Why do seams open and braces come loose?

Sept 2001 -- While apparently simple, acoustic instruments have geometrically complex shapes, bracing that's fragile relative to the stress it endures & materials of construction (wood & glue) that are susceptible to temperature & humidity excursions, string tension, impact & time. Under these circumstances even when they receive good care instruments can develop problems. With marginal or poor care, instruments are almost certain to develop problems; two of the most common are open seams & loose braces. 

 

Temperature: Aliphatic glue, used in guitars, mandolins & other fretted instruments, & hide glue, used in violin-family instruments, are temperature sensitive. At low temperatures, glue becomes brittle & can crack more easily. When the temperature rises, the cracks remain, concentrating stresses (string tension, the tendency of bent sides to straighten, & imperfect contour-matches between the braces & the surfaces that they support). The cracks grow until seams open & braces loosen. At high temperature, glues soften. The rubbery glue, too weak to resist stress, yields & allows glue joints to separate. When the instrument cools, stress concentrates at the separations & opens them farther. So, either hot or cold is potentially damaging. 

 

Humidity: Instrument glues are water soluble. So, in damp/wet conditions, glues absorb moisture, soften & yield more easily to stress. In arid conditions, glues can loose moisture, dry, embrittle & crack. In day-to-day experience, dryness is more likely than humidity to cause glue damage. 

 

String Tension: String tension stresses glue in seams & on braces. Thicker strings require more tension than thinner strings to achieve the same pitch (to tune to an E, a 0.010"-thick steel string requires only 59% of the tension of a 0.013"-thick string: 16.2 vs. 27.4 pounds). Greater tension equals greater stress. While all stringed instruments are susceptible to long-term effects from string tension, those with heavier strings are more prone to seam separations & loose braces. 

 

Impact: Even room-temperature glue at ideal humidity can crack under impact; especially if the impact is especially abrupt. Like Silly Putty , which breaks-off flat if you pull it fast, & stretches ductilely if you pull it slowly, glue is more "brittle" when it experiences abrupt, high-speed impact. And, as described earlier, low temperature & dryness make glue more susceptible to cracking. Therefore, sudden impact has even more potential for damage in low temperature & arid conditions. 

 

Time: Damage from temperature & humidity excursions, string tension & impact is a function of time. With time, glue undergoes permanent molecular changes that can make it more susceptible to failure under stress. The passage of time also gives temperature, humidity, string tension & impact more instantaneous opportunities to loosen seams & braces. 

What Can I Do?

 

To prevent, detect & correct open seams & loose braces: 

 

Protect against temperature & humidity extremes, reduce string tension & avoid impact (a good case helps). Examine seams visually & tap the instrument lightly to listen for rattles caused by loose braces. Seek professional repair for any problems as soon as possible.

 

This article is part of Meadowood Music’s archive of information​ on selection, care, and maintenance of stringed musical instruments.  Some instrument conditions require professional evaluation and repair.