What Causes Cracks?
There are three categories of body cracks. Cracks caused by: improper climate control, those resulting from impacts with foreign objects and cracks forming as a consequence of improperly seasoned wood. All cracks fall into one of two categories: open or closed. A closed crack is one that will leave no gap when the instrument is properly humidified. Conversly, an open crack exhibits a gap when properly humidified.
Most cracks we see here at Finger Lakes Guitar Repair are related to improper climate control or impacts with foreign objects. These cracks typically fall into the category of closed cracks (cracks that have no gap at 50% relative humidity). When an instrument becomes too dry the wood shrinks as it loses its moisture causing cracks to develop in the body of solid wood acoustic instruments and fretboards of all instruments. Stringed instruments require a consistent relative humidity of 50%. Because rapid changes in temperature will cause finish checking as well as cracks in wood, instruments also need to be kept at a steady temperature.
How do You Repair Closed Cracks?
Although there are multiple methods of repairing closed cracks we prefer to glue cracks with hot hide glue. We then reinforce the crack from the inside of the instrument with small diamond pieces of wood (called cleats or cross patches) in order to prevent the crack from reopening in the future. Touching up the finish is usually necessary if the repair is to be invisible.
Open cracks typically form from the improper seasoning of wood or improper climate control during an instrument’s construction or repair. After lumber is harvested it is seasoned. Seasoning essentially brings the moisture content of the wood down to a level that is sustainable at 50% relative humidity throughout the life of a fretted stringed instrument. When many of today’s vintage instruments were constructed, today’s exacting methods of seasoning lumber and climate control wasn’t possible to consistently achieve. Therefore many vintage guitars have dried out over the years and developed open cracks (cracks with a visible gap at 50% relative humidity).
How do You Repair Open Cracks?
Although there are many methods of repairing open cracks the method we here at Finger Lakes Guitar Repair prefer is to inlay a splint into the open crack. Instead of attempting to glue closed a crack that will only reopen it is more desirable to fill the crack with a thin piece of wood or “splint”. Splints should always be touched up in order to hide the repair.
It is possible to remove a plate (top or back of an instrument) and remove the braces in order to essentially make the plate smaller to counter the shrinking of the wood, however, this method is costly and should only be employed when an instrument is a valuable collector’s piece or must be disassembled for other restoration work.