?

Depending on what the whole situation looks like, one can put together a jig to slowly bring the plate back to t's original shape (to fit the ribs) pretty easily. If it's close to 5mm off, I'd be a little worried that bringing it down all at once might put stress on the center joint, or cause other stress to the instrument.

[luthier]

I would say the cupping of the back plate is due to the humidity being lower now than it was when the plate was carved. It would be easy to dampen the inside and see what happens. I bet it would flatten out in no time. It would not be advisable to glue it up damp, though. I've seen a lot of this scenario, living in the desert southwest, where humidity is quite often in the single digits for weeks, even months on end.

[polkat]

No, duane88 didn't say to dampen it, that was my idea. The ribs seem to be perfectly flat. I've let the plate sit loose in a warmish room now for almost two days, but there's no change at all. No, not an expensive instrument.

One thing that I only now remembered was that, if I recall correctly, according to the previous owner, the seam in this back plate apparently opened and was reglued long before the violin got to me. I'm beginning to think that there may have been some 'spring' in the plate when I took it off, perhaps because of a bad reglue angle, although the seam looks good and tight. My idea of dampening was to let the wood reshape a little bit while clamped flat.

But if you guys think that's a bad idea, I'll clamp it flat for a while dry and see what happens.