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.