By Patrick Sullivan
About 40 centuries ago, Egyptian artisans started using a sticky adhesive derived from collagen in animal skins, which we now call hide glue, to craft chairs for the tombs of pharaohs. And for thousands of years after that, hide glue played a crucial role in furniture-making and other woodworking around the world. The invention of synthetic adhesives in the 20th-century drove animal hide glue out of workshop after workshop and factory after factory. Today, this humble slaughterhouse byproduct is increasingly rare.
But hide glue is still holding on strong in the world of violin making and repair. And when luthiers have glue questions, they often turn to Eugene Thordahl, owner of Bjorn Hide Glue in North Carolina.
Thordahl doesn’t play an instrument himself, but he enjoys talking to violin makers. “They’re interesting,” he says. “They want to be precise in what they’re doing and making.”
“High-quality glue is critical for the long-term health of a violin.”
Hide glue is the standard for violin-family instruments, says luthier Christopher Germain, director of the Violin Makers Workshop at Oberlin College. In part that’s because hide glue is reversible—it’s easy to undo a bond without harming wood. That’s pretty handy when you need to open an instrument. Indeed, using other kinds of glue for repairs can wreak havoc on a violin—it’s “the act of a thug who doesn’t care about the future of that fiddle,” as one luthier put it in an online forum.
Hide glue also forms a very stiff bond. “It’s not going to move,” Thordahl says. “The white glues can creep or shift. But hide glues form a bond as strong as the wood itself. It should last the lifetime of the violin.”
Thordahl, now in his 70s, has been in the glue business for decades. He launched his enterprise in the mid-1980s, after the glue company he was working for went out of business.
The Charlotte resident studied food technology in school and developed expertise in quality testing. Glue making, he says, can look messy to outsiders, but it requires precision. It starts with skins from cattle, horses, and other animals. They’re cleaned, treated with chemicals, and slowly heated to extract collagen.
“It takes a lot of water and a lot of time,” he explains. “Then you dry it, grind it, and test it for grading and quality.”
Thordahl doesn’t make his own glue, but he knows a lot about it, which allows him to source high-quality glue and give advice on how to use it. These days, his small company, which has just one or two employees, typically serves artisans, hobbyists, and people restoring antique furniture. Luthiers from America, Canada, and Europe frequently call him with questions about choosing the right grade and extending drying time, as well as local considerations. “Hide glue reacts to temperature, humidity, and other ambient conditions,” Thordahl says. “If you’re building a violin at the North Pole, that’s different than doing it at the South Pole.”
David Burgess, a Michigan luthier, says Thordahl is a good source of glue—and has important information about how to use it. Burgess has called Eugene repeatedly with long lists of questions about glue strength, gel retarders, and other issues. “He seems very willing to take the time and talk through the issues,” Burgess says. “He’s been helpful.”
High-quality glue is critical for the long-term health of a violin, according to another one of Thordahl’s customers, Ontario luthier Mat Roop. “The issue is not whether the glue will hold right now,” Roop says. “It’s what happens in a year or five years or 50.”
And finding high-quality glue is getting harder, according to Hans Johannsson, an Icelandic luthier, who first consulted Thordahl back in the early ’90s. “I contacted him because there aren’t too many good sources of hide glue left,” says Johannsson, whose grandfather was a cabinet maker who taught him how to mix up hide glue—and sometimes added beer to keep it from gelling too quickly.
Johannsson keeps two sacks of Thordahl’s glue in his workshop, alongside some big blocks of century-old hide glue he obtained from a bankrupt furniture factory in Luxembourg. “I have to smash it with a hammer before soaking in cold water,” he says. Johannsson believes that it takes more than profit motive to explain Thordahl’s eagerness to share his expertise. “I think he’s concerned about the knowledge not fading away,” Johannsson says. “I think that’s a priority for him.”
Luthiers pay Thordahl about $60 for a five-pound bag of high-clarity hide glue. That’s enough to last for years, and it keeps more or less indefinitely. The glue comes dried, in flakes. To use it, you start by soaking it in cold water and then heating up the mixture.
Hide glue is typically sold in different gram strengths. “They all stick wood to wood, but they do it differently in different conditions,” Thordahl explains. Higher gram strengths are stronger, and they also dry faster, which means a shorter “open time”—the period you have to work with the glue. Violin makers typically use the 192- and 315-gram strengths, Thordahl says.
Germain explains that makers typically use higher gram-strength glues for top and back joints. “Weaker gram strength glues are effective for rib seams and fingerboards, where the preference is for the joint to pop open under stress,” he says.
Hide glue sets up and gels very quickly, so it must be applied, clamped, and cleaned up very quickly, Germain says. To lengthen working time, hide glue is typically applied to warm surfaces. Johannsson, for example, uses a warm piece of zinc to heat his workpieces. “By the time you’re ready to glue, you want the rib structure and the belly to be quite warm,” the luthier explains. “That gives you time to work.”
And since hide glue reacts to moisture and heat, it can be converted back from solid to liquid form even after a century or more, according to noted American woodworker W. Patrick Edwards. That helps explain why we still have so many working violins that were made hundreds of years ago. But hide glue has another great advantage that also contributes to the longevity of stringed instruments, according to Germain.
“Hide glue can be brittle and if an instrument suffers a shock, the glue joint will often break open cleanly,” he explains, preserving the violin itself from breaking.
Still, for all its advantages, hide glue is vanishing quickly, even among makers of other types of instruments. Guitar makers, for example, have increasingly moved away from its use. “If Eugene stopped selling his hide glue and maybe so did a few others in the US and Europe, what would happen to us?” Johannsson muses. “Maybe we’d have to start making our own.”
Indeed, many glue plants in the US have shut down, and even Thordahl says he’s not sure his product has much of a future. “In another 100 years, I can’t imagine hide glue being in much demand at all,” Thordahl says. “It won’t grow, that’s for sure. But the violin people will probably be some of the last ones still using it.”
This article originally appeared in the July 2018 issue of Strings magazine.