Upper back seam separation

Steve Swan · #1 02-20-2007, 10:04 PM

I saw something in a friend's bass today that has been troubling me all day. The more that I think about it, the more questions come to mind. This bass is German-made, about 6 months old, and has a X-brace with a transverse brace at the upper bout bend. The owner lives very close to the coast in central California, so low humidity is never a problem. He has about 7 or 8 other basses (new and old) in the house and none of the others has ever suffered any trouble with opening seams or cracks while in the house.

When my friend put his bass facing into a corner after playing it for me, I noticed a light color right at the upper bout center seam, above the bend. The seam was indeed wide open for about 6 or 7 inches, bounded by the neck block and the transverse brace at the upper bout bend. I could easily slip a piece of paper into nearly the entire length of it as deep as the thickness of the back plate. There is a wide center strip on the inside of the back that stops the corner of the paper from going any further. Looking at the rest of the back and the top, we looked for signs of stress on the center joints or brace positions, but didn't see any. At its widest opening point, this open seam looked wide enough to put a business card in.

I had my friend call the retailer who sold the bass to him in August. The owner of the retail business seemed remarkably unconcerned about it, saying "flatbacks open up a lot" and "It's not really open because the center strip keeps you from moving the two halves in and out past each other." (???!!!) I asked him if this was a manufacturing warranty problem that he could help with or if he could replace the bass with another one since it was less than 6 months old. He said "No" and excused himself from any responsibility for the bass.

Anybody have any idea of what's going on with this particular bass? The upper bout back shows some concavity above the bend, but most noticeably right at the bend. It is a fairly beefy orchestral type instrument and very clean in its workmanship.

arnoldschnitzer · #2 02-21-2007, 08:00 AM

Steve, you say your friend lives near the central California coast, and lack of humidity is never the problem. I beg to differ. I understand there are periods when the wind blows from the inland toward the sea and the air can be quite dry and hot. If this bass was made in Germany it was probably never subjected to low humidity. My guess is that the bass became very dry and the weakest spot opened up. The concavity in the back is indicative of shrinkage in the wood as well. This is why it is important to acclimate basses to the local climate before for a while they are set-up. If I was the retailer in this case I would likely offer to do a minor repair for free, such as fill in the opening with veneer and glue. I don't expect flatbacks to stay pristine for long...

mojoluthier · #3 02-21-2007, 12:23 PM

We here in California had an unusually deep spike of low humidity this last month. Still nothing like what happened in the east, but I saw a lot of shrinking in the basses I shepherd. I reccommend leaving it alone as long as nothing is rattling.

I consider Flat-Back basses, which are built a bit like guitars where the back is concerned, to be so prone to cracking that it is to be expected, and efforts to avoid it are largely wasted. I tell my customers to accept it, and I tell them going in. The construction goal is get the crack to happen in the center, and to this end there is often a softer wood center seam piece included in the join. This is also done in guitars for the same reason. I learned this early in my guitarmaking career when exposed to the Spanish trdition, though the knowledge seems uncommon among my peers.

Steve Swan · #4 02-23-2007, 09:57 AM

My friend lives just below Monterey, about 1/2 from a beach with a lot of mist coming up from it. His home is in a wooded area with tall trees and lots of ferns. A retiree, he's at home all day with his wife making tea, cooking meals, etc. I don't think that low humidity is ever much of a factor in this home.

The bass came from a dealer 400 miles to the south between Long Beach and San Diego. They get tremendously dry Santa Ana winds coming off of the desert areas to the east periodically from August through the end of October. The bass was purchased in august before these winds really had much of a chance to kick in, so I don't think that it got overly dry at the bass store.

If it were primarily a question of low humidity, I think that I would have seen stress in the lower bout part of the back that is unsupported by the X-brace and some sinking of the summer grain in the top with some sign of stress around the soundpost and tail areas. The lower part of the back is flat. Only the area around the brace for the upper bout break shows that concavity.

Any other ideas on contributing cause(s) for this odd upper back seam separation?

arnoldschnitzer · #5 02-25-2007, 01:57 PM

The crossbars prevent the back wood from being able to move with climate changes. The result is open center seams or splits in the back wood. In a very old bass this is not always a problem as the back wood has had time to become more stable. With new basses, the back wood needs to be extremely dry before using. Even when that is the case, splits and open seams are a fairly common occurance.