admin Ancient Mayan cities don’t seem like places where humans could experiment with their own chemicals. But scientists believe that’s exactly what happened at Copan, an archaeological complex in what is now a mountainous rainforest valley in western Honduras.
According to historians, Copan’s Golden Age began in 427 AD when a king named Yax Kuku Mo entered the valley from the northwest. His dynasty built his one of the jewels of the Mayan world, but it was abandoned in the 10th century, leaving courtyards and squares at the mercy of the jungle. More than 1,000 years later, Copán’s buildings have survived remarkably well despite long periods of baking from the tropical sun and humidity.
The secret may lie in the plaster that the Mayans used to cover Copan’s walls and ceilings. New research suggests that sap from the bark of local trees that Mayan artisans mixed into the plaster helped strengthen the structure. Whether accidentally or intentionally, these Mayan builders created a material similar to mother-of-pearl, the natural element found in seashells.
“The mystery of ancient Mayan stonemasonry has finally been solved,” says Carlos Rodriguez Navarro, a mineralogist at the University of Granada in Spain and lead author of the paper. Rodriguez Navarro and his colleagues published their findings today in the journal Science Advances.
The plasterers followed a very simple recipe. Start with a carbonate rock like limestone. Bake above 1,000 degrees Fahrenheit. Mix the resulting quicklime with water. The mixture is then reacted with carbon dioxide in the air. The final product is what builders call lime plaster or lime mortar.
Civilizations around the world often discover this process independently. For example, Mesoamericans in Mexico and Central America learned around 1,100 BC. Chr. That way. The ancients used it to cover surfaces and to fix bricks, but this simple lime plaster is not particularly durable by modern standards.
However, just as cuisine varies from town to town, so do limestone recipes. “Some of them are better than others,” says Admir Masic, a materials scientist at the Massachusetts Institute of Technology who was not involved in the study. Experts agree that Mayan lime plaster is one of the best.
Rodriguez Navarro and his colleagues wanted to know why. They found their first clue when they examined brick-sized chunks of plaster from Copan’s walls and floor with X-ray and electron microscopy. Traces of organic matter, such as carbohydrates, were found in some fragments.
This piqued their interest, says Rodriguez Navarro. This is because it corroborated early archaeological and written records suggesting that ancient Mayan masons mixed gypsum with plant material. The other standard constituents (lime and water) cannot explain the complex carbon chains.
To follow this example, the authors decided to make their own historical plaster. They consulted the descendants of Freemasons and Mayans living near Copán. A local resident told me about the chucum and jiote trees that grow in the surrounding forest, especially the sap from the bark of the trees.
The authors tested the reaction of sap when mixed with gypsum. This not only made the material harder, but also made the gypsum less soluble in water. This partly explains why Copan has endured the local climate so well.
The microstructure of plant-rich gypsum resembles nacre, or mother-of-pearl, iridescent substances that some mollusks produce to cover their shells. It is not well understood how mollusks produce nacre, but we do know that nacre consists of crystalline plates sandwiched between elastic proteins. This combination makes the outside of the marine life more resilient and protects it from the elements caused by waves.
Closer examination of ancient gypsum specimens and modern analogues reveals that the rock also contains layers of calcite slabs and organic sap material, giving the material the same toughness as nacre. . “They were able to mimic the behavior of living things,” says Rodriguez-Navarro.
“This is really exciting,” Masic says.
Now Rodriguez Navarro and his colleagues are trying to answer another question: from Iberia to Persia to China, to Freemasonry. Could other dependent civilizations have encountered the same secret? For example, it is known that a Chinese lime gypsum manufacturer added glutinous rice broth to it for strength.
Gypsum is not the only ancient substance that scientists have reconstructed. Masic and his colleagues discovered that ancient Roman concrete had the ability to “self-heal.” More than 2,000 years ago, builders across the empire may have added quicklime to rock masses, creating microstructures in the material that helped fill pores and cracks when seawater hit them.
If that feature sounds useful, so do modern engineers. There is a thriving field dedicated to the study and recovery of historical material. The upright structures unearthed from the site have already proven to withstand the ravages of time. As a bonus, the ancients tended to work in a more sustainable way and use less fuel than their industrial counterparts.
“Maya’s paper…is another great example of this [scientific] approach,” says Massic.
Mayan plaster isn’t going to replace the ubiquitous concrete of the modern world, but scientists say it could help preserve and strengthen masonry in pre-industrial buildings. I’m here. Smelling tree sap can extend the life of structures by centuries.
