**Silver birch forest in Gullmarsskogen National Park in Sweden. The trees contain the raw material for the Stone Age equivalent of superglue.** Wikimedia

Stone Age superglue

Birch bark pitch is the oldest all-purpose adhesive in history. Ötzi used birch pitch to affix his arrowheads to the shafts of the arrows; other prehistoric peoples glued broken pottery with birch pitch, or sealed canoes with it. Recently, science has also solved the mystery of how it was produced.

Thomas Weibel

Thomas Weibel is a journalist and Professor of Media Engineering at the Fachhochschule Graubünden and the Hochschule der Künste in Berne.

A human torso protruded from the glacier, slumped forward, the mummified head facedown in the ice. The gruesome find made by German couple Erika and Helmut Simon on 19 September 1991, while they were enjoying a hike on the Tisenjoch, would rapidly become an archaeological sensation. The man who died here, on the border between Austria and Italy, was not a mountaineer who had met with an accident, but a hunter from the late Neolithic Age, and extensive investigations have shown that his death around 5,300 years ago was no accident; it was murder: “Ötzi”, as the Stone Age hunter quickly became known, had been hit in the back by an arrow whose flint tip was embedded two centimetres deep in his left shoulder blade, and the victim bled to death.

TV documentary about Ötzi. YouTube

The Stone Age hunter’s lonely death in the small dip between the Schnalstal and Ötztal valleys has given scientists unexpectedly detailed insights into an age long past. Ötzi’s elaborately sewn clothing, his flint dagger and precious copper axe, the yew bow more than 1.8 metres in length, the deerskin quiver and the 14 arrows it contained – all these finds abruptly cast everyday hunter-gatherer life in the Copper Age in a completely new light. One detail still astonishes archaeologists today. Two of Ötzi’s arrows were ready for shooting, fitted with triple radial fletching and razor-sharp flint points that had been carefully glued to the shaft with birch pitch.

**One of Ötzi’s arrowheads (front and rear faces), which was affixed to the arrow shaft with birch pitch.** Wikimedia

**Reconstruction of Stone Age hunter Ötzi, with his weapons and axe.** Wikimedia

Birch pitch, also known as birch tar, is a black, viscous mass with a strong smell. It has been known for decades that the substance was used as a highly potent all-purpose adhesive as early as 45,000 years ago; other sources believe the Neanderthals were using birch pitch more than 200,000 years ago. It was used to attach stone blades to wooden handles, or to glue broken pottery. Birch pitch was even used for waterproofing canoes and boats. The substance has been found at numerous campsites and settlements dating from the Mesolithic and Neolithic periods – including in Switzerland. During the excavation of a 5,700-year-old settlement at Oberrisch on Lake Zug, for example, 18 large pieces of birch pitch, now on display in the Museum of Prehistory in Zug, were found within the footprint of a Stone Age house.

**Pieces of birch pitch bearing tooth marks from the Hornstaad lake dwelling settlement on Lake Constance.** Archäologisches Landesmuseum Baden Württemberg

But despite its widespread usage, this birch pitch has long been a puzzle to archaeology. Initially, researchers didn’t even know what the strange prehistoric lumps were – they were referred to as “urn resin or tomb resin”, but birch pitch is made from neither the resin nor the sap of the birch tree. Until recently it was thought that it was obtained by carbonising the leathery, white outer layer of the bark in an airtight vessel at temperatures of almost exactly 350 degrees Celsius, a process known in chemistry as “destructive distillation” and also “pyrolysis”. This rather complex process requires the availability of a hermetically sealed ceramic combustion chamber and control of the firing temperature, which suggests that Neolithic craftsmen possessed quite astonishing technological skills.

**Modern experimental attempt to produce birch pitch in a single-pot process: the birch bark is carbonised in a vessel under airtight conditions.** Wikimedia

In 2019, however, a research team from the University of Tübingen discovered that birch pitch can also be produced using a much more straightforward method. It is sufficient to burn birch bark in a hearth or fire pit surrounded by smooth vertical stone surfaces. Even at normal firing temperatures and without sealing in an airtight vessel, the birch pitch is deposited on the stone surface after just three hours and can be scraped off for later use. Today, archaeologists believe that birch pitch was discovered by accident. But its properties were quite astounding. To soften them, the black lumps were heated and then applied; for tools, stone knives and arrowheads, the adhesive joint was usually reinforced with a plant twine lashing. Once it had cooled, the pitch gave a bond almost as strong as a modern two-component adhesive.

**Birch pitch obtained in the single-pot process: the end product consists of tar and the ashes of the bark.** Wikimedia

These characteristics are based on the high content of resin acids and highly volatile aromatic compounds. During production of the birch bark pitch, a polymerisation reaction takes place between the molecules contained in the substance; the chain-like connections turn the previously low-molecular compounds into a polymer. Birch pitch is thus humankind’s very first synthetic material, and it was in use for a very long time. Theophrastus, a student of Aristotle, describes its production, as does the Roman historian Pliny the Elder in his Historia naturalis. Even in the Middle Ages, equipment and tools were glued with birch pitch; it was only the plastics revolution in the 19th century that put an end to the use of this Stone Age adhesive.

**Lump of birch pitch from Syltholm on Lolland, Denmark, with tooth marks of a Stone Age female.** University of Copenhagen

But birch pitch still holds one last puzzle. Human tooth imprints which still contained usable amounts of DNA were discovered on a piece of birch pitch found in Denmark. The lump of pitch, researchers found, was chewed 5,700 years ago by a female who was closely related to hunter-gatherers from continental Europe and had dark skin, dark brown hair and blue eyes. Was she trying to soften the birch pitch by chewing it, was she using it for dental care, or was the first superglue also humankind’s first chewing gum? So far, science has been unable to provide a conclusive answer to this question.

More of: Protohistory Research Nature Article

Stone Age superglue

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