Slate’s archives are full of fascinating stories. We’re republishing this article because it remains a reader favorite. It was originally published Feb. 14, 2012.
Two horses that stumbled while filming racing sequences for the first and seventh episodes of HBO’s Luck were deemed inoperable and euthanized. Dead and dying horses are often said to be “sent to the glue factory.” Why are horses good for making glue?
They have a lot of collagen. Collagen is a key protein in connective tissues (cartilage, tendons, ligaments) as well as hides and bones. It’s also the key ingredient in most animal glues, as it can be made into a gelatin that’s sticky when wet but hardens when it dries. The word collagen actually derives from the Greek kolla, meaning glue, and the suffix -gen, meaning producer. As large, muscled animals, horses contain lots of this glue producer. Horse glue isn’t generally better or stickier than any other kind of animal glue—indeed, an elephant could be used to make even more glue than a horse—and animal glue is more often made from pigs and cattle.
Humans have used animals to make glue for thousands of years. The oldest glue discovered was a collagen-based adhesive that was 8,000 years old and used to hold utensils together. It wasn’t long before these animal glues were used to repair broken pots and, in one instance, glue ivory eyeballs into statues’ eye sockets. Other adhesives were made from egg whites, tree sap, tar, and beeswax, which the ancient Romans used to caulk the planking in ships. In the middle ages, the mysterious author Theophilus compiled instructions for producing various types of glue, including glue made of skins and stag-horns, glue made from fish bladders (now called isinglass), and “the glue of cheese” (now called casein glue). For fish glue, Theophilus recommended the bladder of the sturgeon, but alternatives included eel skin and “the bones of the head of the wolf fish.” The first commercial glue factory, started in Holland in the early 18th century, used animal hides. Glue made from blood, which worked because of blood’s coagulative properties, worked particularly well when bonding plywood, and was commonly used for this purpose until the mid-20th century.
Animal glue, popular for thousands of years, has fallen out of fashion in recent decades. Over the second half of the 20th century, synthetic glues have become more advanced, as they are cheap, uniform in quality, and have longer shelf lives. White all-use glues like Elmer’s are made of rubbery mixtures called polyvinyl acetate emulsions, and while the Elmer’s mascot is a smiling bull, the company says that it doesn’t use any animal parts. Some manufacturers still distribute animal glues. Bookbinders are fond of them because they’re slow to set, allowing binders plenty of time to work. But those manufacturers represent only a small portion of the hundreds of companies that make up the multibillion dollar industry.
These days, dead and unwanted horses aren’t sent to the glue factory as often they are sent across the border, slaughtered, and harvested for their valuable meat. (The United States’ longtime ban on slaughtering horses for human consumption was lifted this past fall, but the practice remains taboo.)* Other horses are rendered into meat for greyhounds and food for large cats at zoos. Hippophiles might cremate favorite horses—in some states it’s illegal to bury them—while others simply take the horse to the local dump.
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Explainer thanks Jerrold Winandy of the University of Minnesota.
Correction, Feb. 15, 2012: This article originally and mistakenly stated that slaughtering horses for human consumption is illegal in the United States. The ban on slaughtering horses for this purpose was lifted this past fall. (Return to the corrected sentence.)