The crack is back!

[chopaholic]

Last winter my new MT2V developed a 3" separation of the glue seam on the birdseye maple back, on the tailpiece end. The kind folks at Collings repaired it by getting glue into the crack and revarnishing. The repair was immaculate, just as you would expect.
Well, winter has returned to Iowa, and so has the crack. I keep the mandolin in a closed case with humidifiers at a constant humidity of 48-49% and about 63-65 degrees. I use a room humidifier that works hard to keep the area around 35-45% humidity, depending on how much my furnace is running. Will playing the mandolin in 35% humidity for an hour or two and then returning it to it's case have such an extreme affect on it? What more can I do?
I'm sure Collings will do the right thing to repair it. I'm just curious what the next more aggressive fix might entail. Thanks for any insight.

[Bob A]

If the back has shrunk, it may be necessary to shim the crack
with a thin wedge of maple. I've had this done to a L&H at one time. Shouldn't be a big deal if done by a competent luthier. Meanwhile, sounds like you're doing all you can short of moving to a rainforest.

[bradeinhorn]

is this a crack or a seam separation?

[Glassweb]

Might just be that particular mandolin... it seems like you've done your part in keeping it adequately humidified. As "old man Fischer", a violin maker up in Bellingham, WA once said to me (in a thick German accent), "Vat do you vant? It's vood!"

[G. Fisher]

You could try using a Damp-it. You can get them at most instrument dealers.

You can get one here:
Humidifiers @ Elderly (http://elderly.com/search/elderly?terms=HUMIDIFIER&step=20&query_start=7&page=1&cname=Search)

[sunburst]

From your description of you care of the mandolin, you've done everything right.

Birdseye figure shows almost exclusively on the "flatsawn" surface of maple, and going by that, I assume your mandolin's back is flatsawn wood. That is the main reason I prefer quartered wood. Quartered wood moves less, and that area, where the back is glued cross-grain to the tailblock, is less likely to crack or develop a seam separation.
Birdseye maple is difficult to machine and joint, so the back center seam may have been less than perfect when the back was joined, and once carved into a mandolin back, getting a good joint for re-gluing is even worse.

Anyway, there are some possible explanations of why, despite your good care of a well made mandolin, you have a recurring structural problem, so what do you do now?
Contact Collings again. They built it, you've taken good care of it, they've fixed it, and they'll almost surely want their customer to be satisfied. If they don't want to fix it, or don't think they can fix it (they are builders, after all, not a repair shop) they can get it to someone who can.

The splint or spline mentioned by Bob A might be a possibility, but better might be to separate the back from the tailblock so the opening can be closed and glued (after cleaning all the old glue out) and then re-glued to the tailblock without the tension that has undoubtedly separated the back twice.

In the end, it might turn out to be a piece of wood that just doesn't want to stay glued. Sometimes that happens. Sometimes it's like Glassweb quoted; "Vat do you vant? It's vood!".

[chopaholic]

Thanks for your responses! Brad, to answer your question, it is more of a seam separation than a crack. I doubt that you could even see light through it. The bad part is that the varnish creeps away or splits which is very noticeable.
Greg, thanks for the Dampit suggestion, but I use one of them already. I'm afraid that John's statement about the fact that some pieces of wood aren't meant to stay together could be the case. I guess all the worry is unwarranted because the folks at Collings will make it right. Gee, maybe they'll throw a one piece back on it!
Thanks again guys.