Blog

Adhesives have been used by humans for a very long time. Not hundreds of years, not thousands of years, but tens of thousands of years. While most modern adhesives are not based on natural materials, people living in the Middle Paleolithic (more commonly known as the Middle Stone Age) didn’t have extensive catalogs of silicone, acrylate, hot melt, and other adhesives to order from and needed to rely on nature-based options.

Adhesive technology is simple, and also complicated.

A report released in late February caught my eye, in no small part because of the years that I spent developing adhesives for a Minnesota-based mining and manufacturing company that will remain unnamed. Adhesive technology can be so simple and so complicated all at the same time with most of the knowledge locked up as corporate secrets. I’m still interested in what’s new with adhesives, or in this case, what’s old.

The researchers claim to have found in Le Moustier, an archaeological site in France’s Dordogne region, the oldest examples of compounded adhesives in Europe. The exact age is hard to pin down, but they are at least 30,000 years old, and likely older. This was at a time when Europe was largely inhabited by Neanderthals (and saber-tooth tigers, cave lions, and woolly rhinos).

Compounding through the ages.

One common characteristic between Stone Age and modern adhesives is that they were compounded. Adhesives are always a balance between adhesive and cohesive strength. Honey is high in adhesive strength and low in cohesive strength, while steel is high in cohesive strength but low in adhesive strength. And just like now, we prefer that any exposed adhesive dries or cures and becomes tack free (so that we don’t end up with a mini version of the La Brea Tar Pits). Enter the compounder.

Chemical analysis showed that ochre clay was mixed with bitumen at a ratio of about 50/50. This makes for a tack-free adhesive that sticks well to flint, the stone used to make the tools. The person making this can easily claim to be Europe’s oldest adhesive compounder.

I chuckled many times reading the report. The researchers are archeologists and not rheologists or adhesives specialists, so they struggle to get the language right and make the best measurements on their modern recreations of the ancient formulations. They even overcooked the bitumen in one case and ruined the batch. (Welcome to the club!)

Quest for cutting-tool handles.

The adhesives were not used to adhere stone tools to a stick, but instead to create a handle so that the sharp stone tools could be more easily manipulated. That need hasn’t changed in 30,000 years. I do a pretty good job of taking off my left fingertips in the kitchen with a handled knife. I can’t imagine how stubby my right hand would be working with a naked blade.

Unsolved mysteries.

This type of adhesive is similar to Stone Age adhesives found in Africa, so there are endless questions about how the technology ended up in Europe. Did people take it as they traveled? Was it passed through word-of-mouth or independently discovered? Since Neanderthals and Homo sapiens didn’t leave precise data about when they traveled where, we will probably never know the full answers.

The most fascinating aspect of this compounding was not the compounding itself, but the quest for the raw materials. The flint for the tools can be found close to the archeological site, but the ochre is found 50 kilometers to the north and the bitumen is found 200 kilometers to the south. Were handles really worth the risk of traveling through land that was inhabited by some pretty dangerous animals? That’s a safety risk that modern compounders never have to consider.