Adhesives, you might say, have a bit of a sticky history. It’s easy to think about glue as we encounter it today: From bookshelves to flower beds to arts-and-crafts projects, we think of glue as an object to repair other things. It’s the stuff that sticks one thing to another–simple enough. Technologically, however, glue represents a complex series of decisions that our genus, Homo, has been making for millennia.
Adhesives imply not only a sophisticated understanding of three-dimensional space, but the fire technology to actually make the stuff. Moreover, making them requires a good working knowledge of the properties of an environment. In other words, a person has to know what he or she wants to have stuck together and how to create something that will inexorably bind two other objects.
Today, the process of actually making glue is outside most people’s experience, but the making and manufacture of all things sticky was an important turning point in the history of technology.
Adhesive technology stretches much further back in humanity’s history than a cursory glance at Home Depot’s glue aisle might suggest. From evidence in the archaeological record, we know that our Pleistocene ancestors in southern Africa made and used glue-like adhesives as early as the Middle Stone Age (280,000-25,000 years ago). Evidence of glue use in the Pleistocene can be found at Stone Age sites in Italy (the use of birch bark tar) and eastern Africa (red ochre stains on stone stools). Archaeological sites at Sibudu Cave and Rose Cottage in South Africa, however, offer a look at a particularly interesting chapter in the history of sticky substances.
Around 70,000 years ago, people made what archaeologists call “compound adhesives”–a means of attaching stone tools to hafts (handles). This technology, archaeologist Dr. Lyn Wadley explains, requires the ability to “mix glue from disparate ingredients, mentally rotate segments, and maintain fire temperature.” Artifacts from Sibudu Cave and Rose Cottage show traces of a plant gum-resin, suggesting to archaeologists that plant-based adhesives, together with twine, allowed the creation of sophisticated tools like spear points.
Wadley and her team of archaeologists argue that the variety of glues found at these sites indicates that different adhesives might have been made for different purposes. For example, a handheld thrusting spear would have needed a glue that could withstand many thrusts without breaking. In contrast, a projectile-like tool–say, a stone-tipped javelin–would have needed an adhesive that allowed the tip would break off after the Pleistocene hunter hit prey, slowly incapacitating the hunted.
To find out what kinds of resources Middle Stone Age people would have used, Wadley concocted a number of experimental glues based on the residue. “My experiments involved mounting stone tools on wooden handles using natural adhesives, all of which had plant gum from Acacia karroo as their base,” she noted in her report. “Simple adhesive comprised Acacia gum alone; compound adhesives combined Acacia gum with powdered ochre and sometimes a small amount of beeswax.”
Even something as “simple” as gum resin becomes complex when made into a tool. Wadley remarks:
The consistency of the gum is variable; drier gums are easier to work with than runny ones, which are very sticky. Gum that is fairly dry can be molded around a tool and air‐dried with no further processing. Wet gum is difficult to control, and it needs to be dehydrated over a fire to prevent it from dripping off the haft, which would cause the stone tool to fall from its haft.
Glue-making in the Pleistocene, then, combined environmental knowledge of a plant-based resin with an understanding of how to create glues by managing fire and temper. “Acacia gum is water soluble and hydroscopic,” Wadley explains, “consequently, damp conditions will cause this simple adhesive to become tacky, allowing the stone tool to fall from its haft.” Perfect for hunting mammoth!
We might look at today’s plethora of sticky suspensions as a demonstration of our technological prowess and cultural sophistication. But, like most technologies, the impetus for glue is nothing new and, in reality, it is a tool that combines a whole host of other technologies. Adhesives, it seems, aren’t going anywhere: We’ve stuck with making glue for over 70,000 years.