Q: Why is hot hide glue preferred by some luthiers, while others use synthetic glues, and what are its advantages and disadvantages? —Doug MacKenzie, Cary, North Carolina
A: Hide glue, animal glue, and protein colloid glue are names for adhesives made from the connective tissues of various animals. Used by woodworkers in a wide variety of forms for millennia, animal glues are made by boiling hide, bones, sinew, or hoofs and refining the residues into granulated solids. Glue is prepared by dissolving granules in warm water to form a gelatinous protein compound that can be refrigerated and stored for future use. When heated in a double boiler, the gelatin liquefies into a workable glue that quickly sets after cooling to room temperature.
Unfortunately, animal glues offer users many opportunities to produce flawed joints. Room temperature and wood surface temperature must be carefully controlled; working time is short, even under shorter, less-than-optimal conditions; glue strength decreases with each reheating; viscosity requires constant adjustment; and gelled glue will eventually degrade even under ideal conditions, or rot if improperly stored. In addition to difficulty of application, even the best animal glue joints are susceptible to failure in hot and humid conditions, and under cold conditions are less tolerant to shock. It’s little wonder that, up until their recent revival, animal glues had largely been replaced by modern adhesives that offer greater ease of application, longer work time, improved shelf life, and greater consistency.
In expert hands, however, disadvantages become assets. Violin makers traditionally use their freshest glue for permanent joints, such as the center joint of carved tops and backs. The same pot of glue can be reheated and used for less critical joints, such as block assemblies, linings, and the like. Glue that has been reheated many times is ideal for joints intended for future disassembly, such as top-to-rim and fingerboard-to-neck joints. Clean disassembly is easily accomplished by the application of moderate heat or by administering a well-directed shock—old glue is entirely removable with warm water and a rag.
Common wisdom holds that modern polyvinyl wood glues impart a damping effect on vibrating surfaces, while animal glues are vibrationally inert. I attempted to test this theory for several years by making a pair of identical guitars, one constructed entirely with hot rabbit-hide glue and one using only Titebond polyvinyl aliphatic resin glue. The two OMs were built simultaneously and with adjacent-cut tops, backs, sides, necks, and braces; tops and backs were voiced as closely as possible. The pair of newly finished guitars was informally blind tested by quite a few players of all levels of ability, many of whom reported hearing subtle or distinct differences. The funny thing is, player preference was nearly evenly divided between the two instruments. (Unfortunately, follow-up testing of well-played guitars is no longer possible.)
A number of makers whose work I hold in the highest regard swear by the sonic virtues of animal glues, and I freely admit that they may know something I don’t. In my shop, animal glue is typically used on our more expensive guitars, the ones that, not incidentally, also get the most desirable woods. I personally love the sound of these guitars, but I attribute their success to more than the glue. I’ve always believed—others may reasonably disagree—that design, selection of individual top and back woods, and individual voicing accounts for perhaps 75 percent of the sound of a guitar. If neck, bridge, brace wood, finish, and countless other factors account for the remaining 25 percent, how much influence can we attribute to glue? Like fine cuisine, a guitar is about its ingredients. But it’s also about how they are combined.
In the end, it’s the meal that matters.