Instrument Seams
Why do seams open? What do I do if I have an open seam?
How do you know a seam has opened? Perhaps there was a “pop”, or your instrument started buzzing. Sometimes it is more obvious, like the large 4” gap between the back and the rib on the bottom of your cello. The seams are the equivalent of the canary in the coal mine for string instruments in the winter. They are the release valve that opens, hopefully, before the tension causes the top or back plate to crack.
What is the “seam”? Luthiers use hide glue to attach top and back plates to the rib assembly. The surface area of the ribs is a few millimeters wide, so the glue must be strong enough, and flexible enough, to keep that “seam” closed. The seam is the surface area where the top or back plate attaches to the rib assembly.
Why do luthiers use hide glue for the seams? Hide glue is an animal-based product with some unique properties. Hide glue is water soluble. We purchase it in granules or plates which we dissolve in water and heat up to melt. The glue bonds can be broken with alcohol. Luthiers build violins with the knowledge that we will have to repair them later. The hide glue bond can be broken with alcohol when applied correctly. The glue comes in different strengths. Luthiers use different strengths of hide glue to attach different parts of the instrument. The neat thing about hide glue is that it seeps into the pores of the wood and can pull two pieces of wood together as it dries. For the center joint on a plate, luthiers use a stronger glue because they don’t want that joint to open, ever. For a seam between the top plate and the rib, a weaker bond allows the seam to “pop” open if the tension on the top plate could cause a crack.
What causes seams to open? Air dried wood that we use to build instruments loses most, but not all, of its moisture content. The final moisture content varies with the ambient humidity the wood is stored in. On the violin, the wood will continue to expand as it absorbs moisture from moist air; or contract as it loses moisture in very dry air. In climates like Vermont, where the humidity levels vary dramatically from summer to winter, the top plate can expand and contract enough to create tension because the glued seam restricts the plate’s ability to expand and contract. If the seam opens, the plate can expand or contract without creating a crack. If the seam does not open, the plate will split, usually along one of the grain lines. The softer spruce wood in the top plate is more likely to crack than the maple back because it reacts to the changes in humidity.
Seams can also open if the surface of the ribs or the plate is uneven. When luthiers take the top off an instrument, the surfaces become uneven. Small slivers of wood can be dislodged when the glue joint is broken. Wood also dries unevenly. A portion of the rib may be shorter than another section, and the plate will only adjust to those changes to some degree. When the surfaces are not even, the glue bond is weaker. The weak bond breaks more easily and the seam “opens.”
How do luthiers fix an open seam? If the seam opens due to humidity changes, the luthiers will reapply hide glue to the open section and clamp the seam (gently) closed. The glue typically takes 4 hours to dry. If the seam opens because the plate and rib are no longer flush, the luthiers will recommend appropriate repairs to create a flat surface area the glue can join.
How do you prevent an open seam?In the winter, we use humidifiers to add moisture.In the summer we use dehumidifiers to reduce moisture.If the instrument is stored in its case, case humidifiers can be used.Common brands include the Stretto, Artino, and Boveda.Humidifiers which require adding water should be used along with a hygrometer to monitor the actual humidity level in the case.Newer humidity packs like the Boveda system, control the moisture level in the case to a certain humidity level (49%) for violins, violas and cellos.Unlike the humidifiers, Bovedas can be used to absorb moisture or release moisture.The size of the Boveda pack, however, works best for a violin or viola case.Multiple packs are needed for a larger cello case.If the instrument is stored outside the case, a room humidifier or dehumidifier is necessary.The size of the room will determine how big a humidifier or dehumidifier you need.