Ancient Maya masons had a smart way to make plaster stronger

Up close, the Mayas' timeless recipe from Copán looks similar to mother-of-pearl.
Ancient Maya idol in Copán, Guatemala
The idols, pyramids, and dwellings in the ancient Maya city of Copán have lasted longer than a thousand years. DEA/V. Giannella/Contributor via Getty Images

An ancient Maya city might seem an unlikely place for people to be experimenting with proprietary chemicals. But scientists think that’s exactly what happened at Copán, an archaeological complex nestled in a valley in the mountainous rainforests of what is now western Honduras.

By historians’ reckoning, Copán’s golden age began in 427 CE, when a king named Yax Kʼukʼ Moʼ came to the valley from the northwest. His dynasty built one of the jewels of the Maya world, but abandoned it by the 10th century, leaving its courts and plazas to the mercy of the jungle. More than 1,000 years later, Copán’s buildings have kept remarkably well, despite baking in the tropical sun and humidity for so long. 

The secret may lie in the plaster the Maya used to coat Copán’s walls and ceilings. New research suggests that sap from the bark of local trees, which Maya craftspeople mixed into their plaster, helped reinforce its structures. Whether by accident or by purpose, those Maya builders created a material not unlike mother-of-pearl, a natural element of mollusc shells.

“We finally unveiled the secret of ancient Maya masons,” says Carlos Rodríguez Navarro, a mineralogist at the University of Granada in Spain and the paper’s first author. Rodríguez Navarro and his colleagues published their work in the journal Science Advances today.

[Related: Scientists may have solved an old Puebloan mystery by strapping giant logs to their foreheads]

Plaster makers followed a fairly straightforward recipe. Start with carbonate rock, such as limestone; bake it at over 1,000 degrees Fahrenheit; mix in water with the resulting quicklime; then, set the concoction out to react with carbon dioxide from the air. The final product is what builders call lime plaster or lime mortar. 

Civilizations across the world discovered this process, often independently. For example, Mesoamericans in Mexico and Central America learned how to do it by around 1,100 BCE. While ancient people found it useful for covering surfaces or holding together bricks, this basic lime plaster isn’t especially durable by modern standards.

Ancient Maya pyramid in Copán, Guatemala, in aerial photo
Copán, with its temples, squares, terraces and other characteristics, is an excellent representation of Classic Mayan civilization. Xin Yuewei/Xinhua via Getty Images

But, just as a dish might differ from town to town, lime plaster recipes varied from place to place. “Some of them perform better than others,” says Admir Masic, a materials scientist at the Massachusetts Institute of Technology who wasn’t part of the study. Maya lime plaster, experts agree, is one of the best.

Rodríguez Navarro and his colleagues wanted to learn why. They found their first clue when they examined brick-sized plaster chunks from Copán’s walls and floors with X-rays and electron microscopes. Inside some pieces, they found traces of organic materials like carbohydrates. 

That made them curious, Rodríguez Navarro says, because it seemed to confirm past archaeological and written records suggesting that ancient Maya masons mixed plant matter into their plaster. The other standard ingredients (lime and water) wouldn’t account for complex carbon chains.

To follow this lead, the authors decided to make the historic plaster themselves. They consulted living masons and Maya descendants near Copán. The locals referred them to the chukum and jiote trees that grow in the surrounding forests—specifically, the sap that came from the trees’ bark.

Jiote or gumbo-limbo tree in the Florida Everglades
Bursera simaruba, sometimes locally known as the jiobe tree. Deposit Photos

The authors tested the sap’s reaction when mixed into the plaster. Not only did it toughen the material, it also made the plaster insoluble in water, which partly explains how Copán survived the local climate so well.

The microscopic structure of the plant-enhanced plaster is similar to nacre or mother-of-pearl: the iridescent substance that some molluscs create to coat their shells. We don’t fully understand how molluscs make nacre, but we know that it consists of crystal plates sandwiching elastic proteins. The combination toughens the sea creatures’ exteriors and reinforces them against weathering from waves.

A close study of the ancient plaster samples and the modern analog revealed that they also had layers of rocky calcite plates and organic sappy material, giving the materials the same kind of resilience as nacre. “They were able to reproduce what living organisms do,” says Rodríguez Navarro. 

“This is really exciting,” says Masic. “It looks like it is improving properties [of regular plaster].”

Now, Rodríguez Navarro and his colleagues are trying to answer another question: Could other civilizations that depended on masonry—from Iberia to Persia to China—have stumbled upon the same secret? We know, for instance, that Chinese lime-plaster-makers mixed in a sticky rice soup for added strength.

Plaster isn’t the only age-old material that scientists have reconstructed. Masic and his colleagues found that ancient Roman concrete has the ability to “self-heal.” More than two millennia ago, builders in the empire may have added quicklime to a rocky aggregate, creating microscopic structures within the material that help fill in pores and cracks when it’s hit by seawater.

[Related: Ancient architecture might be key to creating climate-resilient buildings]

If that property sounds useful, modern engineers think so too. There exists a blossoming field devoted to studying—and recreating—materials of the past. Standing structures from archaeological sites already prove they can withstand the test of time. As a bonus, ancient people tended to work with more sustainable methods and use less fuel than their industrial counterparts.

“The Maya paper…is another great example of this [scientific] approach,” Masic says.

Not that Maya plaster will replace the concrete that’s ubiquitous in the modern world—but scientists say it could have its uses in preserving and upgrading the masonry found in pre-industrial buildings. A touch of plant sap could add centuries to a structure’s lifespan.