Questions: Gluing in India

by Dale Zimmerman

Originally published in American Lutherie #103, 2010

George Mathai from Kerala, India asks:

My friend is a luthier based in the southern state of India, Kerala. For gluing the various parts and braces for his guitars and violins he uses Franklin Titebond glue and Behlen ground hide glue. However, in one of his custom-made flamenco guitars, a rattling sound brought the guitar back to the shop. One of the right fan braces had come off a bit and this was causing the rattling. For that guitar he had used the hide glue to glue the braces on. Kerala is well within the humid equatorial tropics and perhaps the humidity affected the gluing. What is the best glue that can be used to withstand high humidity?

Dale Zimmerman from Franklin International in Columbus, Ohio responds:

Hide glues such as our ready-to-use Titebond Liquid Hide Glue and most hot-pot hide glues are, and remain, quite sensitive to moisture. That affinity for moisture means that they tend to absorb water when exposed to high humidity or damp conditions, and that increase in moisture causes them to swell and weaken. Bonds that are not subject to stress may show little effect, as the glue loses that moisture and regains its strength as it returns to its original dry state. Where the bond is under stress, however, that stress is likely to cause some opening of the joint while the glue is softened. Ultimately then, the amount of change in a bond over time will be influenced by the concentration and duration of the moisture to which it is exposed and the stress, both inherent in the bond and that created as the wood involved changes in dimension in response to those changes in moisture content.

That means that hide glues are generally considered a poor choice for bonds that are likely to be exposed to particularly humid conditions, especially those that are expected to be under meaningful stress. For such situations, yellow glues, which show little effect from dampness or elevated humidity are a decidedly better choice. ◆