Why and where do instruments crack? - Oct 2002


When humidity is under 40%, solid-wood, acoustic instruments dehydrate, shrink & crack. The soundboard, generally made of softer woods, is both weaker & more prone to dehydration and cracking than other parts of an instrument. Therefore, the soundboard is likely to crack first. Cracks usually occur in the wood grain at stress concentrators: 1) at corners of guitar bridges, 2) next to the neck & the saddle of violins, 3) from F-holes in violins & mandolins, & 4) in center seams of book matched tops. 


Soundboards are also vulnerable to cracking when internal bracing fails. For example, the soundboard can crack along its grain at the side of the fingerboard if the block of wood that supports the neck joint breaks loose & shifts position. Similar cracking in the soundboard is possible at other crucial bracing points. The sound post in violin-family instruments, if too long, can put too much pressure on the underside of the soundboard. Excessive tension & abrasion can crack the top along the grain between the treble foot of the bridge & the treble-side F-hole. 


The backs & sides of instruments can also crack. However, those cracks usually result from impact or puncture. Wood in the backs & sides of instruments, while often a little thicker than the tops, is generally only 2-3 millimeters thick. A moderate knock is enough to split the grain. Very hard impact can crack the wood across its grain. Because this kind of damage depends on impact, it can happen anywhere on the body, but is more likely on the outward curves of the instrument. Conversely, a crack on the inside curve of the treble side of the waist of a guitar is almost always from ‘puncture’ instead of impact. When a player presses a guitar onto a sharp, hard object in his/her pocket, very little pressure is needed to crack the side of the guitar; many guitarists have unknowingly cracked guitars on their own keys. 


Cracks also form in glued seams of instruments. In violins & violas, contact between the player’s hand & upper treble bout exposes seams to oils that attack the glue & makes the seams separate earlier than other seams. And, while ideal for violin building & repair, hide glue degrades over time & other seams also eventually separate. Most violins need to have seams re-glued with age. While other instruments also suffer seam separation, the glues in guitars, mandolins & other instruments are less environmentally susceptible, & seam separation usually takes longer & is more frequently related to neglect. Exposure to extreme & sudden heat or humidity variations & exposure to solvents (e.g., insect repellent, cologne, alcohol) accelerate glue failure. 


Cracks are also common at tuning-post holes. On both machine-type & friction-type tuners, string tension torques the posts/pegs & stresses the inside surface of the holes. The holes, themselves stress concentrators, intensify the stress. Any weakness of the wood at the inside surface of the hole can allow cracking. 


Another crack type unique to guitars is ‘pick-guard cracking.’ Synthetic pick-guards shrink with age. Guitar builders who were the first to use synthetic pick-guards were unaware of the shrinkage that would occur, & adhered the pick guards to the raw wood of a soundboard before lacquering the guitar. Without lacquer to buffer the wood from shrinkage, the tension tears soundboard’s wood. Cracks form in the wood grain under & around the pick guard (repair of these cracks are generally done under warrantee). 


What Can I Do?

If your instrument cracks, visit a qualified repair person early. Quick action can prevent crack growth & dirt accumulation that makes repair difficult.