You often hear the expression that a horse past its prime is "sent to the glue factory," but is there any historical basis for turning Mr. Ed into Elmer's? Yes. In fact, up until the second half of the 20th century, most glue was really and truly made from animals. But how did the horse in particular become associated with that sticky goop of childhood lore? It turns out humans have been making animals into glue for thousands of years.
The first commercial glue factory opened in Holland around 1700, using animal hides to make the glue. Because of their size and strength, horses are a great source of collagen, the main ingredient in most animal-based glues. (In fact, the word collagen derives form the Greek word for "glue.") As a result, during the 18th and 19th centuries, ranchers frequently sent their incapacitated equines to glue factories for processing. They've since discontinued the practice, as synthetic glues have become more commonplace and advanced - even that famous all-purpose Elmer's is in fact synthetic. (In truth, most unwanted and dead horses are now sent to the slaughterhouse for their meat, which turns out is a popular food in most places but America.) To fully understand the glue-making process, take a closer look at why and how people made glue out of horses.
Collagen is a key protein found in connective tissues like cartilage, ligaments, tendons, bones, and hides in animals. Although glue could technically be made from any animal, horses are large and muscular, so they tend to have a lot of collagen. Through processing, collagen can be made into a gelatin that sticks when wet yet hardens when dry. In other words: glue.
To begin the process of making glue, glue factories first collected horse parts from various slaughterhouses, tanneries, meat packing companies, and other places specializing in horse hides, skins, tendons, and bones.
After getting collected, the parts were washed to remove any dirt and soaked to soften them up. The resulting material, known as stock, was put through a series of water baths with progressively more lime in each one in order to make the materials swell and break down. Everything was then rinsed, and weak acids were added to neutralize the lime.