Projection issue on a new cello

[Filippo Protani]

Hello everybody,

this is my first post on Maestronet, although I have enjoyed following many discussions.

I'm making a new cello for a customer and I've just completed the varnishing process. When I went to string it up, I realized that the projection had dramatically changed from 80mm to 92mm.

 

As I do for all my instruments, before starting to varnish, I set up the cello and had it played for a week in the white. As everything was fine I removed the whole set up, including the fingerboard and post and began the varnishing process. It took almost two months and half of the time it was under UV light where I kept the cello humidified. Throughout the two months I had issues with some persistent open seams which I attributed to heat in the UV cabinet.

 

Since I discovered the projection problem I removed the front for a thorough inspection and found the front arching quite distorted. All the corners, specially on the treble side, curled up and the cross arching had flattened, causing the f-hole wings to raise above the central area. I measured the arching heights both top and back and found that they had dropped by 3-4 millimiters. Since then I clamped the front onto the ribs to keep it flat as well as setting the old post, which I had to cut down a couple of millimiters in order to stand it up. In the last 5 days I've managed to gradually pull the post to its correct position and that has helped the projection to go down by 5mm, so that it is now 87mm.

 

I was told by the supplier that the wood was seasoned and ready to be used.

I haven't glued the top back yet and am wondering if this has happened to anyone here. Any suggestions would be most welcome!

 

 

www.protaniviolins.com  

[Marilyn Wallin]

Hello and welcome,

The arch becoming distorted to any noticable degree is not something I have ever experienced. This must be very alarming to observe. I have had arching heights change slightly over time, but they usually go up a bit. Can't say that I have ever noticed one go down much, if at all. I have had f-wings rise or fall slightly as well, but over a period of many years, and I usually attribute that to a too tight or too loose post for a period of years.

 

I have had neck projections wander in the varnish and drying box process during the finishing of the 50 cellos I have made. I now stabilize then with an inlay, so they barely mov at all. But even before that, I never had one change as much as 12 mm. The max I have had to deal with is about 4mm on a cello.

So my guess is that the arch distortion is a large contributing problem in the changing neck projection.

 

If this were happening to me, I would probably take the cello mostly apart and let everything settle. For several months. Then return to it to try again. I would secure the neck to a block of wood so that it cannot warp without the fingerboard to support it. Wish I had more immediate fixes at hand, but that is what I would do in your situation.

 

Did you use a water based ground? for example, potassium silicate, known sometimes as waterglass? Did you seal the inside with anything? 

 

Marilyn Wallin

violinmaker

[David Burgess]

Two prime candidates, assuming the wood was as dry as the seller claimed:

 

The drying box wasn't humidified to the extent you thought it was, to keep it near the level the cello was constructed in.

 

You coated the inside or outside with something which contracts significantly, like casein, hot glue, or egg white.

[Conor Russell]

Hi,

 

I would do as Marilyn says, let everything settle down, and then rebuild the cello as you might do with a repair.

 

I would expect to replace the top block and reset the neck, and possibly shorten the bottom ribs too.

[christian bayon]

 

I was told by the supplier that the wood was seasoned and ready to be used.

www.protaniviolins.com  

I'm uses to say "liar as a wood-dealer"

[~ Ben Conover]

Sounds like the front has taken on a life of it's own, maybe ask your supplier for a refund.
How did you prepare the gluing surfaces for the front to ribs ?
 

[David Burgess]

Get a new supplier and make a new front for the Cello.

Ben, whatever is going on, it's probably not just the front. If all the wood was of equal age, and not sufficiently dried in the first place, I'd expect the top wood to be the most dry and stable.

 

Lets say that an instrument is built in a 50% humidity environment, and then spends a lot of time in a 100 degree varnish drying cabinet. Without any added moisture, the drying box will be around 15% humidity. The back will shorten (raising the neck projection), and the top arching will go down (also raising the projection measurement).

 

These things will trend in the direction of normal, after enough time and exposure to normal humidity levels, but varnish can have a tendency to lock things into a shape, and these are wild cards I'd rather not deal with. I've learned (through the school of hard knocks) to keep drying box humidity above 40%, except for brief periods.

[Filippo Protani]

Thanks for your suggestions everybody.

 

The inside of the cello was sealed with a coat of potassium silicate. The outside had a coat of gelatine as suggested by Hammerl for the use of their water stain, all of which I've used on most of my instruments.

[~ Ben Conover]

I edited that just before your post David.

fp,
Who was the supplier ?
As part of a large order last year, which was 99% excellent, I got a gorgeous one piece violin back, but it had a real twist in the grain, so I have not used it and will send it back. Maybe the internal grain stress of the wood was released with sealing or varnishing your Cello.
Got a photo of it ?

 

[DarylG]

How much would a back really shorten in the lightbox? I know an 8ft stud will only shrink a 1/16" from green to 8% MC. I think some of the neck movement (on necks that aren't pinned) comes from the accordion effect of the heel. One side of the heel is glued to the long grain of the neck block and the other side is free to expand and contract like an accordion. 

[NewNewbie]

Can you tell us what the plate's graduations are?

Are the plates too thin?

[NewNewbie]

How much would a back really shorten in the lightbox? I know an 8ft stud will only shrink a 1/16" from green to 8% MC. I think some of the neck movement (on necks that aren't pinned) comes from the accordion effect of the heel. One side of the heel is glued to the long grain of the neck block and the other side is free to expand and contract like an accordion. 

Yes neck accordian has a part to play.

 

Also the problem is one of 'amplification'.

A very small change at the neck root can end-up a big change at the bridge.

 

Look at the process of carving a neck mortise/mortice.

A very small amount of wood, only a shaving or two, removed at either the top or bottom of the motrise/motice will end-up as huge amount of movement at the bridge.

[NewNewbie]

I'm uses to say "liar as a wood-dealer"

Yes I never rely 100% on what anyone says, IF I don't have to, so I track the moisture levels of wood once I get it.

[David Burgess]

How much would a back really shorten in the lightbox? I know an 8ft stud will only shrink a 1/16" from green to 8% MC. 

It's a minor contributor, but easily measureable. I'd need to dig through some old notes to come up with numbers. Chris Dungey was the person who first turned me onto this, how changes in back length were much larger than either of us expected.

[Salve Håkedal]

Strongly flamed wood will change more than plainer wood cut perfectly along the grain.

[DarylG]

Strongly flamed wood will change more than plainer wood cut perfectly along the grain.

 

Ah, that's true. I was thinking that the change in length of the top and back would be similar but the difference between a strongly flamed back and a straight grained top could be enough to show up in the neck projection. Thanks.

[NewNewbie]

Ah, that's true. I was thinking that the change in length of the top and back would be similar but the difference between a strongly flamed back and a straight grained top could be enough to show up in the neck projection. Thanks.

What if the back was seasoned correctly, and the top was not, or vice versa?

[Filippo Protani]

The thickness of the front is 4mm in the central area between the f-holes, graduating down to 3.3mm towards the top and bottom blocks. The 'lung' area is slightly thinner, about 3mm. The edge area ranges between 3.2 to 3.5 but on the C bouts is 3.8-3.9.

 

As for the back, it's not very strongly flamed.

This morning, after your comments, I checked the heel with a straight edge between the button and the thumb stop area and there is a definite dish which wasn't there before, so I suspect the accordion effect suggested by Daryl has happened. Do you think it is a good idea to remove the varnish from the heel so that it is more able to regain some moisture?

 

Another interesting point is that it's been rainy for the last three days and that seems to have helped the projection to drop by another couple of millimeters. I also unclamped the table from the rib structure and the curling of the corners didn't show as dramatically as before. 

Thanks again, it's nice to have such a variety of knowledge.  

[DarylG]

What if the back was seasoned correctly, and the top was not, or vice versa?

 

I think wood that isn't dried and seasoned properly is very unpredictable, especially with a complex shape like a top or back. I was speaking more of the changes properly dried wood undergoes with changes in air humidity.

[NewNewbie]

Yes I see that from your post.

I just was thinking about what could be going on here in this specific case.

Maybe the wood dealer was only half right?

 

Of course we are assuming that the wood has the grain running true through it, which might not be the case.

 

We'll have to wait until we get more information.

[David Burgess]

What if the back was seasoned correctly, and the top was not, or vice versa?

Some accomodation to shrinkage would come by the arching being pulled down, but there comes a point where the edges, constrained from shrinking by the ribs, become prone to developing cracks, particularly on the top.

 

From what I've been told, the Beckers would open some edges when they transfered fiddles they were making between humid Lake Pickerel, and the extreme indoor dryiness of winter in Chicago.

[Bruce Carlson]

Some accomodation to shrinkage would come by the arching being pulled down, but there comes a point where the edges, constrained from shrinking by the ribs, become prone to developing cracks, particularly on the top.

 

From what I've been told, the Beckers would open some edges when they transfered fiddles they were making between humid Lake Pickerel, and the extreme indoor dryiness of winter in Chicago.

Hans Weisshaar did the same thing when he imported school grade cellos from Germany to Los Angeles for exactly the same reason.

 

Bruce

[David Burgess]

Heh heh, those were some of the treasures they started me out on.

 

Hans put you on translating the Saconni book right after you got there, didn't he?

[Melvin Goldsmith]

Hi Filippo

Welcome!

First I must say that I am sure everything will be fine with your cello but I understand your
feeling.

 

I notice you said you removed the fingerboard before starting the varnishing process. I assume you
fitted a 'false fingerboard to the neck immediately once the proper
ebony one was removed? If one was not fitted I would expect the neck
to move to the degree you describe and in other ways too!

I re read this thread a few times. I doubt from what I read so far that your wood is unseasoned. I also
think you are experienced enough to sense this earlier during the building
process. I would expect free plates with differing internal and
external coatings to distort a bit temporarily after prolongued light
box exposure and anyway the outside of the plates could generally be
expected to be drier causing the effects you describe. I would only
advise to measure the arch heights with the whole instrument fully
glued together.

As others have already said it will be
a good idea to let this instrument settle for a while and then get it
into the shape you want it. Probably it will be all the more stable
in the long run after all this...and the outcome will be positive in
many ways.

 

And also please don't forget that some makers ( including me) will
actually be wanting to re do quite a lot of things including
projection, grads, archings etc way after the varnishing process so
you have some company in your work!

 

[Bruce Carlson]

Heh heh, those were some of the treasures they started me out on.

 

Hans put you on translating the Saconni book right after you got there, didn't he?

That's right, initially the chapter on varnish ....